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Investigation of the Correlation between Odd Oxygen and Secondary Organic Aerosol in Mexico City and Houston
Citation:
Wood, E. C., M. R. Canagaratna, S. C. Herndon, Timothy ONASCH, J. H. KROLL, C. E. KOLB, D. R. WORSNOP, W. Knighton, R. L. SEILA, M. Zavala, L. T. Molina, P. F. DECARLO, J. JIMENEZ, A. J. Weinheimer, D. J. Knapp, B. Jobson, W. C. Kuster, AND E. J. Williams. Investigation of the Correlation between Odd Oxygen and Secondary Organic Aerosol in Mexico City and Houston. Atmospheric Chemistry and Physics. Copernicus Publications, Katlenburg-Lindau, Germany, 10(18):8947-8968, (2010).
Impact/Purpose:
The National Exposure Research Laboratory′s (NERL) Human Exposure and Atmospheric Sciences Division (HEASD) conducts research in support of EPA′s mission to protect human health and the environment. HEASD′s research program supports Goal 1 (Clean Air) and Goal 4 (Healthy People) of EPA′s strategic plan. More specifically, our division conducts research to characterize the movement of pollutants from the source to contact with humans. Our multidisciplinary research program produces Methods, Measurements, and Models to identify relationships between and characterize processes that link source emissions, environmental concentrations, human exposures, and target-tissue dose. The impact of these tools is improved regulatory programs and policies for EPA.
Description:
Many recent models underpredict secondary organic aerosol (SOA) particulate matter(PM) concentrations in polluted regions, indicating serious deficiencies in the models' chemical mechanisms and/or missing SOA precursors. Since tropospheric photochemical ozone production is much better understood, we investigate the correlation of odd-oxygen ([Ox] ≡[O3]+[NO2] and the oxygenated component of organic aerosol (OOA), which is interpreted as a surrogate for SOA. OOA and Ox measured in Mexico City in 2006 and Houston in 2000 were well correlated in air masses where both species were formed on similar timescales (less than 8 h) and not well correlated when their formation timescales or location differed greatly. When correlated, the ratio of these two species ranged from 30μg m-3/ppm (STP) in Houston during time periods affected by large petrochemical plant emissions to as high as 160 μg m-3/ppm in Mexico City, where typical values were near 120 μg m-3/ppm. On several days in Mexico City, the [OOA]/(Ox ratio decreased by a factor of ∼ 2 between 08:00 and 13:00 local time. This decrease is only partially attributable to evaporation of the least oxidized and most volatile components of OOA; differences in the diurnal emission trends and timescales for photochemical processing of SOA precursors compared to ozone precursors also likely contribute to the observed decrease. The extent of OOA oxidation increased with photochemical aging. Calculations of the ratio of the SOA formation rate to the Ox production rate using ambient VOC measurements and traditional laboratory SOA yields are lower than the observed [OOA]/[Ox] ratios by factors of 5 to 15, consistent with several other models' underestimates of SOA. Calculations of this ratio using emission factors for organic compounds from gasoline and diesel exhaust do not reproduce the observed ratio. Although not successful in reproducing the atmospheric observations presented, modeling P(SOA)/P(Ox) can serve as a useful test of photochemical models using improved formulation mechanisms for SOA.
URLs/Downloads:
Investigation of the Correlation between Odd Oxygen and Secondary Organic Aerosol in Mexico City and Houston (PDF, NA pp, 5 KB, about PDF)Atmospheric Chemistry and Physics
