Institute of Chemical Engineering Sciences, Foundation for Research & Technology-Hellas, Patras 26504, Greece.
Institute of Atmospheric Sciences and Climate, Italian National Research Council, Bologna 40129, Italy.
Proc Natl Acad Sci U S A. 2020 Dec 29;117(52):33028-33033. doi: 10.1073/pnas.2010365117. Epub 2020 Dec 14.
Oxidized organic aerosol (OOA) is a major component of ambient particulate matter, substantially impacting climate, human health, and ecosystems. OOA is readily produced in the presence of sunlight, and requires days of photooxidation to reach the levels observed in the atmosphere. High concentrations of OOA are thus expected in the summer; however, our current mechanistic understanding fails to explain elevated OOA during wintertime periods of low photochemical activity that coincide with periods of intense biomass burning. As a result, atmospheric models underpredict OOA concentrations by a factor of 3 to 5. Here we show that fresh emissions from biomass burning exposed to NO and O (precursors to the NO radical) rapidly form OOA in the laboratory over a few hours and without any sunlight. The extent of oxidation is sensitive to relative humidity. The resulting OOA chemical composition is consistent with the observed OOA in field studies in major urban areas. Additionally, this dark chemical processing leads to significant enhancements in secondary nitrate aerosol, of which 50 to 60% is estimated to be organic. Simulations that include this understanding of dark chemical processing show that over 70% of organic aerosol from biomass burning is substantially influenced by dark oxidation. This rapid and extensive dark oxidation elevates the importance of nocturnal chemistry and biomass burning as a global source of OOA.
氧化有机气溶胶（OOA）是环境颗粒物的主要组成部分，对气候、人类健康和生态系统有重大影响。OOA 在阳光的存在下很容易产生，并且需要数天的光氧化才能达到大气中观测到的水平。因此，预计夏季 OOA 的浓度会很高；然而，我们目前的机械理解无法解释在低光化学反应活性的冬季期间，与强烈生物质燃烧时期同时发生的 OOA 升高现象。结果，大气模型对 OOA 浓度的预测值低了 3 到 5 倍。在这里，我们表明，暴露于 NO 和 O（NO 自由基的前体）的生物质燃烧的新鲜排放物在实验室中仅需几个小时且无需阳光即可迅速形成 OOA。氧化的程度对相对湿度敏感。所得 OOA 的化学成分与主要城市地区实地研究中观察到的 OOA 一致。此外，这种暗化学反应处理会导致二次硝酸盐气溶胶的显著增加，其中估计有 50%至 60%是有机的。包括这种暗化学处理理解的模拟表明，超过 70%的生物质燃烧产生的有机气溶胶受到暗氧化的显著影响。这种快速和广泛的暗氧化提高了夜间化学和生物质燃烧作为 OOA 全球来源的重要性。
