Evolution of the light-absorption properties of combustion brown carbon aerosols following reaction with nitrate radicals
Evolution of the light-absorption properties of combustion brown carbon aerosols following reaction with nitrate radicals
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DOI：
10.1080/02786826.2020.1726867
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发表时间：
2020-02-28
影响因子：
5.2
通讯作者：
Saleh, Rawad
中科院分区：
文献类型：
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作者：
Cheng, Zezhen;Atwi, Khairallah M.;Saleh, Rawad
多环芳烃(PAHs)是褐碳(BRC)的重要组成部分，可被羟基自由基(OH)和硝酸根(NO3)等气相氧化剂进行大气处理。虽然BRC的羟基氧化已被广泛研究，但对NO3氧化的研究还很有限。在这里，我们从含有光吸收多环芳烃的复杂混合物的甲苯的燃烧中生成BRC，并在氧化流动反应器中研究了它们在暴露于NO2后的化学组成和光吸收性质的变化。研究了三种不同类型的BRC，根据折射率(K)的虚部将其分类为：浅BRC(k532 nm，k(532)=0.008)，中等BRC(k(532)=0.026)和暗BRC(k(532)=0.091)。暴露于硝酸可使亮、中BRC的k(532)增加30%，暗BRC的k(532)减少5%。这种差异归因于两个相互竞争的效应：1)NO3诱导的非均相氧化增加了发色官能团；2)气相PAH+NO3氧化产物的缩合，其吸收能力低于颗粒状PAHs。对气溶胶化学演化的分析表明，在暗BRC实验中，效应(2)更为重要。我们进行了光学计算来分离效应(1)，结果表明，对于所有的BRC类型，非均相氧化导致了类似于50%的k(532)增加。这些结果表明，NO3诱导的非均相氧化使某些类型的大气BRC变暗，从而抵消了OH氧化引起的漂白效应。版权所有(C)2020美国气溶胶研究协会
Polycyclic aromatic hydrocarbons (PAHs) are important constituents of brown carbon (BrC) that are subject to atmospheric processing by gas-phase oxidants such as the hydroxyl radical (OH) and the nitrate radical (NO3). While OH oxidation of BrC has been investigated extensively, studies of NO3 oxidation are limited. Here, we generated BrC from the combustion of toluene containing a complex mixture of light-absorbing PAHs and investigated changes in their chemical composition and light-absorption properties following exposure to NO3 in an oxidation flow reactor. Three types of BrC were studied, with varying light-absorption properties that were classified in terms of the imaginary part of the refractive index (k) as: light BrC (k at 532 nm, k(532) = 0.008), medium BrC (k(532) = 0.026), and dark BrC (k(532) = 0.091). Exposure to NO3 led to similar to 30% increase in k(532) of the light and medium BrC and similar to 5% decrease in k(532) of the dark BrC. This discrepancy is attributed to two competing effects: 1) addition of chromophoric functional groups by NO3-induced heterogeneous oxidation and 2) condensation of gas-phase PAH + NO3 oxidation products that were less-absorbing than the particulate PAHs. Analysis of the aerosol chemical evolution revealed that effect (2) was more important in the dark BrC experiments. We performed optical calculations to isolate effect (1) and showed that heterogeneous oxidation led to similar to 50% increase in k(532) for all the BrC types. These results indicate that NO3-induced heterogeneous oxidation darkens some types of atmospheric BrC, which can counterbalance bleaching effects induced by OH oxidation. Copyright (c) 2020 American Association for Aerosol Research