Aqueous-phase mechanism for secondary organic aerosol formation from isoprene: application to the southeast United States and co-benefit of SO2 emission controls
Aqueous-phase mechanism for secondary organic aerosol formation from isoprene: application to the southeast United States and co-benefit of SO2 emission controls
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DOI：
10.5194/acp-16-1603-2016
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发表时间：
2016-01-01
影响因子：
6.3
通讯作者：
McNeill, V. F.
中科院分区：
文献类型：
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作者：
Marais, E. A.;Jacob, D. J.;McNeill, V. F.
植物排放的异戊二烯是二次有机气溶胶的重要前驱物质，但其产生机制和产率尚不清楚。在异戊二烯排放区典型的潮湿条件下，气溶胶主要是含水的。在这里，我们开发了异戊二烯SOA形成的水相机制，并结合了详细的气相异戊二烯氧化方案。其机理是基于水溶性异戊二烯氧化产物的气溶胶反应吸收系数(伽马)，包括对气溶胶酸度和亲核剂浓度的敏感性。我们将这一机制应用于2013年夏季美国东南部上空飞机(SEAC(4)RS)和地面(SOAS)观测的模拟，使用GEOS-Chem化学输送模式。美国东南部地区氮氧化物(NOx=NO+NO2)的排放使得异戊二烯氧化产生的过氧基(ISOPO2)与NO(高NOx途径)和HO2(低NOx途径)发生显著反应，导致不同的异戊二烯SOA前体。结果表明，异戊二烯氧化的平均SOA产率为3.3%，与观测到的总有机气溶胶(OA)和甲醛(异戊二烯氧化产物)的关系一致。异戊二烯SOA的产生主要由两个直接的气相前体贡献，即来自低NOx途径的异戊二烯环氧二醇(IEPOX，占异戊二烯SOA58%)和来自低NOx途径和高NOx途径的乙二醛(28%)。这种形态与从SOAS和SEAC4RS观察到的IEPOX SOA是一致的。观测结果表明，IEPOX SOA和硫酸盐气溶胶之间有很强的关系，我们解释这是由于硫酸盐对气溶胶的酸度和体积的影响。异戊二烯浓度随着NOx排放量的减少而增加(有利于异戊二烯氧化的低NOx途径)，但随着SO2排放量的减少(由于硫酸盐对气溶胶酸度和体积的影响)，异戊二烯浓度下降的幅度更大。美国环境保护署(EPA)预计，2013-2025年，人为排放的NOx将减少34%(导致异戊二烯SOA增加7%)，SO2减少48%(异戊二烯SOA减少35%)。减少SO2排放将以类似的幅度减少硫酸盐和异戊二烯SOA，相当于SO2排放控制对PM2.5的共同效益的2倍。
Isoprene emitted by vegetation is an important precursor of secondary organic aerosol (SOA), but the mechanism and yields are uncertain. Aerosol is prevailingly aqueous under the humid conditions typical of isoprene-emitting regions. Here we develop an aqueous-phase mechanism for isoprene SOA formation coupled to a detailed gas-phase isoprene oxidation scheme. The mechanism is based on aerosol reactive uptake coefficients (gamma) for water-soluble isoprene oxidation products, including sensitivity to aerosol acidity and nucleophile concentrations. We apply this mechanism to simulation of aircraft (SEAC(4)RS) and ground-based (SOAS) observations over the southeast US in summer 2013 using the GEOS-Chem chemical transport model. Emissions of nitrogen oxides (NOx = NO + NO2) over the southeast US are such that the peroxy radicals produced from isoprene oxidation (ISOPO2) react significantly with both NO (high-NOx pathway) and HO2 (low-NOx pathway), leading to different suites of isoprene SOA precursors. We find a mean SOA mass yield of 3.3% from isoprene oxidation, consistent with the observed relationship of total fine organic aerosol (OA) and formaldehyde (a product of isoprene oxidation). Isoprene SOA production is mainly contributed by two immediate gasphase precursors, isoprene epoxydiols (IEPOX, 58% of isoprene SOA) from the low-NOx pathway and glyoxal (28 %) from both low-and high-NOx pathways. This speciation is consistent with observations of IEPOX SOA from SOAS and SEAC4RS. Observations show a strong relationship between IEPOX SOA and sulfate aerosol that we explain as due to the effect of sulfate on aerosol acidity and volume. Isoprene SOA concentrations increase as NOx emissions decrease (favoring the low-NOx pathway for isoprene oxidation), but decrease more strongly as SO2 emissions decrease (due to the effect of sulfate on aerosol acidity and volume). The US Environmental Protection Agency (EPA) projects 2013-2025 decreases in anthropogenic emissions of 34% for NOx (leading to a 7% increase in isoprene SOA) and 48% for SO2 (35% decrease in isoprene SOA). Reducing SO2 emissions decreases sulfate and isoprene SOA by a similar magnitude, representing a factor of 2 co-benefit for PM2.5 from SO2 emission controls.