Comparison of predicted secondary organic aerosol formation from isoprene to aerosol formation from monoterpene reactions in southeast Texas

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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