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Figure 1 |
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Tropospheric Chemistry
Atmospheric Radical Studies
The Atmospheric Radical Studies (ARS) group (Christopher Cantrell) is involved in the measurement and interpretation of peroxy radical levels in laboratory and atmospheric measurement situations. Activities this year have included analysis and reporting of results from earlier studies and deployment in a NASA-sponsored campaign. The main analytical tool for these studies is the Peroxy radical Chemical Ionization Mass Spectrometer system, which is based on the chemical conversion of ambient peroxy radicals (HO2 and RO2) into a unique ion (HSO4-) which can be quantified by mass spectrometry. This instrument has been developed, characterized and improved over the past several years and has now been deployed in three field campaigns
Intensive efforts have been made toward the analysis, interpretation and reporting of measurements of peroxy radicals and other species from the Calspan Chamber Study campaign that took place in the autumn of 1998, and the Tropospheric Ozone about the Spring Equinox (TOPSE) study from the winter and spring of 2000. There was also some progress on papers from the International Photolysis frequency Measurement and Modeling Intercomparison (IPMMI) campaign, in which the group had a significant presence in collaboration with the Atmospheric Radiation Investigations and Measurements (ARIM) group of ACD and groups throughout the radiation community.
The Calspan Chamber study involved a number of experiments designed to understand aerosol formation, cloud processing and gas-to-particle transfer of a number of sulfur and carbon-containing systems. ARS led in a study of peroxy radical uptake by aerosols and cloud droplets which allowed quantification of parameters usable in the numerical modeling of atmospheric processes. A first paper was submitted in August; papers describing further analysis of the rich data set may follow.
The TOPSE experiment was an ACD-led campaign with significant university and national laboratory collaboration that was designed to understand the evolution of ozone and other trace gases during the winter-to-spring transition in northern middle to high latitudes. Cantrell was a co-Principal Investigator on this experiment, and led the activities to request the aircraft facility and coordinate the package of instruments that were deployed. The aircraft platform was deployed seven times of approximately one week from early February to mid-May. These included flights from Jefferson County airport to Churchill, Manitoba on to Thule, Greenland and north over the Arctic Ocean. Important measurements of a number of photochemical, tracer and remotely-sensed species were made that allow constraint of numerical models and tests of our understanding of tropospheric photochemistry and transport. A first paper describing the peroxy radical budgets and ozone photochemistry is nearly ready for submittal to a special TOPSE section of the Journal of Geophysical Research - Atmospheres. Examples of summaries of data collected and results from a simple steady-state model are shown in Figure 1.
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Figure 1 |
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The Transport and Chemical Evolution over the Pacific (TRACE-P) campaign was NASA-sponsored, and the measurement phase took place from February to mid-April, 2001. The experiment involved detailed measurement packages aboard the NASA P-3B and DC-8 aircraft designed to investigate the transport and chemical evolution of eastern Asian anthropogenic and biogenic emissions. The PerCIMS instrument was deployed as in TOPSE, with several improvements designed to increase its reliability, minimize interference with the OH measurements, and allow for in-flight radical calibrations. Most of these efforts were highly successful, with more work remaining to achieve accurate, reliable in-flight calibrations. Final data is in preparation for submittal to the archive, and production of papers will soon begin.
Gavin Edwards (postdoctoral visitor with ACD) has undertaken a series of detailed laboratory experiments to verify the performance of the PerCIMS under simulated operating conditions. HO2 and a series of RO2 radicals were produced by the photolysis of chlorine/hydrocarbon mixtures. The response of the instrument in the HO2 and HOxROx modes has been determined and compared with theoretical estimates (see Figures 2 and 3).
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Figure 2 |
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Figure 3 |
Applying the observed response ratios to theoretical RO2 distributions of TOPSE indicates that the instrument responds on average to 0.965 of the HOxROx and 1.063 of the HO2. The effect of humidity on the response of the PerCIMS has also been investigated. In contrast to the chemical amplifier instrument, the PerCIMS shows no humidity dependence within the uncertainty of the measurements (Figure 4). Detailed absolute calibrations have been performed using photolysis of water vapor at 184.9 nm, and the newly designed N2O actinometry. These results show a high degree of stability over a period of several years.
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Figure 4 |
Development of this instrument will continue while we explore further ways to separate HO2 from RO2. Future activities will include comparison of PerCIMS with the chemical amplifier, intercomparison exercises with LIF techniques, and in flight calibration refinements.
These activities are highly relevant to ACD's goal of understanding the details of tropospheric photochemistry. The deployment of this instrument, the interpretation of the results, and efforts to improve its performance all can lead to better understanding of the processes by which peroxy radicals are formed and destroyed and their role in the production of intermediate-lived atmospheric species. These are demonstrated in part through new papers prepared this year (Cantrell, et al., J. Geophys. Res., submitted) and (Cantrell et al., J. Geophys. Res.,in preparation).
