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Vapor-Pressure Pathways Initiate but Hydrolysis Products Dominate the Aerosol Estimated from Organic Nitrates
Citation:
Zare, A., K. Fahey, G. Sarwar, R. Cohen, AND H. Pye. Vapor-Pressure Pathways Initiate but Hydrolysis Products Dominate the Aerosol Estimated from Organic Nitrates. ACS Earth and Space Chemistry. American Chemical Society, Washington, DC, 3(8):1426-1437, (2019). https://doi.org/10.1021/acsearthspacechem.9b00067
Impact/Purpose:
When biogenic VOCs react in the presence of nitrogen oxides from combustion, they can form organic nitrates. These organic nitrates are semi-soluble and semivolatile and thus partition into PM2.5 and other condensed phases. In the particulate phase, organic nitrates retain the nitrogen signature of their chemistry, but hydrolysis can convert them to another form and release the nitrogen signature. We find that the low vapor pressure of organic nitrates results in significant amounts of particulate organic nitrates that rapidly hydrolyze. This type of chemistry is needed in models to accurately predict PM2.5, ozone, and deposition of nitrogen.
Description:
Organic nitrates contribute significantly to the total organic aerosol burden. However, the formation and loss mechanisms of particulate organic nitrates (PONs) remain poorly understood. In this study, with the CMAQ modeling system, we implement a detailed biogenic volatile organic carbon gas phase oxidation mechanism and an explicit representation of multiphase organic nitrate formation and loss, including both aqueous-phase uptake and vapor-pressure driven partitioning into organic aerosol as well as condensed-phase reactions. We find vapor-pressure dependent partitioning is the leading mechanism for formation of PONs and hydrolysis is a major loss mechanism for PON resulting in substantial amounts of organic aerosol that originate as an organic nitrate. Partitioning and hydrolysis together can produce high concentrations (up to 5 μg/m3) of PON-derived aerosols over the southeast United States. The main source of PON-derived aerosols is monoterpene nitrates that have been chemically processed to lose their nitrate functionality through aqueous chemistry. In contrast, the major portion of aqueous aerosol and in-cloud PON, which retains its nitrate moiety, are soluble isoprene nitrates. We evaluate the model using the observations from the Southern Oxidant and Aerosol Study (SOAS) campaign in the Southeast US in summer 2013 and show implementing aerosol-phase pathways for organic nitrates dramatically improves the magnitude of total alkyl nitrates (ANs) in CMAQ. The contribution of PONs to the total ANs at the SOAS site is estimated to be ∼20%, a value in the range of the measurements. The predicted AN composition is shifted from monoterpene to isoprene and anthropogenic organic nitrates.
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DOI: Vapor-Pressure Pathways Initiate but Hydrolysis Products Dominate the Aerosol Estimated from Organic Nitrates