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Photodegradation of Secondary Organic Aerosols by Long-Term Exposure to Solar Actinic Radiation
ACS Earth and Space Chemistry ( IF 2.9 ) Pub Date : 2020-06-01 , DOI: 10.1021/acsearthspacechem.0c00088 Vahe J. Baboomian 1 , Yiran Gu 1 , Sergey A. Nizkorodov 1
ACS Earth and Space Chemistry ( IF 2.9 ) Pub Date : 2020-06-01 , DOI: 10.1021/acsearthspacechem.0c00088 Vahe J. Baboomian 1 , Yiran Gu 1 , Sergey A. Nizkorodov 1
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Sunlight-driven chemical transformations of secondary organic aerosol (SOA) are important for understanding the climate- and health-relevant properties of atmospheric particulate matter, but these photochemical processes are not well understood. We measured the photodegradation rates of SOA by observing condensed-phase photochemical processes over many days of UV exposure. The experiments relied on a quartz crystal microbalance to quantify the mass loss rate from SOA materials prepared by ozonolysis of d-limonene and α-pinene and photo-oxidation of toluene under either high or low NOx conditions. We observed that 254 nm irradiation degraded SOA almost entirely after 24 h. The mass loss rates were higher for toluene-derived SOA, which absorbs strongly at 254 nm. Irradiation at 305 nm, which is more relevant for the troposphere, resulted in larger mass loss rates from SOA generated from α-pinene and d-limonene, even though toluene-derived SOA had a higher absorption coefficient. In all 305 nm irradiation experiments, the initial mass loss rate was high (corresponding to 1–5% fractional mass loss per hour), but it slowed down after 24 h of irradiation, with a photorecalcitrant fraction of SOA degrading much slower (<1% fractional mass loss per hour). The mass loss rates were observed to increase at a higher relative humidity because volatile photoproducts could diffuse out of SOA faster. Long-term changes in the chemical composition of limonene ozonolysis SOA were examined using high-resolution electrospray ionization mass spectrometry and revealed a more complex mixture of species after photodegradation compared to the initial SOA. The compounds in the photodegraded sample had on average lower molecular weights, lower H/C ratios, and higher O/C ratios compared to the compounds in the un-photolyzed sample. These experiments confirm that condensed-phase photochemistry is an important aging mechanism for SOA during long-range transport.
中文翻译：
长期暴露于太阳光化辐射中对次要有机气溶胶的光降解作用
阳光驱动的次级有机气溶胶（SOA）的化学转化对于理解大气颗粒物与气候和健康相关的特性很重要，但是对这些光化学过程的了解却很少。我们通过观察紫外线暴露多天的凝结相光化学过程来测量SOA的光降解速率。实验依赖于石英晶体微量天平量化从SOA材料质量损失率制备的臭氧分解d或高或低NO下-limonene和α蒎烯和甲苯的光氧化X条件。我们观察到254 nm辐射在24 h后几乎完全降解了SOA。甲苯衍生的SOA的质量损失率更高，后者在254 nm处强烈吸收。与对流层更相关的305 nm辐照导致α-pine烯和d生成的SOA产生更大的质量损失率-柠檬烯，即使来自甲苯的SOA具有较高的吸收系数。在所有305 nm的辐照实验中，初始质量损失率都很高（相当于每小时1–5％的部分质量损失），但辐照后24小时它减慢了速度，而SOA的光难分解部分降解得慢得多（<1每小时百分比质量损失百分比）。观察到质量损失率在较高的相对湿度下会增加，因为挥发性的光产物可以更快地从SOA扩散出去。使用高分辨率电喷雾电离质谱分析了柠檬烯臭氧分解SOA的化学组成的长期变化，发现与初始SOA相比，光降解后物种的混合物更加复杂。光降解样品中的化合物平均具有较低的分子量，较低的H / C比，与未光解样品中的化合物相比，O / C比更高。这些实验证实，固相光化学是长距离运输过程中SOA的重要老化机理。
更新日期：2020-07-16
中文翻译：
长期暴露于太阳光化辐射中对次要有机气溶胶的光降解作用
阳光驱动的次级有机气溶胶（SOA）的化学转化对于理解大气颗粒物与气候和健康相关的特性很重要，但是对这些光化学过程的了解却很少。我们通过观察紫外线暴露多天的凝结相光化学过程来测量SOA的光降解速率。实验依赖于石英晶体微量天平量化从SOA材料质量损失率制备的臭氧分解d或高或低NO下-limonene和α蒎烯和甲苯的光氧化X条件。我们观察到254 nm辐射在24 h后几乎完全降解了SOA。甲苯衍生的SOA的质量损失率更高，后者在254 nm处强烈吸收。与对流层更相关的305 nm辐照导致α-pine烯和d生成的SOA产生更大的质量损失率-柠檬烯，即使来自甲苯的SOA具有较高的吸收系数。在所有305 nm的辐照实验中，初始质量损失率都很高（相当于每小时1–5％的部分质量损失），但辐照后24小时它减慢了速度，而SOA的光难分解部分降解得慢得多（<1每小时百分比质量损失百分比）。观察到质量损失率在较高的相对湿度下会增加，因为挥发性的光产物可以更快地从SOA扩散出去。使用高分辨率电喷雾电离质谱分析了柠檬烯臭氧分解SOA的化学组成的长期变化，发现与初始SOA相比，光降解后物种的混合物更加复杂。光降解样品中的化合物平均具有较低的分子量，较低的H / C比，与未光解样品中的化合物相比，O / C比更高。这些实验证实，固相光化学是长距离运输过程中SOA的重要老化机理。
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