The
Atmospheric Chemistry Division
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The
Atmospheric Chemistry Division
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DIVISION NARRATIVE
Atmospheric Odd Nitrogen Group
The Atmospheric Odd Nitrogen (AON) Group's (Brian Ridley, Frank Flocke, Andrew Weinheimer, Denise Montzka, David Knapp, and Frank Grahek) expertise is in measurements and analysis of NO, NO2, total reactive nitrogen (NOy), O3, peroxyacetyl nitrate (PAN) and related homologues, and other organic nitrates. This past year the group focused on NO-NOy instrument development, analysis of Cirrus Regional Study of Tropical Anvils and Cirrus Layers - Florida Area Cirrus Experiment (CRYSTAL-FACE) results, kinetics of alkyl nitrates, refined characterization of the fast PAN- Gas Chromatography (GC), and characterization of a novel Thermal Dissociation - Chemical Ionization Mass Spectrometer (TD-CIMS) for PANs.
PAN, PPN, PiBN and MPAN were measured on board the NASA P-3B aircraft during the TRACE-P mission (February to April, 2001) with 20 flights spanning a longitude range of 100 to 300 °E and a latitude range of 10 to 40° N. Local flights were based out of Hong Kong (4) and Yokota Air Base, Japan (5) as well as one night flight out of Midway Islands. PAN mixing ratios ranged from below the limit of detection in the Pacific marine boundary layer to almost 3 ppbv, observed near Tokyo and in the Shanghai urban plume. Strongly elevated mixing ratios of PANs were also observed over the Sea of Japan, the Yellow Sea and the East and South China Seas, associated with continental outflow from Asia. Elevated PAN was also observed far removed from the continent over the central and Eastern Pacific Ocean, when pollution plumes originating from Asia were encountered. The observed ratios of different PAN species were generally consistent with anthropogenic pollution rather than biogenic sources.
The export of PANs from polluted regions is generally viewed as a means of transporting NOx to remote regions where it may impact the chemistry of ozone upon subsidence of an air mass. Indeed, profiles of PAN and NOx observed over the central Pacific by the P3 in TRACE-P suggest that the descent of air parcels and the attendant warming may foster the conversion of PAN to NOx. The median vertical profiles of PAN and NOx for three longitude bins (Asia/Western Pacific, Central Pacific and Eastern Pacific) were simulated in box models runs with imposed density and temperature time series to simulate descent. Also simulated was the impact of the NOx so liberated on the profile of O3.
AON Figure 1a (upper panel) shows a comparison of the measured profiles of PAN, NOx, and ozone with those obtained by modeling the descent of an air parcel from 6 km that is initialized with median measured values and which descends at 1.2 km/day through the observed temperature profile. As the air parcel descends in the model, PAN dissociates at an increasing rate, resulting in a release of NOx, which becomes available to affect ozone chemistry. The ozone profile, however, shows little variation, in both the model and the observations. The lower panel (AON Figure 1b) illustrates the effect of the initial mixing ratio of PAN on ozone. The sensitivity of low-level O3 to initial PAN at 6 km, prior to descent of the air parcel, is about 1 ppbv O3 per 100 pptv PAN. The median value at 6 km for 160-200°E is 200 pptv, so the effect of PAN is to counteract the net destruction that occurs during descent by an amount of 2 ppbv. The upper panel of Figure 1 also shows two model runs with the measured aldehyde mixing ratios and with the aldehydes set to zero. The measured aldehyde mixing ratios of about 150 pptv for acetaldehyde and 75 pptv for propionaldehyde do not agree with the observations of PANs and NOx over the remote Pacific Ocean. Such high aldehyde mixing ratios would remove all NOx in a period of less than a day and convert it back to PAN. Similar circumstances were observed in the Arctic boundary layer during TOPSE. Ambient measurements of aldehydes in remote areas must be reevaluated with respect to inlet artifacts or other interferences.
AON Figure 1a (upper panel): A comparison of the measured profiles of PAN, NOx, and ozone with those obtained by modeling. 1b (lower panel): The effect of the initial mixing ratio of PAN on ozone.
In an attempt to improve instrument performance, and with an eye toward the development of a new, two-channel instrument for the HIAPER aircraft, the group performed laboratory tests of photomultiplier tubes and reaction vessel design. In addition, computer simulations of photon collection were conducted to guide potential reaction vessel modifications. For the new HIAPER instrument, a computer for data acquisition and instrument control was purchased and was interfaced with some components. Work was started on the design of the mechanical layout of the new HIAPER instrument in order to fit into the new aircraft. In a related effort, the group provided instrument design specifications to Purdue University (Paul Shepson, Kimberley Hill) to facilitate their measurement capabilities for NO and NOy.
A goal of CRYSTAL-FACE was to measure the condensed-phase amounts of HNO3 and NOy on ice and other particles. On the WB-57, the NCAR NOy and the NOAA (David Fahey, Ru-Shan Gao, Peter Popp, Tim Marcy) HNO3 instruments were each equipped with similar pairs of inlets so that each would have a forward-facing inlet to collect particles in an enhanced manner (enhancing the particle signal relative to that from the gas phase), plus a second inlet to collect primarily gas-phase species only. The measurements of condensed-phase NOy and condensed-phase HNO3 were generally in agreement with each other, with a couple of interesting exceptions. However, there was no compelling evidence for the presence of significant amounts of condensed-phase NOy species other than HNO3. Overall the measured condensed-phase HNO3 is reasonably well fit with a Langmuir adsorption model and is generally much greater than that which could be explained by dissolution in the solid. Alternatively, the condensed-phase amounts may be calculated using a multi-layer adsorption model, with the measured adsorption being 10-100% of that calculated. The fraction of total HNO3 found in the condensed phase varied with the ice particle surface area with small (<10%) amounts for low surface areas up to 100% for large surface areas.
The difference in the condensed-phase signals from the HNO3 and NOy instruments and the greater sensitivity of the HNO3 instrument to particles smaller than 4 microns suggests the possible presence of HNO3-containing aerosols on one flight, during which the obesrved aerosols competed effectively with ice for the uptake of HNO3. On another flight, an interesting exception to the model-predicted temperature-dependence occurred. Condensed-phase NOy did not vary in correlation with an 18 K temperature change, suggesting that a simple steady-state adsorption model is not adequate to account for all of the observations.
In collaboration with Geoff Tyndall, John Orlando, and Adele Chuck, a visitor from the University of East Anglia, Flocke and Swanson conducted laboratory experiments on the yield of alkyl nitrates from the photo-oxidation of simple branched alkanes. Particular emphasis was placed on the yield of tertiary alkyl nitrate species, since there was suspicion from experiments conducted earlier by Flocke, Tyndall and Orlando that the yields reported in the literature may be incorrect. The yield of a specific alkyl nitrate species from a parent hydrocarbon depends on 1) the rate constant ki for the attack of OH on the specific site forming the alkyl peroxy radical precursor to the alkyl nitrate molecule in question and 2) the branching ratio α of the reaction of the RO2 radical with NO which can result in alkyl nitrate production (minor channel) and in the production of NO2 and an RO radical:
RO2 + NO à RO + NO2 ki·(1-α)
RO2 + NO à RONO2 ki·α
Alkyl nitrate isomer formation from isobutane, isopentane, 2-methylpentane and 3-methylpentane was investigated. We could show that, for a given carbon number, the branching ratios α for primary and tertiary RO2 are very close to those of secondary species. This is in contrast to the values reported in the literature, where primary and tertiary RO2 have about a factor of two smaller branching ratios than the secondary species of the same carbon number. There is a general increase of α with carbon number which was reported previously. We also found that tertiary alkyl nitrates seem to be irreversibly adsorbed onto or destroyed on stainless steel surfaces, which is the likely reason for the incorrect branching ratios reported in the literature and also for the absence of these species in most air samples.
The dual channel fast gas chromatograph (fast PAN-GC) with electron capture detection was cross-calibrated against a total nitrogen oxide (NOy) instrument for the measurement of the peroxyacyl nitrates (PANs). The intercalibration determined the sensitivity of the fast PAN-GC for C2-C4 PANs by using a secondary preparative GC to purify synthesized liquid diffusion standards, which were subsequently analyzed on the fast PAN-GC and the NOy instrument. Results demonstrated a 95.5 ± 0.5 % conversion efficiency for PAN from the photolytic calibration source, and defined the response factors relative to that of PAN as 0.90, 0.64, and 0.55 for PPN, MPAN, and PiBN, respectively. This work was carried out in collaboration with James Roberts at NOAA AL. Results will be presented along with a full analytical description of the fast PAN-GC system in an article that is in preparation for submission to the Journal of Analytical Chemistry.
The current work of the PAN group focuses on the development and characterization of a novel Thermal Dissociation - Chemical Ionization Mass Spectrometer (TD-CIMS) method to replace the fast PAN-GC for the atmospheric measurement of peroxyacetyl nitrate compounds. The TD-CIMS system was newly developed by a collaborative group of scientists at NCAR and the Georgia Institute of Technology (Slusher et al., 2003). The ionization scheme uses a 10 mCi Po210 source to ionize CH3I to I- by alpha particle bombardment. The I- reacts with acyl peroxy radicals generated by the thermal dissociation of the PAN compounds in front of the inlet to form the acyl ion which is detected in the mass analyzer. The very low ionization potential of I- leads to a very selective ionization process that only reacts with the acyl peroxy radical species, and a few nitrogen oxide species (NO3 and N2O5). This method currently demonstrates a 3-10 pptv detection limit with a 1-10 second time resolution for PAN, PPN, PiBN, and APAN. Therefore the method will be ideally suited for fast measurements of fluxes in the boundary layer or for emission plumes on aircraft platforms. Atmospheric concentrations of PAN and PPN, as measured by the TD-CIMS instrument, showed good agreement (±10%) with those of our fast PAN-GC under field conditions.
A detailed laboratory characterization of the TD-CIMS instrument has been carried out in preparation for deployment on the NOAA P-3 research aircraft for the NOAA’s Northeast- North Atlantic (NENA) campaign in summer 2004. The TD-CIMS has been cross-calibrated against the fast PAN-GC and against the NOy instrument to determine calibration factors and relative sensitivities for the PAN homologues (PAN, PPN, PiBN, and MPAN) on the TD-CIMS. In addition, the PAN homologues have been produced in a 50 liter reaction chamber at the NCAR/ACD kinetics lab, in collaboration with Geoff Tyndall and John Orlando, where the TD-CIMS was cross calibrated against the long path FTIR connected to the reaction chamber. Additionally, some novel PAN homologues (1-hydroxyl-peroxyacetyl nitrate (HPAN), methoxyformyl nitrate (MoPN, and peroxybenzoyl nitrate (PBzN)) were also produced in the chamber to investigate the TD-CIMS measurement capabilities. HPAN appears to be very short lived under ambient conditions and the sensitivity could not be determined, while MoPN and PBzN had equivalent sensitivities to PAN on the TD-CIMS.
AON Figure 2: Time series of PANs measurements using the new TD-CIMS showing low background counts (left half) while sampling zero air and sensitive response (right half) when sampling outside air. A calibration signal from the PAN photosource is intermittently superimposed while on zero air. Note the logarithmic scale on the vertical axis.
Measurement Of Pollution In The Troposphere Group
The Measurement Of Pollution In The Troposphere (MOPITT) experiment has been operating for nearly three years now, providing atmospheric scientists with a multi-year, global view of carbon monoxide profile concentrations in the troposphere. The mission is a joint Canadian-US effort, and NCAR/ACD is responsible to NASA for the continued development of the data reduction algorithms and for operational data processing at every stage from instrument counts to calibrated radiances, through to globally retrieved carbon monoxide vertical profiles and methane total columns. MOPITT Version 3 validated tropospheric CO profiles are currently being processed and delivered to NASA for use by the international community. This is the first dataset of its kind, and represents a significant advance in satellite remote sensing of the troposphere.
The MOPITT group, including Jarmei Chen, Merritt Deeter, David Edwards, Proj Ldr, Louisa Emmons, Gene Francis, John Gille, Prog Mgr, PI, Ben (Shu-Peng) Ho, Lawrence Lyjak, Debbie Mao, Steve Massie, Daniel Packman, Barbara Tunison, Valery Yudin, and Daniel Ziskin, has been active in 3 main areas this year: (1) continued improvement of the data reduction algorithms, data processing, and delivery of the final products to NASA for later distribution to the community; (2) validation of the MOPITT data with independent correlative measurements to accurately assess data quality; and (3) use of the MOPITT data in tropospheric chemistry and transport studies. More information about the MOPITT project can be found at http://www.eos.ucar.edu/mopitt/ and http://www.atmosp.physics.utoronto.ca/MOPITT/home.html.
The new retrieval algorithm that was developed this year has required careful validation. Validation activities are essential at each level of the data processing to ensure a full understanding of the in-flight MOPITT performance, to allow characterization of measurement accuracy, precision, and resolution, and to point the way to needed improvements. The validation of the Version 3 MOPITT Level 2 data product (mixing ratio profiles and total column amounts) was performed by comparison with numerous in-situ aircraft measurements from fixed sites by Paul Novelli (NOAA CMDL), and from measurements taken during intensive field campaigns. As a result of this validation work, the status of the MOPITT Level 2 product has been upgraded from “provisional” to “validated”. This status change is an indication to the community users of the high quality of the data that has been provided by the NCAR MOPITT team. A paper presenting the validation results has been submitted to JGR (Emmons et al., Validation of MOPITT CO retrievals with aircraft in-situ profiles.)
The results of the validation of MOPITT CO with aircraft in-situ measurements from CMDL sites and from intensive field campaigns are shown in MOPITT Figure 1. This indicates the bias (MOPITT minus aircraft, in percent) for each validation profile, and shows that the general correlation is very strong. This work is continuing, and is important for characterizing the effect of the retrieval algorithm reconfiguration that was required following the loss of one of the instrument coolers and half the channels in April 2001. The Phase 2 product shows only small differences compared to that of Phase 1, with no apparent loss of vertical resolution. The culmination of the algorithm development effort, the retrieval reconfiguration, and the validation work, is that a multi-year MOPITT record is now available for examination. MOPITT Figure 2 shows the zonal average.
MOPITT Figure 1: Scatter plot of MOPITT versus aircraft data for each retrieval level and column, for Phase 1 data from the five CMDL sites (March 2000-May 2001). The in-situ aircraft data have been transformed with the averaging kernel and a priori profile. The error bars indicate the inter-quartile range for each MOPITT overpass. The dashed line is the 1:1 line and the correlation coefficient (R) is given.
MOPITT Figure 2: Zonal mean MOPITT 700 hPa mixing ration (ppbv) as a function of year since the launch of the Terra satellite.
The combination of satellite retrievals of CO, NO2, aerosol, fire and lightning flash counts, can provide a powerful method for investigating the production of ozone precursors. Edwards, Lamarque, Emmons, and Gille, have used these sensor data in conjunction with in-situ measurements and chemical transport modeling to investigate the impact of biomass burning on tropical chemistry and to help explain the observed tropospheric ozone distribution over the Atlantic.
There has been considerable interest in the recent literature regarding the apparent Atlantic tropical tropospheric ozone “paradox” (Thompson et al., 2000; 2001). This can be summarized as follows: Most of the African northern hemisphere (NH) biomass burning in January and February occurs north of the intertropical convergence zone (ITCZ), while the maximum in most of the satellite-derived tropical tropospheric ozone (TTO) columns is observed in the southern hemisphere (SH) tropical Atlantic, south of the ITCZ. Apart from requiring a mechanism of significant ozone precursor production and inter-hemispheric transport across the ITCZ, it is also hard to reconcile this behavior with modeling studies and in-situ measurements which generally show high tropospheric ozone in regions of intense burning.
The mean MOPITT CO mixing ratio over Africa and the Atlantic for January 20-27, 2001, at 700 hPa, is shown in MOPITT Figure 3. The importance of biomass burning as a source of CO over northern equatorial Africa is readily apparent, and the northern extent of the plume correlates well with satellite observations of savanna fires. Emissions from the region of maximum burning in eastern Africa are generally transported southwest by the prevailing Harmattan flow to the ITCZ where convection takes place, and the plume ends up over southern central Africa or out in the Gulf of Guinea. Emissions from the fires in western Africa are advected westwards out into the Atlantic, suggesting that the ITCZ in this region presents a stronger barrier to interhemispheric transport. Once in the free troposphere, the CO from west Africa forms a strong plume that is caught in the tropical easterlies. MOPITT data show that this plume is persistent during the months December 2000 - April 2001, and that long-range transport carries CO to South America. Part of this plume crosses to the Pacific Ocean, while part is circulated over Amazonia. The absence of a significant CO plume in the southern tropical Atlantic suggests that it is unlikely that the north African fires are the origin of the observed Total Ozone Mapping Spectrometer (TOMS) TTO maximum.
Further information about ozone precursor distributions is provided by the ERS-2/GOME instrument. The January 2001, mean residual tropospheric NO2 vertical column is shown in MOPITT Figure 4. This can be compared directly with the MOPITT CO, and shows strong correlation with the peak CO concentrations over the fire locations. The westward extent of the NO2 plume over the equatorial Atlantic is not as great due to the shorter lifetime. Also evident are the significant levels of NO2 over southern Africa where very few fire counts are observed at this time. Elevated levels are also seen over South America and out into the southern Atlantic. The absence of significant low altitude CO plumes in these regions suggests a possible lightning source of NO2 in the mid-troposphere. Further evidence of this is provided by the fact that the areas of highest lightning flash density from other satellite observations corresponds well with the GOME NO2 observations. The resulting ozone would be advected westwards into the Atlantic, and may be one component of the ozone maximum observed in the TOMS TTO. This conclusion has been confirmed with a modeling analysis using the Model for Ozone and Related Chemical Tracers (MOZART)-2 Chemical Transport Model (CTM). This investigation has included the collaboration of Jean-Luc Attie and Jean-Pierre Cammas from the Observatoire Midi Pyrenees, Toulouse, France, and Andreas Richter of the University of Bremen, Germany. A paper detailing these findings was published in JGR, Edwards et al., Tropospheric ozone over the tropical Atlantic: A satellite perspective. This paper prompted press releases by, NASA, AGU, and NCAR, and was the subject of a number of reports in the electronic press.
MOPITT Figure 3: MOPITT CO (ppbv) distribution at 800 hPa. Mean values for January 20-27, 2001. Data resolution is 0.5° longitude x 0.5° latitude.
MOPITT Figure 4: The January 2001 mean GOME residual tropospheric NO2 vertical column. Data resolution is 0.5° longitude x 0.5° latitude.
One of the major science activities by Gille, Valery Yudin, and Larry Lyjak, has been the use of assimilated data to investigate the transport of CO across the Pacific. An initial study defined the periods of major transport, in the Spring, and the primary latitudes involved. Subsequent analyses showed the source regions and transport paths in latitude and altitude. Not infrequently, plumes are observed to reach the West Coast of North America, although these do not appear to penetrate inland. Intercontinental transport to and from Europe has also been examined by Pfister using a combination of data from MOPITT, aircraft, and in-situ measurements in conjunction with MOZART-2 modeling studies.
MOPITT data has also been used by Lamarque and Edwards to examine the regional impact of localized emissions. During the summer of 2000, large wildfires burned more than a million acres over the Northwest and Rocky Mountain regions of the USA, with particularly high intensity in Idaho and Montana. The plumes from these fires are clearly evident in the MOPITT CO distributions, and indicate that the emissions spread across the USA reaching down into Florida in a manner consistent with the prevailing meteorology. The MOZART-2 CTM was used to investigate the CO emissions, initiated at the location of the fires as provided by Advanced Very High Resolution Radiometer (AVHRR) observations analyzed by the United States Forest Service, that were required to reproduce the CO enhancement above background levels that were observed by MOPITT. This analysis indicated that in the month of August 2000, the Western fires were responsible for some 9 Tg of CO, which is equivalent to about half the US anthropogenic emissions of CO for the same month. This work has now been published in JRL, Lamarque et al., Identification of CO plumes from MOPITT data: Application to the August 2000 Idaho-Montana forest fire. Similar studies are continuing to investigate the CO emissions and plume transport from the Colorado Hayman and Arizona Chedeski fires of the summer of 2002.
The MOPITT modeling team (Boris Khatattov, Yudin, Gabrielle Petron) has continued work on the assimilation of MOPITT data, profiles and total column independently, in the MOZART-2 global chemistry-transport model. The forecast model error specification has been revised in the assimilative code, and the effect of uncertainties in the analyzed winds, boundary layer venting, cloud mass fluxes, surface fluxes, and chemical production and loss continue to be examined. Work is starting on the direct assimilation of MOPITT Level 1 radiances into the MOZART-2 model. This will have the advantage of eliminating the traditional retrieval step in the data processing and allow a fully-consistent treatment of a priori information.
Gabrielle Petron, an NCAR/ASP
scientist working in the MOPITT team, has led an investigation of inverse
modeling using the MOPITT data and the MOZART-2 model to constrain surface
fluxes of carbon monoxide and assess the accuracy of emission inventories. This
work has also involved the collaboration of Claire Granier (University
of Paris). As a result of this investigation, a new set of monthly mean
surface CO emissions was produced for a full year and is available for
use in numerical simulations. Model results obtained with the new emissions
show significantly better agreement with independent measurements.
One of the most important findings is that anthropogenic emissions from both North America and Asia are in reality significantly larger than previously thought. Petron recently defended her PhD thesis with honors using results of this work. MOPITT Figure 5 shows the global anthropogenic CO budget separated by geographical region and emission type for the a priori estimates and the a posteriori estimates resulting from the inverse modeling of the MOPITT data.
Work on the assimilation of MOPITT data is also being carried out by Jean Luc Attie at the Observatoire Midi Pyrenees, Toulouse, France. Attie was previously a long-term visitor in the MOPITT group. The observational data are being assimilated into the Meteo-France chemical transport model MOCAGE (Modele de Chimie Atmospherique a Grande Echelle) using a Kalman-Bucy filter. This work is being used to assess the impact of the MOPITT CO assimilation on the simulated ozone field, by comparing first with model results without assimilation and with aircraft data from the MOZAIC campaign.
MOPITT Figure 5: Anthropogenic CO emissions by different sources and geographical regions from previous inventory estimates, a priori and after performing a linearized Kalman filter inversion of the MOPITT CO data at 700 hPa in conjunction with the MOZART-2 chemical transport model, a posteriori
Edwards is leading a study to investigate emissions from biomass burning in Africa and South America. This is a major source of pollution in the tropical troposphere and a major forcing of tropospheric chemistry. The outflow of both aerosol and CO from the widespread fires in southern Africa and South America during September-November, 2000, is being studied using observation from the Terra satellite, CO profiles from MOPITT and aerosol optical depth from MODIS. Recent developments in aerosol retrieval allow the distinction to be made between fine and coarse mode particles. The fine mode particles are produced by the same anthropogenic combustion processes that emit CO, and comparison of the fine mode aerosol and CO distributions provide information about biomass plume aging and the associated advection and convection of the emissions. The long range transport of these pollutants and the impact on atmospheric air quality in remote oceanic regions is also being studied, along with the seasonal changes in the pollutant distributions by comparing with the observations of CO and fine mode aerosol made during the December, 2000-February, 2001, period. A paper describing this work will soon be submitted to JGR.
MOPITT data are currently being made available to the National Institute for Space Research in The Netherlands (Ilse Aben and Anne Grete Straume) for use in CO validation studies for the recently launched European Environmental Satellite/SCanning Imaging Absorption SpectroMeter for Atmospheric CHartographY (Envisat/SCIAMACHY) instrument. A similar collaboration is underway with Larrabee Strow and Wallace McMillan at University of Maryland, Baltimore County (UMBC) who are investigating the potential for retrieving CO from the Aqua/ Atmospheric Infrared Sounder (AIRS) instrument. MOPITT data will also provide key validation of the CO retrievals expected from the Aura/Tropospheric Emission Spectrometer (TES) instrument to be launched next year.
As already noted, MOPITT data are particularly useful for providing a global context to localized measurements. In this respect, collaborations are underway with a number of groups involved in field campaigns. Validation of MOPITT measurements was a goal of the 2001 TRACE-P mission. The NCAR MOPITT team collaborated with the group of Daniel Jacob at Harvard University to compare with aircraft in-situ measurements taken during this campaign and to investigate Asian outflow into the Pacific. A paper detailing this work, Hearld et al., Asian outflow and transpacific transport of carbon monoxide and ozone pollution: An integrated satellite, aircraft and model perspective, is now in press at JGR. MOPITT data have also been provided to Paulo Artaxo (Sao Paulo University, Brazil) for use in the analysis of the results in the Smoke Aerosols, Clouds, Rainfall and Climate (SMOCC) campaign in Rondonia, Amazonia. There is also considerable interest in the role of mega-cities in pollutant production and a potential involvement of MOPITT in the ACD MIRAGE initiative in Mexico City.
MOPITT Operational Studies
The NCAR MOPITT team has been engaged in the ongoing data reduction process to produce geophysical quantities for scientific use by the community from the instrument count data. The elements of this processing capability and the people responsible, are: the data handling interfaces and protocols between NCAR and the NASA centers which receive and archive the satellite data and the ancillary meteorological data (Daniel Ziskin, Jarmei Chen); the Level 0-1 processor which calibrates the instrument counts to produce geolocated radiances (Ziskin, Debbie Mao, Chen); and the Level 1-2 processor which comprises a forward model which provides a full simulation of the MOPITT measurement, a cloud detection algorithm, and a retrieval algorithm (David Edwards, Gene Francis, Juying Warner, Merritt Deeter, Ben Ho, and Gille). The retrieval combines information from the measurements, the forward model, and previous measurements which define the current understanding of the atmosphere, to obtain the most likely CO profile or CH4 column consistent with the measured MOPITT signal. The Moderate Resolution Imaging Spectroradiometer (MODIS) cloud mask is also used operationally in the current data processing, and ensuring the smooth transfer of this data from NASA to NCAR in a timely manner has taken considerable effort on the part of Ziskin and Chen.
The algorithms continue to be developed as greater confidence in the instrument performance characterization is acquired over time. The data for the mission Phase 1, before the instrument cooler failure in April 2001, have been designated as a validated product. The data for the post-April 2001 period, Phase 2, are currently provisionally validated and will be upgraded to fully validated in the near future. Work has continued to characterize the instrument performance in the Phase 2 period, and to develop the retrieval algorithms to ensure maximum utilization of the instrument signal to retrieve CO vertical structure. A paper describing the operational MOPITT CO retrieval algorithm, Deeter et al., Operational carbon monoxide retrieval algorithm and selected results for the MOPITT instrument, was published in JGR in July.
The algorithms are now being prepared for the next data version 4. This includes a new forward model with improved description of the MOPITT gas correlation cells, and a new cloud clearing technique. Work has also continued on the characterization of MOPITT instrument noise issues to ensure that CO retrievals are not biased for different stares. The validation of MOPITT retrieved surface skin temperature using collocated MODIS measurements has been a major effort led by Ho. This has involved studies to quantify the sensitivity of the MOPITT CO, surface temperature, and emissivity retrievals to various underlying surface conditions, and has led to the development of an iterative retrieval algorithm using the MOPITT forward model together with collocated MODIS surface skin temperature to produce a global 4.7 µm emissivity database. This will be used in the new version 4 algorithm.
The MOPITT team has also begun production of a gridded level 3 product. This is intended mainly as a browse product since most modelers prefer to make their own gridded products according to their particular model resolution. The production of a gridded and time-averaged product is not as straight-forward as it might appear at first sight due to inherent differences between retrieval characteristics day to night, and between land and ocean. The MOPITT website continues to be developed. This is regarded as an important means of community outreach and education. The website currently includes details of how to obtain the data and how to use it correctly, along with other MOPITT references.
A major effort within the group in recent months was the definition of the Team Leader proposal to NASA, headed by Gille, to continue the refinement and improvement of the MOPITT data, and make further progress on the methane retrieval. Several areas in the CO retrievals were identified as needing further work, including the improved surface emissivities, correction for systematic errors in the forward radiance model, verification of error characteristics, and exploration of the use of variable a priori data.
Recently, there have been significant advances in the understanding of the high noise levels that were observed in the 2.2 micron solar reflectance channels that are used to obtain information about the CO and CH4 total atmospheric columns. This effort is being led by Gabriele Pfister, a visiting Erwin Shrodinger Fellow from Austria. Problems have been identified that prevent methane retrieval and potential corrections are being devised. The main issues relate to: polarization effects in the instrument, which affect the measurements mostly over ocean sunglint regions; high instrumental noise contribution in the signals over the ocean; stray-radiation in the instrument; and a spectrally varying surface reflectance over land. The latter causes complications in the retrieval because the retrieval theory requires the surface reflectance to be constant over the channel bandpass. It is hoped that the identification and characterization of these issues will enable a useful CH4 product to be obtained in the near future.
Characterization of the MOPITT calibrated radiances is a critical element of MOPITT validation. In the past year, a project was completed to validate the Level 1 radiances, and the results are described in a paper now in press by JGR. During this investigation, a correlation was discovered between CO concentration and radiance bias, possibly indicating a problem with the operational radiative transfer model. Resolving this problem should significantly improve the quality of future MOPITT CO products. The vertical resolution of the MOPITT CO retrievals is a key indicator of retrieval performance and is of interest to many end-users. However, vertical resolution is highly variable, and depends on retrieval configuration, surface temperature and emissivity, atmospheric temperature profile, etc. One index for quantifying vertical resolution is termed “Degrees of Freedom for Signal” (DFS). DFS approximately represents the number of layers in the retrieved profile which are retrieved independently. A project is now underway to quantify DFS and use it to investigate the dependence of retrieval performance on retrieval configuration (i.e., the subset of radiances activated in the retrieval algorithm), daytime versus nighttime, geographic surface types, etc.
The MOPITT Algorithm Test Radiometer (MATR) aircraft instrument team (Alan Hills, Janguo Nui, Deeter) has continued studies using the 2001 flight data taken (1) during April and May over the Oklahoma Cloud and Radiation Testbed (CART) site; and (2) during November over three Western U.S. cities (Los Angeles, Las Vegas, and Denver). These flights were supported with in-situ measurements by Paul Novelli (NOAA CMDL). Analysis of retrieval results for recent MATR measurements over the western cities show plumes of CO whose direction and distribution appear to agree well with the prevailing meteorology. A manuscript describing the MATR operation, recent flights, and scientific analysis of the data is in preparation.
Analytical Photonics and Optoelectronics Laboratory
The Analytical Photonics & Optoelectronics Laboratory (APOL) (Alan Fried, James Walega, and Dirk Richter) was established this past year at the NCAR Foothills complex. The new APOL group and facility is a joint Atmospheric Technology (ATD) / Atmospheric Chemistry (ACD) endeavor. The new state-of-the-art APOL facility was created to enable NCAR and visiting scientists, including national and international visitors, students, and teachers from area high schools, undergraduates, and graduate students to take advantage of recent advances in photonic and telecommunication laser-based technology for developing the next generation of advanced optical systems for ground and airborne atmospheric research. The APOL group has two overarching goals: a) To develop new technologies and advanced instruments for improved ground-based and airborne measurements, and b) To utilize the high quality data thus obtained to further advance our understanding of atmospheric processes and chemical transformations.
The new APOL facility was established with help from the NCAR Director’s Opportunity fund, and ACD and ATD shared responsibility for the support laboratory operations this past year. The new APOL facility required extensive modifications to an existing laboratory at the Foothills complex, and APOL personnel were involved in all aspects of this major endeavor, including: the planning, design and oversight of major laboratory modifications; the purchase of required infrastructure items (laboratory benches, workbenches, cabinets, tools, and scientific equipment); and the implementation of the subsequent move from the Mesa Laboratory to the new facility.
In addition to the requisite infrastructure support now available, the new APOL facility has approximately three times more useable space than the former Mesa laboratory. This aspect is particularly important for sustaining current and future simultaneous multiple research and development efforts. The new APOL facility, moreover, provides a new restricted access clean room for eye-safe development and characterization of new laser sources.
Scientists at the NASA Johnson Space Flight Center Toxicology Group have established spacecraft maximum allowable concentrations (SMACs) for a variety of selected airborne contaminants onboard the International Space Station. One such contaminant gas, formaldehyde (CH2O), is of particular concern since it emanates from a wide variety of common materials, including various foams and epoxies employed on the International Space Station. Despite the fact that these materials have been extensively tested and deemed safe by NASA and other groups, CH2O concentrations have continued to increase on the space station, and present levels now exceed the SMAC value of 40 parts-per-billion for exposure durations greater than 7 days. Upon learning of APOL’s capabilities to measure with high accuracy ambient CH2O levels three orders of magnitude lower than the SMAC level, NASA scientists immediately contacted the APOL group to seek advice and help. As a result, the APOL group formed a partnership with scientists from the University of Hartford to conduct a series of laboratory tests at the APOL facility to a) Measure CH2O emission rates from various foam and epoxy samples of the type employed on the space station, and b) Verify the veracity of the CH2O measurement approach (chemical badges) employed on the space station and its flow dependence. The latter is important to ascertain whether or not large differences in CH2O levels observed in different areas of the space station are real or are due to differences in badge response to different airflows.
These measurements were successfully carried out employing APOL’s airborne CH2O instrument and calibration standards. One particular type of foam used throughout the space station, melamine, was found to emit large quantities of CH2O and could account for the levels observed. The high sensitivity of APOL’s instrument was essential for the success of these measurements. Notwithstanding the high space station levels of CH2O, the emission flux concentrations from individual small foam samples tested were very low (less than 1.5 parts-per-billion and more typically an order of magnitude lower). In previous attempts to measure CH2O foam emission rates, the instruments employed could only resolve CH2O levels in the 10’s of parts-per-billion range, far too high for accurate quantification of CH2O sources.
The flow dependence of the chemical badge results was very small. However, the absolute values of the retrieved concentrations from these simple devices was nearly 100% too high when compared to APOL’s CH2O standards, which were used to test the badges. The high verifiable accuracy of APOL’s CH2O standards was essential to these tests. A report to NASA is in preparation summarizing these findings, and mitigation strategies will be devised in the future based upon these findings.
The trace gas CH2O is one of many important atmospheric species involved in ozone production and radical formation, (see “Advances in Tropospheric Chemistry,” below) and the APOL group has devoted a significant effort over the past several years to carry out ever more accurate and precise measurements of this gas. Although the present airborne system, which employs a liquid nitrogen-cooled lead salt diode laser, yields satisfactory results for most regions of the atmosphere, improved sensitivity is clearly needed in the background atmosphere where CH2O levels approach 30 to 50 parts-per-trillion (pptv). The present instrument sensitivity (15 – 50 pptv, 1s precision for 1 minute of averaging) needs to be improved for routine measurements in the background atmosphere, particularly in the upper troposphere/lower stratosphere where CH2O decomposition becomes a major source of reactive hydrogen radicals. To address this critical need as well as the need to develop smaller, lighter, and autonomous-operation instruments for future HIAPER campaigns, the APOL group has been developing a new high performance airborne instrument based upon difference frequency generation (DFG). In this approach, mid-infrared (IR) laser light at 3.53-mm, a spectral region where there are strong CH2O absorption lines, is generated employing well characterized room temperature operation near-IR telecommunication lasers. The mid-IR light retains all the high quality spectral and spatial qualities of the near-IR pump lasers (Richter et al., Applied Physics B, 2002), a significant advantage when compared to more traditional lead-salt diode lasers. Moreover, the ultimate instrument size will be significantly smaller and more rugged than our present lead-salt diode system. APOL Figure 1 shows an optical schematic of the laser sources and the non-linear crystal used in generating the difference frequency.
APOL Figure 1: Set-up of optical fiber based DFG-source: distributed feedback diode laser (DFB-DL);
wavelength division multiplexer (WDM); temperature controller (TEC-CTRL.)
This new DFG system has been extensively characterized in the laboratory over the past year. An important source of optical noise has been identified and present strategies are being implemented to circumvent this problem. Preliminary performance measurements employing one solution are extremely encouraging, and more rigorous laboratory tests are underway.
In addition to progress in the development of new and improved laser sources for the near-IR spectral, where a host of gases like CH2O exhibit strong absorption features, significant progress has also been achieved this past year in the development and testing of a new multipass absorption cell. The long optical pathlengths of multipass absorption cells (100 – 200 meters) are required to achieve detection sensitivities in the pptv range. In such cells, the laser beam is reflected back and forth hundreds of times, and this requires very high spatial beam quality as well as high beam pointing stability and cell alignment stability. The new DFG laser source addresses the first two requirements and significantly helps in achieving the third (to be discussed). By contrast, presently employed lead-salt diode laser systems and multipass cells fall short in all three areas on airborne platforms. In addition to poor beam quality, lead-salt laser systems, including our state-of-the-art airborne system, must employ numerous discrete optical elements before and after the multipass cell. As a result of cabin pressure changes, aircraft vibrations, and changes in aircraft attitude, the present components, including the multipass cell, all undergo slight alignment changes. Such changes degrade instrument performance and require human intervention for active rectification. Although we have been quite successful operating with these problems, a mechanically more robust optical system would reap significant benefits.
The APOL group has expended a great deal of time and effort this past year to address optical mechanical stability of the entire optical system, including the multipass cell and transfer optics. To our knowledge, a very limited number of groups throughout the world are actively addressing this very important issue. In addition to the very high spatial beam quality and beam pointing stability offered by DFG laser sources, the DFG module can be directly coupled to the multipass cell in a pressure-stabilized semi-sealed compartment. This not only avoids dust contamination on the transfer optics but also eliminates pressure-induced alignment changes. To be completely successful, this approach also required the development of a new pressure-insensitive multipass absorption cell. A visiting graduate engineering student, Christoph Dyroff, from the University of Applied Sciences, Emden, Germany has completed a 6-month project designing, constructing, and testing such a new cell for this purpose. In addition, he has carried out extensive sensitivity tests, including alignment sensitivity of the multipass cell output beam to the shape of the initial input beam. The high quality DFG input beam was found to be far superior to that from lead-salt diode lasers in this regard.
APOL Figure 2 shows a schematic of the new absorption cell with carbon fiber stabilizing rods and the new DFG laser stage directly coupled to the end of the cell. A number of preliminary tests have been carried out with this new cell in the laboratory, and the results directly indicate improvements in stability with changes in pressure. Additional studies are planned for this new cell as well as the entire optical system, including the new pressure-stabilized DFG module shown in APOL Figure 2. Once completed, a comprehensive refereed journal paper will be submitted this upcoming year and the new system will be employed next year on NASA’s INTEX airborne study.
APOL Figure 2: New ultra-stable multipass absorption cell with sealed DFG laser/detector module directly mounted to the endplate of the cell.
Ultimately, we anticipate that this new laser source/multipass absorption cell system will require no active alignment in flight, a requisite for autonomous operation on HIAPER. In addition to CH2O, new DFG laser sources similar to that shown here can be devised throughout the 2.7 to 5-mm spectral region to access many other atmospheric gases of importance. As many research groups employ airborne IR laser absorption systems, the new approaches being developed by the APOL group and the subsequent technology transfer will ultimately lead to significant advances in measurement capabilities.
The APOL group has embarked upon a 3-year NSF-funded effort to exploit the developments in new DFG laser sources discussed above for high precision carbon dioxide isotopic ratio measurements. New approaches are urgently needed to acquire such measurements in real time. Present ultra high precision measurements acquired by flask sampling and mass spectrometry can only process a limited number of samples. The APOL group in collaboration with partners from the University of Colorado and Rice University have made considerable progress in the development of a new instrument. APOL Figure 3 illustrates the optical schematic of such a system, which is presently under construction. The signals from two cells containing the sample gas and an isotopic reference standard are rapidly compared using the setup shown in this figure.
APOL Figure 3: Optical schematic of a DFG-based system for high precision carbon dioxide isotopic ratio measurements.
In addition to the optics, a comprehensive inlet system was designed, which allows multiple combinations of calibration reference gases and/or sample gases to be directed into one or both cells. This system is presently under construction, and comprehensive tests will be carried out this upcoming year. A graduate student from Rice University, Chad Roller, has joined our group and is working on multiple aspects of the new system, including the data acquisition and retrieval algorithms.
As part of this new effort, the APOL group hosts each summer two teachers and students from local area high schools to help APOL scientists work on all phases of this project. This effort is in partnership with NCAR’s Education and Outreach Program. The first summer has just been completed, and the students/teachers were exposed to a number of new concepts, including carbon dioxide research and the utility of high precision isotopic ratio measurements; absorption spectroscopy and how one employs laser light sources to obtain quantitative results; and telecommunication lasers, fiber optic technology, and new non-linear mixing processes.
The students/teachers were very enthusiastic about their experiences and are in the process of preparing a presentation/demonstration for the Colorado Science Convention. A demonstration system is being constructed in an effort to allow high school teachers to bring advances in carbon dioxide research, such as the new system being developed at NCAR, into the classroom.
The APOL group has been involved over the past several years in efforts to advance our understanding of tropospheric chemistry. This effort has continued this past year in the newly formed joint ATD/ACD group. Specifically, highly accurate and sensitive measurements of CH2O, acquired during the 2001 NASA-funded TRACE-P campaign, were analyzed and published in a recent special issue of the Journal of Geophysical Research (Fried et al., 2003). A number of new findings from this study have been discussed in last year’s ACD Annual Scientific Report, and one finding not included in that report is presented here.
Comparisons of CH2O measurements with those from box model calculations in all atmospheric regimes are an important component for further assessing our understanding of atmospheric processes. Unfortunately, measurement-model comparisons, even for remote background conditions where CH4 oxidation is the primary CH2O precursor, have sometimes exhibited both positive and negative deviations. These discrepancies clearly point to gaps in our understanding of CH2O production and destruction pathways, and hence in tropospheric oxidation processes. One such regime is uptake of CH2O in clouds and on marine aerosols. Although such uptake has been predicted and has been modeled, many complicating factors tend to obfuscate the observation of such uptake. In some cases where large CH2O uptake has been suspected, questions regarding measurement accuracy have been raised. For example, Jacob et al. (JGR, 1996) report large CH2O model overpredictions by a factor of 4 in airborne measurements carried out near the ocean surface over the South Atlantic Ocean. Jacob et al. raise the possibility of a large unknown oceanic sink or measurement error.
The newly published study by the APOL group (Fried et al., 2003) report for the first time clear evidence of CH2O uptake in clouds. In one case, measurement-model comparisons revealed a peak uptake of 85% upon entering a cloud where the complicating effects of pollution were not present. This same study also revealed large CH2O uptake in the lower marine troposphere in the presence of haze. These observations are important for furthering our understanding of radical chemistry and transport over the oceans and how such chemistry may be altered by the presence of clouds and marine aerosols.
Atmospheric Radiation Investigation and Measurement Group
Atmospheric Radiation Investigations and Measurement (ARIM) Group (Richard Shetter, Teresa Campos, Edward Riedel, Samuel Hall, and Barry Lefer) concentrated much of their effort this year on data reduction and analysis from several campaigns undertaken last 18 months. Additional effort was placed in developing new instrumentation for actinic flux measurements that will be applicable to new aircraft platforms like HIAPER and aircraft direct beam irradiance measurements.
ARIM Highlights
A new instrument to measure direct solar beam irradiance as a function of wavelength from aircraft was developed with NSF and NASA funding. This instrument was flown on the NASA SAGE-III (Stratospheric Aerosol and Gas Experiment – III) Ozone Loss Validation Experiment – II (SOLVE-II) mission to determine overhead ozone column and aerosol optical depths (as a function of wavelength) in support of the validation activities for the SAGE-III satellite instrument. Data was collected on 10 science flights probing the Arctic polar vortex with the NASA DC-8 based out of Kiruna, Sweden. There were numerous encounters with polar stratospheric clouds (ARIM Figure 1) and intense auroral activity (ARIM Figure 2).
Data collected is currently being finalized and will be compared with other solar instruments deployed on the DC-8 and with various satellite datasets. Details of the instrument development and data examples appear below.
ARIM Figure 1: Polar stratospheric cloud off the coast of Iceland observed from the NASA DC-8 during SOLVE-II
ARIM Figure 2: Auroral activity viewed from the NASA DC-8
Sunlit snow has been shown to be one of the most photochemically active, and strongly oxidizing, regions of the entire troposphere, rather than simply a passive sink for the products of tropospheric chemical processing. Photolysis of nitrate initiates very active chemistry that leads to the release of a number of important trace gases. Initial measurements suggest that just above sunlit snow the production of HOx from photolysis of HCHO, HOOH, CH3CHO and HONO are all significant, and collectively dominate over photolysis of O3. The net result is a large enhancement of OH and HO2 in air just above the snow. Oxidation by OH is the main sink for a number of gases important for climate change and stratospheric O3 depletion, so this enhancement may perturb chemistry in much of the free troposphere, and also modify the chemical records of atmospheric composition ultimately preserved in glacial ice. In an NSF Polar Programs-funded Collaborative Research Project with 5 different university investigators and one national laboratory investigator, the ARIM group developed and deployed a new automated instrument to measure the UV radiation flux profile in the surface snowpack. This instrument was deployed for two months in the Summer of 2003 to Summit, Greenland. The Summit base camp is show in ARIM Figure 3. All measurements were actually collected at a remote photochemistry camp to eliminate the possibility of contamination from the base camp generators and other human activities (see below for instrument description and preliminary data).
ARIM Figure 3: Summit, Greenland base camp
A spectroradiometer to determine the direct beam solar irradiance from 290 to 700 nm has been developed with joint NSF and NASA funding. Direct beam irradiance data can be used to determine the slant path ozone columns and wavelength dependent total and aerosol optical depths. The instrument is comprised of 3 subsystems: a narrow field of view optical collector, an active solar tracking system, and monochromator detection systems. The instrument is represented schematically in ARIM Figure 4. The DIAS system was deployed for the first time on the NASA DC-8 aircraft during the SOLVE 2 mission for use in validation of the SAGE III satellite instrument. The aircraft installation on the NASA DC-8 is shown in ARIM Figure 5.
The narrow field of view collection optic accepts the direct solar beam while excluding almost all of the atmospherically-scattered radiation. While the entire solar disk could be sampled with a ~0.5° field of view, a 2.6° field of view allows for some inaccuracy or short time lags in the response of pointing system. The collection optic is mounted to the pointing system at the center of rotation of the x-axis and y-axis and is optically connected to the detection system with a flexible UV fiber optic.
The solar tracking system contains a commercially available 2-axis gimbal, a position sensing system, and a custom embedded controller system. The Sagebrush Technology Model-20 servo gimbal has a large range of motion, positional resolution of 0.004° and angular travel rates up to 120° per second. The gimbal is controlled by communication with RS-232 commands or a commercial grade joystick. The position sensing detector system consists of a focusing lens, neutral density filter, and a duolinear position sensing module connected to a position sensing amplifier mounted on the gimbal platform. A lens focuses the image of the solar disk on the position sensing detector and the amplifier produces x and y-axis voltages which are directly proportional to the image position on the detector. Changing solar intensity is accommodated by the multiple amplifier gain ranges available on the amplifier module. The embedded controller control and data acquisition system is based on a Motorola 68338 single board computer (Persistor CF-1). This low power computer is interfaced with a 4 channel 16-bit analog to digital card (OES AD16S) and a 4 port serial interface card (OES U4S) and collects the X and Y analog data, issues RS-232 commands to the gimbal, and records positional data and times to a compact flash card
The double monochromator detection systems employ a fused silica fiber optic bundle, f matching optic, 1/8 meter scanning double monochromator (CVI CM112), UV sensitive photomultipliers, custom 4 channel electrometer/amplifier, and rack mount data acquisition and control system. This system is based on an instrument used by Shetter and Müller [1999] for wavelength dependent actinic flux measurements from aircraft. The full width at half maximum (FWHM) of the double monochromators is 1.0 nm using 2400 g/mm gratings and 600 micron entrance and exit slits which produces a symmetrical triangular slit function.
The diode array monochromator detection system employs a fused silica single fiber optic, a fixed monolithic monochromator (Zeiss MCS), a cooled windowless 512 pixel diode array detector, and a rack mount data acquisition and control system. The system produces spectra from 300 to 700 nm with a 2.4 nm FWHM.
ARIM Figure 4: Schematic representation of the Direct beam Irradiance Airborne Spectroradiometer (DIAS)
ARIM Figure 5: Installation of DIAS on the NASA DC-8
Instrument sensitivity calibrations of the 3 systems were done in the ARIM laboratory before and after the SOLVE II mission with NIST traceable 1000 watt QTH radiation sources. During the deployment, system sensitivities were tracked with secondary calibration sources before and after each flight. Since the major uncertainty in the direct beam irradiance determinations is the uncertainty in the radiation source flux, these calibrations are critical. In addition to source uncertainties transferring calibrations from the source flux to a solar flux that is orders of magnitude higher induces additional error. In an attempt to understand these uncertainties ARIM staff have performed additional calibrations using the Langley technique which uses the sun as a source. The instruments were taken to Rocky Mountain National Park at elevations of ~12,000 feet. When the conditions included clear skies and clean air masses from the west we were able to obtain representative Langley plots to independently determine instrument sensitivities. These calibrations are being compared with the lamp calibrations done in the lab and during SOLVE II.
Wavelength calibrations using a mercury discharge line source were also performed before and after each flight during the mission. These calibrations provided data which demonstrate the differences in the wavelength assignment of the monochromators from the accepted literature values. These differences vary between <0.1 nm to 1.1 nm and can vary with wavelength. Wavelength errors are especially important in areas of the solar spectrum where there are large intensity changes over short wavelength ranges, as seen in the UV-B to UV-A region. ARIM has done some preliminary comparisons between the wavelength assignment differences from the mercury calibrations and those calculated using a convolution/deconvolution algorithm (Slaper et al. 1995) and found the results to be quite comparable. Therefore we will run the algorithm on all of the SOLVE II spectra taken with the 3 detectors to correct the wavelength assignments.
Development of a new data reduction algorithm to calculate ozone column and line of sight ozone data from the Chappius band direct beam irradiance data has been necessary since the original algorithm developed for ozone column determinations was based on UV-B wavelength pairs and very little data in the UV-B was collected during SOLVE II, due to the large solar zenith angles (mostly 88.5° to 90.2°) during the solar runs for the SAGE III occultations. See ARIM Figure 6 for some preliminary ozone column results from the new algorithm.
ARIM Figure 6: Ozone column measured from NASA DC-8 during SOLVE-II
The algorithm for calculation of wavelength dependent total and aerosol depths has been refined and preliminary data was submitted to the mission archive. Final data reduction and analysis using the new calibration values is in progress. See ARIM Figure 7 for some preliminary Aerosol Optical Depth (AOD) results.
Direct comparisons of the DIAS data with data from the other solar instruments on the DC-8 and the SAGE III satellite for AODs are planned.
ARIM Figure 7: Aerosol Optical Depths for different wavelengths during SOLVE-II
Sunlit snow has been shown to be one of the most photochemically active, and strongly oxidizing, regions of the entire troposphere, rather than simply a passive sink for the products of tropospheric chemical processing. Photolysis of nitrate initiates very active chemistry that leads to the release of a number of important trace gases. Initial measurements suggest that just above sunlit snow the production of HOx from photolysis of HCHO, HOOH, CH3CHO and HONO are all significant, and collectively dominate over photolysis of O3. The net result is a large enhancement of OH and HO2 in air just above the snow. Oxidation by OH is the main sink for a number of gases important for climate change and stratospheric O3 depletion, so this enhancement may perturb chemistry in much of the free troposphere, and also modify the chemical records of atmospheric composition ultimately preserved in glacial ice. While recent work has shown that photochemical and physical processes in the snowpack can impact the chemistry and composition of both the atmosphere and snowpack, these processes are, in general, poorly understood. This is especially true for the processes that produce and consume OH and HO2.
In an effort to better understand this photochemical environment, the ARIM group has developed a new profiling spectroradiometer to determine the intensity of solar UV radiation at 5 different depths in the surface snowpack (ARIM Figure 8). This instrument serially samples the radiation field from 290 to 420 nm from 5 different fiber optic cables inserted into the snowpack at different depths between 1 and 150 cm. These snowpack profiles of solar irradiance were used to determine the attenuation rate of UV in the snowpack and how this varied with changes in environmental variables such as snowpack density, soot content, solar zenith angle (SZA) and overhead ozone column. Given the isotropic nature of the radiation field in the snowpack the measured irradiance spectra were converted to actinic flux spectra and the photolysis frequencies of several important snowpack photochemical reactions were calculated for the different sampling depths. This data has helped determine that the photic zone (or photochemically active region) of the snowpack is only the top 15 cm of the Greenland snowpack. This instrument was deployed for the months of June and July 2003 at the Greenland Summit Ice Camp (ARIM Figure 9) as part of the NSF-funded Collaborative Research Grant titled: “Impact of snow photochemistry on atmospheric radical concentrations at Summit, Greenland.”
ARIM Figure 8: NCAR Snow Profiler in Greenland
The NCAR Snow Profiling spectroradiometer (NSPS) instrument consists of a small cosine-corrected irradiance probe (Ocean Optics), 5m fiber-optic with high UV throughput (Ocean Optics), a precision computer controlled dovetail slide (Velmex), a double monochromator (CVI Digikrom CM 112), a photomultiplier tube with a bialkali photocathode (Electron Tubes, LTD), a custom four-stage current-to-voltage amplifier and a PC computer for fully automated data acquisition and system control. Spectra from 280 to 420 nm are scanned every 30 seconds. The spectral band pass full width at half maximum (FWHM) for this optical system is 1.0 nm. Absolute spectral calibrations are performed with a National Institute of Standards and Technology (NIST) traceable 1000 Watt QTH irradiance standard in the NCAR laboratory before and after instrument field deployment. Field calibrations are performed with 250 W secondary QTH lamps every 4 to 5 days to assess the relative stability of the instrument sensitivity. Wavelength calibrations of the monochromator are performed in conjunction with each sensitivity calibration using the emission lines from a mercury lamp to track any drift in the monochromator wavelength assignment.
ARIM Figure 9: Summit, Greenland snow photochemistry camp
An NCAR Scanning Actinic Flux Spectroradiometer was also deployed to Greenland to measure the UV radiation incident to the snowpack surface. The quartz optical collector for this system was mounted on the roof of the mobile sampling laboratory (ARIM Figure 10) and had a 30 cm artificial horizon to limit the field of view to 2pi steradians to minimize the influence of reflected light from the other sampling inlets.
It is quite difficult to measure snowpack radiation levels due to issues of self-shading by the optical sampling inlet. To this end, our lab has selected miniaturized Teflon sampling inlets (approximately 1 cm in diameter and less than 2.5 cm long), which attach directly to the end of a fiber optic using an SMA connector. These miniature-sampling inlets were inserted into the snowpack at a location near the gas sampling activities at 5 different depths between the surface and 1.5 meters. Each sampling inlet was attached to a 5m fiber optic cable that was connected to the same spectroradiometer. The snow spectroradiometer serially sample each of these 5 depths, thus collecting a complete snow profile every 2.5 minutes. The above-snow spectroradiometer continuously measured the actinic flux reaching the snow surface, which can account for any changes in the light environment (e.g., due to clouds) while a specific snow profile is being collected.
ARIM Figure 10: Above-snow SAFS optic with rime ice deposition
Profile measurements of the downwelling UV actinic radiation in the snowpack at Summit, Greenland show an exponential decrease with depth in the snowpack. The e-folding depths (1/e attenuation) calculated from the profile shown in ARIM figure 11 shows a wavelength dependence of the penetration of UV/VIS radiation ranging from 20 cm at 420 nm to approximately 14 cm at 310 nm. These results suggest that 85% (i.e. 2 e-folding depths) of the photochemical activity is occurring in the top 28-40 cm of this snowpack. This project will continue with a second field season at the Summit Ice Camp from March to April 2004.
ARIM Figure 11: Preliminary e-folding measurements from Summit, Greenland
The ARIM group has deployed ground based and airborne Scanning Actinic Flux Spectroradiometer (SAFS) instruments for the determination of actinic fluxes and photolysis frequencies for the last 6-7 years. These have operated well but have limited time resolution (~10 seconds for a spectrum) and require frequent calibration and maintenance. A new instrument based on a monolithic monochromator with a UV enhanced windowless CCD detector is being developed. This instrument is smaller and lighter with a new PC-104+ data acquisition and control system, should be quite reliable, and be able to collect actinic flux spectra from 280 to 680 nm at 1-10 Hz allowing for fast photochemistry investigations. The smaller design will allow for deployment on aircraft platforms with limited space for instruments and operators like the NSF High-performance Instrumented Airborne Platform for Environmental Research (HIAPER) aircraft. The instrument is currently being tested and will be intercalibrated with the existing scanning monochromator systems in the near future.
The ARIM group analyzed the actinic flux dataset taken during the NASA Transport and Chemical Evolution over the Pacific (TRACE-P) Mission from the spring of 2001. This analysis consisted of comparisons of modeled and measured actinic flux and the calculated photolysis frequencies. Chemical transport model (CTM) output for the cloud and aerosol parameters was used as inputs to the radiative transfer model (RTM). During clear-sky periods, the instrumental data and RTM photolysis frequencies were compared to evaluate the CTM aerosol output parameters. The measured and modeled photolysis frequencies were also input into a photochemical box model to look at the impact of clouds and aerosols on ozone photochemistry. These results were published in two collaborative papers in the Journal of Geophysical Research.
In an effort to improve the state of the science for the measurement of actinic flux radiation and the calculation of photolysis frequencies, the ARIM group hosted an informal international actinic flux spectroradiometer intercomparison for 6 weeks in August and September 2003 at the NCAR Marshall Field site (ARIM Figure 12). The ARIM group deployed three Scanning Actinic Flux instruments, a diode array instrument, and 4 j-NO2 filter radiometers and also provided 1-minute temperature, pressure, and cloud camera images (ARIM Figure 13) to the other intercomparison participants. The other participants included scientists from the Institut fur Tropospharenforschung-Leipzig that compared 2 diode array spectroradiometers, as well as scientists from the NOAA Aeronomy Laboratory-Boulder that compared a dual channel double monochomator spectroradiometer with a CCD detector as well as 2 j-NO2 filter radiometers. Data for final comparisons is currently being analyzed.
ARIM Figure 12: View of some instruments participating in intercomparison at Marshall Field Site
ARIM Figure 13: ARIM Cloud Camera during the intercomparsion.
Photochemical Oxidation and Products Group
The Photochemical Oxidation and Products (POP) Group studies fast photochemistry, sulfur chemistry, ion chemistry, particle nucleation, and the chemical composition of ultrafine particles. Its members include Fred Eisele, Roy Leon Mauldin, James Smith, Edward Kosciuch, Bruce Henry, and Katharine Moore (ASP).
A linear ion trap mass spectrometer has recently been developed and successfully tested. It offers high ion throughput and trapping efficiencies and can resolve individual ion masses over a wide range and in a short time relative to the ion collection time (POP Figure 1). This makes it an ideal instrument for rapid, near simultaneous measurements of numerous organic compounds. Preliminary progress has also been made in fragmenting ion clusters and the subsequent measurement of daughter ions on this same instrument.
POP Figure 1: Mass spectrum of NO3- and its first nitric acid cluster obtained using a recently developed linear ion trap
In an attempt to improve the planning process for the future ACD MIRAGE study, an NCAR Advanced Study Program (ASP) / University of Colorado graduate student was sent to Mexico City this past spring to perform initial measurements of ultrafine aerosol particles. This study was also a joint effort with Peter McMurry’s group at the University of Minnesota and resulted in the measurement of many relatively intense aerosol nucleation events, an example is shown in POP Figure 2. This study provided the first direct evidence of nucleation events in and near Mexico City and will be used to help determine future instrument needs and the aerosol study location to be used during MIRAGE.
POP Figure 2: Nanoparticle size distribution measurements as a function of local time performed at Santa Ana, a rural town located at the southeastern edge of the Mexico City Federal District. The red color at the bottom of this figure shows the initial results of an intense nucleation event. The subsequent growth of these aerosols over the next several hours is then shown as the red moves to larger sizes and later in time.
The POP Group has undertaken a number of research efforts during the past year. In the area of gas phase measurements, the final data analysis and manuscript preparation were completed for the NASA/Global Tropospheric Experiment (GTE) Transport and Atmospheric Chemistry near the Equator (TRACE-P) mission and the NSF/Polar Programs Investigation of Sulfur Chemistry in the Antarctic Troposphere (ISCAT) 2000 mission. Three TRACE-P papers are either in press or published, and one ISCAT paper is ready for submission. Some preliminary work on an instrument to measure gas phase ammonia concentrations is presently underway. A redesign of the prototype was recently completed and will be tested shortly. This research is being supported in large part by funding from the NASA/GTE program, but the resulting instrument will be used to study ammonia and its role in particle nucleation and growth in both future NASA (most likely Intercontinental Chemical Transport Experiment [INTEX]) and NSF (MIRAGE and probably UTLS) field campaigns.
The group has also put considerable effort into both improving present instruments and at the same time testing new techniques that will allow smaller instruments to be designed in the future. These smaller instruments would be able to measure more quickly and also study a broader range of compounds. The initial phase of this effort during the past year involved the replacement of an electrostatic ion lens system with an octopole radio frequency (RF) lens system in a two-channel mass spectrometer system. This resulted in about a four-fold increase in ion signal under relatively low vacuum pressure conditions. This was then followed by the design of a conical octopole lens system to be installed in a differentially pumped section of a mass spectrometer vacuum chamber. This will be tested shortly on the OH channel of the 4 channel mass spectrometer. If successful, it would allow a significant reduction in the size and vacuum pumping requirements of future mass spectrometer systems.
A major goal of future studies is to provide new, more sensitive and more chemically-specific measurements of numerous organic compounds, both in the gas and aerosol phases. To accomplish this, tandem mass spectrometers and an ion trap mass spectrometer with a combined spectrometry (MS/MS) capability are obvious candidates. While tandem mass spectrometers provide high ion throughput, they measure parent and fragment ions in a serial mode and thus require significant time to acquire a spectrum. Standard ion traps acquire spectra in a quasi-parallel manner by trapping and storing a wide range of ion masses and then analyzing them in a time short compared to their collection. The overall trapping and analysis efficiency of a standard ion trap, however, is quite low. Thus, recent research efforts have led to the conversion of a quadrupole mass spectrometer, already designed for atmospheric studies, into a linear ion trap mass spectrometer. Mass spectra were first measured with this new technique a few months ago, and the linear trap offers both high ion throughput and high trapping efficiency (see POP Figure 1). A preliminary attempt to fragment ions and measure daughter ion spectra was also successful, but thus far it has only been successful with weakly bound ion clusters. The next step will be to fragment more stable ion species.
Other areas of instrument upgrades include the development of a better, more field-portable method for calibrating UV diodes, which are in turn used to calibrate OH and sulfuric acid measurements during field studies. A gas phase calibration box for calibrating ammonia measurements was also improved as part of Okason Morrison’s SOARS project, and the OH aircraft calibration system is being fully automated.
A large effort has also been put into improving and expanding aerosol measurement capabilities. In order to develop improved techniques for measuring the chemical composition of ultrafine aerosols and at the same time better understand the local aerosol environment, several one- to two-month-long continuous measurements were performed of air from just outside the laboratory during each season. This allowed seasonal changes to be tracked and at the same time provided an opportunity to optimize instrument performance under varied conditions. Various parts of the Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS) system used for these studies are still undergoing multiple improvements, so the ability to directly compare measurements from different seasons is still somewhat limited, but new understanding is nonetheless being gained. Among the surprises observed in this measurement set was a large source of nitrate ions which were part of a relatively stable complex, but do not appear to originate from ammonium nitrate. Techniques for better understanding the source of this nitrate are being investigated. Several nucleation events have also been observed and provide a test of the ability to measure the chemical composition of very small particles.
One major modification of the TDCIMS instrument was also made this past year. The electrostatic precipitator, thermal desorption filament, and housing assembly were all reoriented to reduce tur