Secondary organic aerosol formation from anthropogenic air pollution: Rapid and higher than expected

Abstract

[1] The atmospheric chemistry of volatile organic compounds (VOCs) in urban areas results in the formation of 'photochemical smog', including secondary organic aerosol (SOA). State-of-the-art SOA models parameterize the results of simulation chamber experiments that bracket the conditions found in the polluted urban atmosphere. Here we show that in the real urban atmosphere reactive anthropogenic VOCs (AVOCs) produce much larger amounts of SOA than these models predict, even shortly after sunrise. Contrary to current belief, a significant fraction of the excess SOA is formed from first-generation AVOC oxidation products. Global models deem AVOCs a very minor contributor to SOA compared to biogenic VOCs (BVOCs). If our results are extrapolated to other urban areas, AVOCs could be responsible for additional 3-25 Tg yr À1 SOA production globally, and cause up to À0.1 W m À2 additional topof-the-atmosphere radiative cooling. Citation: Volkamer, R.,

An evaluation of real‐time air quality forecasts and their urban emissions over eastern Texas during the summer of 2006 Second Texas Air Quality Study field study

“…This relatively fast OA formation rate is consistent with the observations of rapid PM 2.5 formation downwind of the Houston ship channel in 2000 [Brock et al, 2003]. Rapid OA formation was also observed less than 50 km downwind of Mexico City by Volkamer et al [2006], who used SOA's close correlation with glyoxal to argue that primary VOC oxidation steps must be responsible for much of the anthropogenic SOA. Though a large fraction of the model/ observed discrepancy in Figure 13 is due to overestimates of CO emissions, one can infer (e.g., Figure 12) at least a factor of 2 underestimate (for CMAQ/NAM, CHRONOS and STEM) in SOA formation over Dallas, and larger discrepancies for Houston and other models.…”

“…Moreover, none of the models capture the higher PM 2.5 /NO y for Houston relative to Dallas, and if the low emission ratios for WRF/Chem are representative of true emission ratios, a strong photochemical source of organic PM 2.5 missing from the models is required to explain the observations. Possible sources of missing OA in global-and regional-scale models have been the subject of recent inquiry [Heald et al, 2005;Volkamer et al, 2006] with no clear consensus on biogenic versus anthropogenic origin of the missing sources [Weber et al, 2007;de Gouw et al, 2008]. 5.2.5.…”

Known and Unexplored Organic Constituents in the Earth's Atmosphere

“…A large and sustained source of SOAs from oxidation of VOCs, which are long-lived enough to be transported into the free troposphere and not included in current models, could explain this discrepancy (36). In another example in Mexico City, observed SOA growth in the real urban atmosphere was a factor of 8 higher than predicted by state-of-the-art SOA models based on oxidation of the simultaneously observed gas-phase VOCs (37). This observation suggests either higher than expected SOA yields or unmeasured precursors that together could be even more important for SOA formation than the measured compounds.…”

Reactions at surfaces in the atmosphere: integration of experiments and theory as necessary (but not necessarily sufficient) for predicting the physical chemistry of aerosols

“…[63][64][65] This would help to reconcile the difference between predicted SOA concentrations in air and the measured values, which can be as much as an order of magnitude greater than predicted. 66,67 Finally, it had been thought for many years that the uptake of gases into particles and their subsequent reactions in the condensed phase in the atmosphere could be treated in terms of well-known physical (e.g., diffusion in the gas and liquid phases, mass accommodation at the surface) and chemical processes. Even this relatively simple picture is complicated by the dramatic differences in gas uptake that have been seen for surfactant-coated liquids where the coating can either enhance or inhibit gas uptake, or have no effect, depending on the specific gas/surfactant combination.…”