Chemistry of Volatile Organic Compounds in the Los Angeles Basin: Formation of Oxygenated Compounds and Determination of Emission Ratios Journal of Geophysical Research: Atmospheres

We analyze an expanded data set of oxygenated volatile organic compounds (OVOCs) in air measured by several instruments at a surface site in Pasadena near Los Angeles during the National Oceanic and Atmospheric Administration California Nexus study in 2010. The contributions of emissions, chemical formation, and removal are quantified for each OVOC using CO as a tracer of emissions and the OH exposure of the sampled air masses calculated from hydrocarbon ratios. The method for separating emissions from chemical formation is evaluated using output for Pasadena from the Weather Research and Forecasting-Chemistry model. The model is analyzed by the same method as the measurement data, and the emission ratios versus CO calculated from the model output agree for ketones with the inventory used in the model but overestimate aldehydes by similar to 70%. In contrast with the measurements, nighttime formation of OVOCs is significant in the model and is attributed to overestimated precursor emissions and overestimated rate coefficients for the reactions of the precursors with ozone and NO3. Most measured aldehydes correlated strongly with CO at night, suggesting a contribution from motor vehicle emissions. However, the emission ratios of most aldehydes versus CO are higher than those reported in motor vehicle emissions and the aldehyde sources remain unclear. Formation of several OVOCs is investigated in terms of the removal of specific precursors. Direct emissions of alcohols and aldehydes contribute significantly to OH reactivity throughout the day, and these emissions should be accurately represented in models describing ozone formation. Plain Language Summary We report new measurements of volatile organic compounds (VOCs) in ambient air in the Los Angeles basin. Chemical reactions between VOCs and nitrogen oxides form ozone and fine particles, two important pollutants in Los Angeles smog. It is therefore important to understand VOC emission sources. In this work, we derive the composition of VOC emissions using ambient measurements at Pasadena in 2010. The study is complicated due to rapid chemical reactions that can form and remove VOCs in between the time of emission and measurement. After correcting for this chemistry, it is shown that emissions of many oxygen-containing VOCs are important for the formation of ozone.

本研究分析了2010年美国国家海洋和大气管理局（National Oceanic and Atmospheric Administration）加利福尼亚联网研究（California Nexus study）期间，洛杉矶附近帕萨迪纳某地面站点通过多台仪器测得的空气中含氧挥发性有机物（oxygenated volatile organic compounds, OVOCs）扩充数据集。本研究以一氧化碳（carbon monoxide, CO）作为排放示踪剂，并基于烃类比值计算采样气团的羟基（OH）暴露量，以此量化每种含氧挥发性有机物的排放、化学生成与清除贡献。

随后，利用天气研究与预报-化学（Weather Research and Forecasting-Chemistry, WRF-Chem）模型针对帕萨迪纳的输出结果，对区分排放与化学生成的分析方法进行评估。本研究采用与实测数据一致的方法对该模型结果开展分析，结果显示：模型输出得到的酮类排放比与一氧化碳的比值与模型所用排放清单相符，但醛类的该比值被高估约70%。

与实测结果不同，模型模拟得到的夜间含氧挥发性有机物生成量显著，该现象可归因于前体物排放被高估，以及前体物与臭氧、硝酸根（NO₃）反应的速率系数被高估。多数实测醛类在夜间与一氧化碳呈强相关关系，提示其存在机动车排放来源。然而多数醛类与一氧化碳的排放比高于机动车排放文献报道的数值，醛类的具体排放来源仍不明确。

本研究还针对特定前体物的清除过程，探究了数种含氧挥发性有机物的生成机制。醇类与醛类的直接排放在全天对羟基反应活性均有显著贡献，因此在描述臭氧生成的模型中，应准确表征这类排放的相关参数。

【通俗摘要】本研究报道了洛杉矶盆地环境空气中挥发性有机物（volatile organic compounds, VOCs）的新实测结果。挥发性有机物与氮氧化物（nitrogen oxides）发生化学反应会生成臭氧与细颗粒物（fine particles），二者均为洛杉矶烟雾中的核心污染物，因此明确挥发性有机物的排放来源具有重要意义。本研究基于2010年帕萨迪纳站点的环境实测数据，推导得到挥发性有机物排放的组成特征。由于在排放至被采样测量的时段内，挥发性有机物会发生快速化学反应并实现生成或清除，这给研究带来了显著复杂性。经过化学过程校正后，结果表明多数含氧挥发性有机物的排放对臭氧生成具有关键作用。