A larger pool of secondary organic aerosol precursors in continental air

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Description

Measurements of aerosol in urban and marine atmospheres suggest that production of secondary organic aerosol (SOA) cannot be explained by oxidation of organic gases that are routinely measured. Laboratory experiments demonstrated exhaust from diesel-powered vehicles (DPVs) produced more SOA than the amount predicted from oxidation of volatile organic compounds (VOCs) in the exhaust that are known precursors of SOA. A complex mixture of semi-volatile organic compounds (SVOCs) is suspected of producing the additional SOA. The SVOCs and suspected oxidation products are not routinely measured in ambient air or are measured with insufficient sampling frequency to evaluate SOA formation rates. Here, gas-aerosol partitioning characteristics of the likely semi-volatile precursors of SOA are evaluated, an SVOC emission profile for a roadway tunnel is developed, and the average contribution of SVOCs to SOA precursors emitted by vehicles in the United States is estimated. The results indicate approximately 81% and 12% of the suspected SOA precursors emitted by DPVs and gasoline-powered vehicles (GPVs), respectively, are semi-volatile. The magnitude of the OH reactivity of SOA precursor emissions from DPVs and GPVs are similar; however, approximately 82% and 15% of the OH reactivity from DPVs and GPVs, respectively, are contributed by SVOCs. The SVOCs from vehicle tailpipe emissions are more reactive with OH than VOCs, and thus, SVOCs will make a greater contribution to SOA formation nearby sources. In the high NOx conditions of urban areas, SVOC oxidation is likely to produce a complex mixture of δ-hydroxynitrates, dinitrates, and hydroxynitrates that are also poorly measured by current technologies. Using transportation statistics on annual DPV and GPV mileage in the United States, the emission profile of SVOCs in DPV and GPV exhaust, and the reported yield of SOA from photo-oxidation of diesel exhaust, we estimate the annual contribution of SVOCs to SOA precursors emitted by vehicles in urban areas of the United States might be as high as 20%. The SVOCs are expected to partition into the gas phase downwind of urban areas as total suspended aerosol concentrations diminish by dilution, and thus, the total contribution of SVOCs to SOA precursors is likely to be even greater.