HOx Free Radicals
The Photochemical Oxidants and Products group makes use of chemical ionization mass spectrometry to quantify the concentrations of free radical species and some of the products of free radical reactions. Measurements on ground and aircraft platforms have been performed for decades. Recent field campaigns include:
| Campaign | Date | Platform | Observations |
|---|---|---|---|
| ANTCI II | December 2005 | Twin Otter | OH, H2SO4 |
| MIRAGE | March 2006 | NSF C-130 | OH, H2SO4, MSA, HO2, RO2 |
| INTEX-B | April-May 2006 | NSF C-130 | OH, H2SO4, MSA, HO2, RO2 |
| Niwot Ridge | September 2006 | ground | OH, H2SO4 |
| Finland Forest | May 2007 | ground | OH, H2SO4, OH reactivity |
| PASE | August 2007 | NSF C-130 | OH, H2SO4, MSA, HO2, RO2 |
| ARCTAS A | April 2008 | NASA DC-8 | OH, H2SO4, MSA, HO2, RO2 |
| NIFTY | May 2008 | ground | OH, H2SO4, OH reactivity |
| ARCTAS B | June-July 2008 | NASA DC-8 | OH, H2SO4, MSA, HO2, RO2 |
Acronyms: ANTCI II: Antarctic Tropospheric Chemistry Investigation, Part 2; MIRAGE: Megacities Impacts on Regional and Global Environment; INTEX-B: Intercontinental Chemical Transport Experiment, Phase B; PASE: Pacific Atmospheric Sulfur Experiment; ARCTAS: Arctic Research of the Composition of the Troposphere, Phases A and B; NIFTY: Nucleation In ForesTs.
These campaigns focused on a variety of goals from photochemistry in Polar Regions to aerosol formation in forests to sulfur photochemistry in the tropics. Free radicals play important roles in all of the processes under study. The oxidation product of sulfur chemistry, H2SO4, is important in the formation and growth of tropospheric aerosols.
Tropospheric photochemistry can be viewed as a slow combustion process in which hydrocarbons, carbon monoxide and other species are oxidized to carbon dioxide and water by reactions involving HOx free radicals.
Figure 1.
Diagram showing the reactions of HOx radicals in the troposphere.
Acronym definitions: HP: hydroperoxides; AN: alkyl nitrates; PN: peroxy nitrates;
RR’CO: aldehydes and ketones from hydrocarbon oxidation.
To understand the behavior of free radicals, various types of numerical models are used. A box model with detailed chemistry that is constrained by measurements of the controlling variables (NOx, CO, hydrocarbons, j-values, temperature, pressure, etc.) is one useful tool. The model estimated radical concentrations are compared with the observations as functions of various variables. For example, shown below are the measurement-model ratios versus the observed NOx levels for data from the MIRAGE campaign.
Figure 2.
Measurement-model ratios of HO2 (red) and HO2+RO2 (blue) versus NOx concentrations
from MIRAGE (2006) as measured aboard the NSF C-130 aircraft.
The large measurement-model differences at high NOx concentrations indicate problems with the understanding of the chemistry of peroxy radicals under these conditions, a problem with the measurement technique at high NOx, or a combination of both. Explanations are being actively evaluated.