Secondary organic aerosol formation and source apportionment in Southeast Texas
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The latest version of US EPA's Community Multi-scale Air Quality (CMAQ v4.7) model with the most recent update on secondary organic aerosol (SOA) formation pathways was adapted into a source-oriented modeling framework to determine the contributions of different emission sources to SOA concentrations from a carbon source perspective in Southeast Texas during the 2000 Texas Air Quality Study (TexAQS 2000) from August 25 to September 5, 2000. A comparison of the VOC and SOA predictions with observations shows that anthropogenic emissions of long chain alkanes and aromatics are likely underestimated in the EPA's Clean Air Interstate Rule (CAIR) inventory and the current SOA mechanism in CMAQ still under-predicts SOA. The peak SOA concentrations measured at La Porte are more accurately predicted by increasing the emissions of the anthropogenic SOA precursors by a factor of 5 although the overall precursor concentrations are better predicted by increasing the emissions by a factor of 2. A linear correlation between SOA and odd oxygen (SOA/Ox = 23.0-28.4 g m-3/ppm Ox) can be found when they are formed simultaneously in the air masses passing the urban Houston area on high SOA days.Based on the adjusted emissions (a factor of 2 increase in the alkane and aromatics precursor emissions), approximately 20% of the total SOA in the Houston-Galveston Bay area is due to anthropogenic sources. Solvent utilization and gasoline engines are the main anthropogenic sources. SOA from alkanes and aromatics accounts for approximately 2-4% and 5-9% of total SOA, respectively. The predicted overall anthropogenic SOA concentrations are not sensitive to the half-life time used to calculate the conversion rate of semi-volatile organic compounds to non-volatile oligomers in the particle phase. The main precursors of biogenic SOA are sesquiterpenes, which contribute to approximately 12-35% of total SOA. Monoterpenes contribute to 3-14% and isoprene accounts for approximately 6-9% of the total SOA. Oligomers from biogenic SOA account for approximately 30-58% of the total SOA, indicating that long range transport is an important source of SOA in this region. 2011 Elsevier Ltd.
