Aqueous-phase mechanism for secondary organic aerosol formation from isoprene: application to the southeast United States and co-benefit of SO2 emission controls Atmospheric Chemistry and Physics

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.

植被排放的异戊二烯是二次有机气溶胶（secondary organic aerosol, SOA）的重要前体物，但其生成机制与产率尚未明确。在异戊二烯排放区域典型的潮湿环境中，气溶胶普遍以水相状态存在。本研究构建了耦合详细气相异戊二烯氧化过程的水相异戊二烯SOA生成机制，该机制基于水溶性异戊二烯氧化产物的气溶胶反应摄取系数（gamma, γ），并考量了气溶胶酸度与亲核试剂浓度的敏感性。我们将该机制应用于2013年夏季美国东南部地区的航空观测（SEAC(4)RS）与地面观测（SOAS）的模拟，模拟所用模型为GEOS-Chem化学传输模型。美国东南部的氮氧化物（nitrogen oxides, NOx = NO + NO2）排放水平使得异戊二烯氧化生成的过氧自由基（ISOPO2）可同时与NO（高NOx路径）和HO2（低NOx路径）发生显著反应，进而生成不同类别的异戊二烯SOA前体物。本研究得到异戊二烯氧化的平均SOA质量产率为3.3%，该结果与实测细粒有机气溶胶（organic aerosol, OA）与异戊二烯氧化产物甲醛的相关关系相符。异戊二烯SOA的生成主要由两种即时气相前体物贡献：低NOx路径生成的异戊二烯环氧二醇（isoprene epoxydiols, IEPOX，占异戊二烯SOA的58%），以及高低NOx路径均会产生的乙二醛（占比28%）。该组分分布特征与SOAS和SEAC4RS观测到的IEPOX SOA结果一致。观测数据显示IEPOX SOA与硫酸盐气溶胶存在显著相关关系，我们将其归因于硫酸盐对气溶胶酸度与体积的影响。异戊二烯SOA浓度随NOx排放降低（有利于异戊二烯氧化的低NOx路径）而升高，但随SO2排放降低（因硫酸盐对气溶胶酸度与体积的影响）而显著下降。美国环境保护署（Environmental Protection Agency, EPA）预测，2013-2025年人类活动排放的NOx将减少34%（对应异戊二烯SOA升高7%），SO2将减少48%（对应异戊二烯SOA下降35%）。减少SO2排放可使硫酸盐与异戊二烯SOA以相近幅度降低，这意味着二氧化硫排放管控可为PM2.5带来两倍的协同效益。