Photooxidants from brown carbon and other chromophores in illuminated particle extracts
Photooxidants from brown carbon and other chromophores in illuminated particle extracts
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DOI：
10.5194/acp-19-6579-2019
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发表时间：
2019-05-17
影响因子：
6.3
通讯作者：
Anastasio, Cort
中科院分区：
文献类型：
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作者：
Kaur, Richie;Labins, Jacqueline R.;Anastasio, Cort
虽然光氧化剂在大气凝聚相中很重要，但对颗粒物 (PM) 的测量却很少。在这里，我们测量了加州戴维斯冬季粒子的光照水提取物中的光吸收和三种光氧化剂的浓度 - 羟基自由基（(OH)-O-中心点）、单线态分子氧 (O-1(2)*) 和有机物的氧化三重激发态 (C-3*)。 O-1(2)* 和 C-3* 是由棕碳 (BrC) 光激发形成的，之前尚未在 PM 中测量过。在提取物中，溶解有机化合物 (MAC(DOC)) 在 300 nm 范围内 13 000 至 30 000 cm(2) (g C)(-1) 之间的质量吸收系数大约是戴维斯雾中先前值的两倍。粒子提取物中的平均 (+/- 1 sigma)(OH)-O-中心点稳态浓度为 4.4(+/- 2.3) x 10(-16) M，这与雾、云和雨中的先前值非常相似：尽管我们的粒子提取物更浓缩，但 (OH)-O-中心自然汇浓度的相应增强基本上抵消了 (OH)-O-中心点光生成速率的增强点。相比之下，与戴维斯雾相比，颗粒提取物中主要由棕碳形成的两种氧化剂（即 O-1(2)* 和 C-3*）的浓度均有所提高，这是提取物中溶解的有机碳浓度更高和光吸收速率更快的结果。 PM提取物中O-1(2)*的平均浓度为1.6(+/- 0.5) x 10(-12) M，比过去的雾测量高7倍，而氧化三联体的平均浓度为1.0(+/- 0.4) x 10(-13) M，几乎是平均戴维斯雾值的两倍。此外，O-1(2)* 和 C-3* 光生成速率均与阳光吸收速率密切相关。由于我们无法在环境颗粒水条件下通过实验测量光氧化剂，因此我们测量了 PM 稀释对氧化剂浓度的影响，然后外推到环境颗粒条件。随着提取物中颗粒质量浓度的增加，(OH)-O-中心点的测量浓度保持相对不变，O-1(2)* 线性增加，而 C-3* 浓度增加小于线性，可能是由于溶解的有机物猝灭。根据我们的测量，并考虑到在 PM 条件下应该重要的其他源和汇，我们估计颗粒中的 [(OH)-O-中心点] 略低于稀云/雾滴中的含量，而 PM 中的 [C-3*] 比滴中的高 30 至 2000 倍，PM 中的 [O-1(2)*] 与滴中的相比增强了大约 2400 倍。由于 O-1(2)* 和 C-3* 浓度的增强，一些高溶解度有机物在颗粒液态水中的寿命似乎比在雾/多云条件下短得多。根据我们测量的 PM 提取物形成速率的推断，BrC 衍生的单线态分子氧和三重态激发态总体上是颗粒液态水中有机化合物的主要汇，每种氧化剂的总反应速率大约比具有 (OH)-O-中心点的有机物的总反应速率高 200-300 倍。对于个别的、高度溶解的反应性有机化合物，O-1(2)* 似乎通常是颗粒水中的主要汇，这是一个新发现。三重态激发态可能对单个颗粒有机物的命运也很重要，但评估这一点需要额外测量三重态与天然样品中溶解的有机碳的相互作用。
While photooxidants are important in atmospheric condensed phases, there are very few measurements in particulate matter (PM). Here we measure light absorption and the concentrations of three photooxidants - hydroxyl radical ((OH)-O-center dot), singlet molecular oxygen (O-1(2)*), and oxidizing triplet excited states of organic matter (C-3*) - in illuminated aqueous extracts of wintertime particles from Davis, California. O-1(2)* and C-3*, which are formed from photoexcitation of brown carbon (BrC), have not been previously measured in PM. In the extracts, mass absorption coefficients for dissolved organic compounds (MAC(DOC)) at 300 nm range between 13 000 and 30 000 cm(2) (g C)(-1) are approximately twice as high as previous values in Davis fogs. The average (+/- 1 sigma)(OH)-O-center dot steady-state concentration in particle extracts is 4.4(+/- 2.3) x 10(-16) M, which is very similar to previous values in fog, cloud, and rain: although our particle extracts are more concentrated, the resulting enhancement in the rate of (OH)-O-center dot photoproduction is essentially canceled out by a corresponding enhancement in concentrations of natural sinks for (OH)-O-center dot. In contrast, concentrations of the two oxidants formed primarily from brown carbon (i.e., O-1(2)* and C-3*) are both enhanced in the particle extracts compared to Davis fogs, a result of higher concentrations of dissolved organic carbon and faster rates of light absorption in the extracts. The average O-1(2)* concentration in the PM extracts is 1.6(+/- 0.5) x 10(-12) M, 7 times higher than past fog measurements, while the average concentration of oxidizing triplets is 1.0(+/- 0.4) x 10(-13) M, nearly double the average Davis fog value. Additionally, the rates of O-1(2)* and C-3* photoproduction are both well correlated with the rate of sunlight absorption.Since we cannot experimentally measure photooxidants under ambient particle water conditions, we measured the effect of PM dilution on oxidant concentrations and then extrapolated to ambient particle conditions. As the particle mass concentration in the extracts increases, measured concentrations of (OH)-O-center dot remain relatively unchanged, O-1(2)* increases linearly, and C-3* concentrations increase less than linearly, likely due to quenching by dissolved organics. Based on our measurements, and accounting for additional sources and sinks that should be important under PM conditions, we estimate that [(OH)-O-center dot] in particles is somewhat lower than in dilute cloud/fog drops, while [C-3*] is 30 to 2000 times higher in PM than in drops, and [O-1(2)*] is enhanced by a factor of roughly 2400 in PM compared to drops. Because of these enhancements in O-1(2)* and C-3* concentrations, the lifetimes of some highly soluble organics appear to be much shorter in particle liquid water than under foggy/cloudy conditions. Based on extrapolating our measured rates of formation in PM extracts, BrC-derived singlet molecular oxygen and triplet excited states are overall the dominant sinks for organic compounds in particle liquid water, with an aggregate rate of reaction for each oxidant that is approximately 200-300 times higher than the aggregate rate of reactions for organics with (OH)-O-center dot. For individual, highly soluble reactive organic compounds it appears that O-1(2)* is often the major sink in particle water, which is a new finding. Triplet excited states are likely also important in the fate of individual particulate organics, but assessing this requires additional measurements of triplet interactions with dissolved organic carbon in natural samples.