Formation and fate of secondary organic aerosol produced from nitrate radical oxidaton of monoterpenes
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Atmospheric aerosol, or particulate matter, has important implications for climate, visibility and health. A large portion of atmospheric aerosol is comprised of secondary organic aerosol (SOA), which is formed by the oxidation of volatile organic compounds (VOCs) with atmospheric oxidants, including the nitrate (NO3) radical. Reducing the impact of human pollution on aerosol formation is challenging, in large part because of the complexities in the mechanisms for the formation of organic aerosol through the oxidation of VOCs. The oxidation of biogenic volatile organic compounds (BVOCs) by NO3 radical, formed by the reaction of O3 with anthropogenic NO2, provides one example of the effect of human activities on atmospheric aerosol. Due to the high aerosol mass yields of BVOC+NO3 reactions, this pathway can contribute to a large fraction of atmospheric aerosol. In addition to high aerosol mass yields, the BVOC+NO3 reaction produces a large mass of particulate and gas-phase organic nitrates. Organic nitrate formation, which involves loss of atmospheric NOx (NO+NO2), can either act as a permanent sink for NOx or a temporary reservoir for NOx. The role of organic nitrates on the NOx cycle is highly dependent on the types and eventual fate of organic nitrates, whether by deposition, hydrolysis to nitric acid, or release of NOx upon photooxidation or photolysis. The effect of organic nitrates on atmospheric NOx will impact ozone production, SOA mass and composition, and formation of atmospheric nitric acid. Nevertheless, the fate of atmospheric organic nitrates and their effect on the NOx cycle are highly uncertain. The goals of this work are to improve our understanding of the formation of atmospheric aerosol and organic nitrates through the BVOC+NO3 reaction. The formation of secondary organic aerosol (SOA) from the oxidation of BVOCs via NO3 radical is investigated in the Georgia Tech Environmental Chamber facility (GTEC). This work focuses primarily on the monoterpenes β-pinene and limonene, two globally abundant VOCs with high mass yields upon NO3 radical oxidation. For β-pinene, aerosol yields are determined for experiments performed under both dry (RH < 2%) and humid (RH = 50% and RH = 70%) conditions. To further probe the β-pinene+NO3 reaction, the effects of peroxy radical (RO2) fate on aerosol formation were investigated by reacting the RO2 radicals with either NO3 radicals or hydroperoxy (HO2) radicals, simulating both polluted and cleaner environments. We find that the β-pinene+NO3 reaction leads to formation of gas-phase organic nitrate species (with molecular weights of 215 amu, 229 amu, 231 amu, and 245 amu—which are assigned molecular formulas of C10H17NO4, C10H15NO5, C10H17NO5, and C10H15NO6, respectively). Additionally, the SOA yields (defined as the mass of aerosol formed per mass of hydrocarbon reacted) in the “RO2+NO3 dominant” and “RO2+HO2 dominant” experiments and across all relative humidities are comparable. The aerosol mass yield is calculated to be 27.0-104.1% for a wide range of organic mass loadings, and approximately 45-74% of this organic aerosol is composed of organic nitrate species. It has been estimated that approximately 50% of the nighttime organic aerosol production in the southeastern United States could be due to the monoterpene oxidation by NO3, a substantial portion of which is from the β-pinene+NO3 reaction. This observation helps to elucidate one of the control strategies for reducing atmospheric aerosol. While it is clear that the β-pinene+NO3 reaction contributes to atmospheric aerosol, its impacts on organic nitrate formation and the NOx cycle are less clear. Although humidity does not appear to affect SOA yields, there is evidence of particle-phase hydrolysis of organic nitrates, which are estimated to compose 45-74% of the organic aerosol. It is estimated that about 90 and 10% of the organic nitrates formed from the β-pinene+NO3 reaction are primary organic nitrates and tertiary organic nitrates, respectively. While the primary organic nitrates do not appear to hydrolyze, the tertiary organic nitrates undergo hydrolysis with a lifetime of 3-4.5 hours. Organic nitrate species that do not hydrolyze are likely to either deposit and remove atmospheric NOx or may potentially re-release NOx into the atmosphere upon photolysis or photooxidation. The fate of these organic nitrate species must be better understood to determine their impact on the NOx cycle. Given the long lifetime with respect to hydrolysis of organic nitrates formed by the BVOC+NO3 reaction, it is possible that organic nitrates formed at night will remain in the atmosphere during the night-to-day transition. The night-to-day transition is accompanied by rising surface temperatures and aerosol dilution caused by an expansion of the boundary layer. Both of these processes can potentially lead to aerosol evaporation, which must be accurately accounted for in atmospheric models in order to predict aerosol concentrations and their effect on climate, visibility, and health. We find that for the limonene+NO3 system, very little aerosol evaporation is observed through dilution due to the non-volatile nature of SOA produced by the limonene+NO3 reaction. To determine evaporation by heating, the enthalpy of vaporization for the limonene+NO3 reaction is calculated over a wide range of atmospherically relevant mass loadings. The enthalpy of vaporization is highly dependent on the aerosol mass loading and can be as high as 237 kJ mol-1 for loadings below 18 μg m-3. We also find that there is a contrast in mass and composition between aerosol formed at 25 °C and heated to 40 °C and aerosol formed at 40 °C, indicating a resistance to aerosol evaporation. Evidence from mixtures of SOA from the limonene+NO3 and β-pinene+NO3 reactions suggests that high volatility species that dominate the outer edge of the aerosol hinders the evaporation of the low volatility species underneath. This resistance to evaporation can greatly affect the phase of SOA and organic nitrates form the BVOC+NO3 system and can thus affect the organic nitrate fate. Human activities can influence SOA formation through the BVOC+NO3 reaction. It is clear that reductions in NOx should therefore lead to a decrease in the contribution of SOA and organic nitrates from the BVOC+NO3 reaction. Less certain is the fate of the organic nitrates formed from nitrate radical oxidation of BVOCs. Current evidence suggests that organic nitrates produced via nitrate radical may not be susceptible to hydrolysis, but the reaction of organic nitrates through photooxidation and photolysis are still highly uncertain. As these processes are better understood, modeling of pollutants such as O3 and nitric acid will improve as well as the concentrations of aerosol and their impacts on climate and human health.