Kaur Richie, Anastasio Cort
Environ Sci Technol. 2018 May 1;52(9):5218-5226. doi: 10.1021/acs.est.7b06699. Epub 2018 Apr 9.
Photooxidants chemically transform organic compounds in atmospheric drops and particles. Photooxidants such as hydroxyl radical (OH) and singlet molecular oxygen (O*) have been characterized in cloud and fog drops, but there are no measurements of the triplet excited states of organic matter (C*). These "triplets", which are formed from excitation of chromophoric dissolved organic matter (CDOM), i.e., brown carbon, are difficult to measure because they are a mixture of species instead of a single entity. Here, we use a two-probe technique to measure the steady-state concentrations, rates of photoformation, and quantum yields of oxidizing triplet states during simulated-sunlight illumination of bulk fog waters. Concentrations of C* are (0.70-15) × 10 M with an average (±σ) value of 5.0 (±5.1) × 10 M. The average C* photoformation rate is 130 (±130) μM h, while the average quantum yield is 3.7 (±4.5)%. Based on our previous measurements of OH and O* in the same fog samples, the ratio of the steady-state concentrations for O*:C*:OH is approximately 3:1:0.04, respectively. At our measured concentrations, triplet excited states can be the dominant aqueous oxidants for organic compounds such as phenols from biomass combustion.
光氧化剂可使大气中的液滴和颗粒物中的有机化合物发生化学转化。诸如羟基自由基（OH）和单线态分子氧（O*）等光氧化剂已在云雾滴中得到表征，但尚未对有机物的三线态激发态（C*）进行测量。这些“三线态”由发色溶解有机物（CDOM）即棕碳激发形成，由于它们是多种物质而非单一实体的混合物，因此难以测量。在此，我们使用双探针技术来测量在模拟阳光照射下大雾水体中氧化三线态的稳态浓度、光生成速率和量子产率。C的浓度为（0.70 - 15）×10⁻¹⁰ M，平均（±σ）值为5.0（±5.1）×10⁻¹⁰ M。C的平均光生成速率为130（±130）μM/h，而平均量子产率为3.7（±4.5）%。根据我们之前对相同雾样本中OH和O的测量，O:C*:OH的稳态浓度比分别约为3:1:0.04。在我们测量的浓度下，三线态激发态可能是生物质燃烧产生的酚类等有机化合物的主要水相氧化剂。
