挥发性有机物(VOCs)的不同化学去除途径:城市与区域站点的对比

VOCs在大气中主要是与OH自由基、NO₃自由基和O₃等反应氧化去除，部分OVOCs的自身光解也是重要的化学去除途径.本研究基于2018年和2019年秋季在珠三角地区的城市和区域站点的外场观测实验，使用VOCs、常规痕量气体及气象参数的观测数据，对烷烃、烯烃、芳香烃和OVOCs等VOCs组分不同化学去除途径的去除速率进行分析.结果表明，烷烃和芳香烃主要通过与OH自由基反应去除，最高占比超过99%.与NO₃自由基和O₃的反应可贡献烯烃去除速率的80%以上，特别是一些天然源的烯烃（如单萜烯）与NO₃自由基的氧化去除是贡献最大的氧化途径.光解是甲醛最重要的去除途径，在两个站点均达到了50%以上，酮类的光解贡献会高于其他OVOCs类物质.OH自由基的氧化去除途径在城市和区域站点的人为源及天然源VOCs去除中占主导地位.区域站点，烯烃尤其是天然源的烯烃物种，与NO₃自由基和O₃反应的贡献要高于城市站点.本研究对促进不同VOCs物种在大气中的去除途径以及其空间差异的认识有重要意义.

Different chemical removal pathways of volatile organic compounds (VOCs): Comparison of urban and regional sites

Volatile organic compounds (VOCs) are mainly removed from the atmosphere by oxidation reactions with hydroxyl (OH) radicals, nitrate (NO₃) radicals, and ozone. Photolysis also plays an important role in the chemical removal of some oxygenated volatile organic compounds (OVOCs). The contributions of different chemical removal pathways for various VOCs species are evaluated based on the data sets collected from two field campaigns in fall of Pearl River Delta (PRD):an urban site in 2018 and a downwind receptor site in 2019, respectively. Our result shows that chemical removal of alkanes and aromatics hydrocarbons are dominated by reactions with OH radicals, accounting for as high as 99% of their chemical losses, while reactions with ozone and NO₃ radicals can account for more than 80% of loss rates of alkenes. In particular, some biogenic alkenes, such as monoterpenes, are mainly removed from the atmosphere through reactions with NO₃ radicals. Photolysis contributes to over 50% of formaldehyde removal at both sites. The contributions of photolysis to loss rates of ketones are significant, which is usually higher than aldehydes with the same carbon number. As a whole, OH radicals play dominant roles in chemical removal pathways for both anthropogenic and biogenic VOCs at urban and regional sites. The contributions of ozone and NO₃ oxidations to losses rates of alkenes, especially biogenic alkenes, are significantly higher at the regional site than at the urban site. Our study provides important information for understanding oxidation reaction pathways of VOCs and the variations in different environments.