Browsing by Subject "Atmospheric aerosols--Texas"

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Comparison of predicted secondary organic aerosol formation from isoprene to aerosol formation from monoterpene reactions in southeast Texas
(2005-08-15) Lasseter, Jennifer Schiffelbein; Allen, David T.
In the eastern half of Texas, the atmospheric reactions of hydrocarbons released by vegetation can lead to significant quantities of fine particulate matter. Previous analyses have indicated that the reaction of ozone with monoterpenes (especially α-pinene) is the dominant chemical pathway for the formation of fine particulate matter from biogenic hydrocarbon emissions. These analyses neglected the reactions of isoprene, however, and isoprene represents roughly 80% of the mass of biogenic volatile organic compound (BVOC) emissions in eastern Texas. Conversion of even a small fraction of the isoprene emissions to condensable products could therefore make a significant contribution to biogenic secondary organic aerosol (SOA) formation. This work utilizes two approaches to estimate SOA formation from the atmospheric reactions of isoprene with the hydroxyl radical. One approach involves a simple SOA yield model, which is based on SOA yield data from environmental chamber experiments. The other approach uses molecular reaction mechanisms, based on postulated reaction pathways and estimated rate parameters. The two SOA formation models were incorporated into the photochemical grid model, CAMx 3.10, and used to predict SOA formation from isoprene during the period of August 28 to 31, 2000. Both models predict SOA formation in southeast Texas from the reaction of isoprene with OH to be considerably less than the yield of SOA from the reactions of monoterpenes (α-pinene/O₃, β-pinene/ NO₃ and β-pinene/OH). The daily SOA formation, due to isoprene reactions, estimated using the yield model is about 5% of the yield from the dominant monoterpene reaction pathway - α-pinene/O₃. The SOA formation from isoprene predicted using the molecular mechanism, is about 10% of the isoprene SOA yield model estimates
Predicting secondary organic aerosol formation rates and concentrations in southeast Texas
(2003) Russell, Matthew Maclean; Allen, David T.; Liljestrand, Howard M.
Elevated concentrations of atmospheric particulate matter are a significant public health concern, yet the sources, composition and formation mechanisms for this class of pollutants remain poorly understood. This work addresses these knowledge gaps, especially for Southeast Texas. This dissertation reports: 1) the development of a conceptual model of fine particulate matter (less than 2.5 µm in diameter, PM2.5) for Southeast Texas including estimates of primary and secondary organic aerosol (SOA) concentrations, 2) the development of a photochemical grid model tool that implements a flexible chemical mechanism and a module to predict SOA formation, and 3) the application of the tool to Southeast Texas to estimate the spatial distribution, temporal distribution, and precursors of SOA formation. PM2.5 concentrations in Southeast Texas during 2000-2001 were found to be close to, but not in excess of, the annual National Ambient Air Quality Standard for PM2.5. PM2.5 mass concentrations, composition and diurnal patterns were found to be relatively consistent throughout Southeast Texas and from season to season. The major components of PM2.5 were found to be ammonium sulfate and organic carbon; the majority of organic carbon is primary yet secondary organic carbon is significant during the ozone season. The conceptual model contributes greatly to understanding PM2.5 pollution in this area. A state-of-the-science photochemical grid model was modified to include a flexible chemical mechanism and a module to predict SOA formation. The tool was used in this work to model SOA formation rates from aromatics and monoterpenes in Southeast Texas during the ozone season, using a new chemical mechanism designed for this purpose. SOA formation was found to come predominantly from biogenic monoterpenes, particularly from α-pinene/ozone and β-pinene/nitrate-radical reactions. SOA formation rates were predicted to peak in the evening during the episode considered here. The levels, spatial distribution, and biogenic nature of the SOA formation are consistent with the limited ambient SOA information collected during this period. Sensitivity simulations showed that SOA formation is proportional to existing particulate matter concentrations and proportional to precursor emission rates. The model results are, most importantly, a guide for identifying knowledge gaps to model SOA air pollution.