Müller Silvan, Giorio Chiara, Borduas-Dedekind Nadine
Department of Environmental Systems Science, ETH Zurich, Zurich 8092, Switzerland.
Yusuf Hamied Department of Chemistry, University of Cambridge, Cambridge CB2 1EW, United Kingdom.
ACS Environ Au. 2023 Mar 17;3(3):164-178. doi: 10.1021/acsenvironau.2c00055. eCollection 2023 May 17.
Organic aerosols affect the planet's radiative balance by absorbing and scattering light as well as by activating cloud droplets. These organic aerosols contain chromophores, termed brown carbon (BrC), and can undergo indirect photochemistry, affecting their ability to act as cloud condensation nuclei (CCN). Here, we investigated the effect of photochemical aging by tracking the conversion of organic carbon into inorganic carbon, termed the photomineralization mechanism, and its effect on the CCN abilities in four different types of BrC samples: (1) laboratory-generated (NH)SO-methylglyoxal solutions, (2) dissolved organic matter isolate from Suwannee River fulvic acid (SRFA), (3) ambient firewood smoke aerosols, and (4) ambient urban wintertime particulate matter in Padua, Italy. Photomineralization occurred in all BrC samples albeit at different rates, evidenced by photobleaching and by loss of organic carbon up to 23% over a simulated 17.6 h of sunlight exposure. These losses were correlated with the production of CO up to 4% and of CO up to 54% of the initial organic carbon mass, monitored by gas chromatography. Photoproducts of formic, acetic, oxalic and pyruvic acids were also produced during irradiation of the BrC solutions, but at different yields depending on the sample. Despite these chemical changes, CCN abilities did not change substantially for the BrC samples. In fact, the CCN abilities were dictated by the salt content of the BrC solution, trumping a photomineralization effect on the CCN abilities for the hygroscopic BrC samples. Solutions of (NH)SO-methylglyoxal, SRFA, firewood smoke, and ambient Padua samples had hygroscopicity parameters κ of 0.6, 0.1, 0.3, and 0.6, respectively. As expected, the SRFA solution with a κ of 0.1 was most impacted by the photomineralization mechanism. Overall, our results suggest that the photomineralization mechanism is expected in all BrC samples and can drive changes in the optical properties and chemical composition of aging organic aerosols.
有机气溶胶通过吸收和散射光以及激活云滴来影响地球的辐射平衡。这些有机气溶胶含有发色团，称为棕碳（BrC），并且会发生间接光化学作用，影响其作为云凝结核（CCN）的能力。在此，我们通过追踪有机碳向无机碳的转化（即光矿化机制）及其对四种不同类型BrC样品CCN能力的影响，研究了光化学老化的作用：（1）实验室生成的（NH）₂SO₄ - 甲基乙二醛溶液，（2）从苏万尼河富里酸（SRFA）中分离出的溶解有机物，（3）环境柴烟雾气溶胶，以及（4）意大利帕多瓦市环境城市冬季颗粒物。光矿化在所有BrC样品中均有发生，尽管速率不同，这通过光漂白以及在模拟17.6小时阳光照射下高达23%的有机碳损失得以证明。这些损失与高达4%的CO₂以及高达初始有机碳质量54%的CO的产生相关，通过气相色谱法进行监测。在BrC溶液辐照过程中还产生了甲酸、乙酸、草酸和丙酮酸的光产物，但产率因样品而异。尽管发生了这些化学变化，BrC样品的CCN能力并未发生实质性改变。事实上，CCN能力由BrC溶液的盐含量决定，对于吸湿性BrC样品而言，这一因素胜过光矿化对CCN能力的影响。（NH）₂SO₄ - 甲基乙二醛、SRFA、柴烟雾和帕多瓦市环境样品溶液的吸湿性参数κ分别为0.6、0.1、0.3和0.6。正如预期的那样，κ为0.1的SRFA溶液受光矿化机制的影响最大。总体而言，我们的结果表明，光矿化机制在所有BrC样品中都可能出现，并可推动老化有机气溶胶光学性质和化学成分的变化。
