典型城市大气气固两相水溶性有机物来源与气固分配机制

Gas/particle partitioning of water-soluble organic compounds is an important formation pathway of secondary organic aerosols (SOA), but the related mechanisms remain elusive, resulting in a significant bias of modelling results compared to the field measurements. Currently, there is little data that characterizes the total gas to particle partitioning of ambient SOA in China. Air pollution from volatile organic compounds (VOCs) in China is very severe; concentrations of water-soluble organic compounds, as the oxidation products of VOCs, are very high in comparison with those in other developed countries. However, studies on water-soluble organic compounds in China have focused only on their aerosol fraction and simultaneous measurements of gas/particle phases of water-soluble organic compounds in China have not been reported. In the current proposal, high-time resolution (1-hr) of temporal and spatial distributions of gas and particle phases of water-soluble organic compounds will be characterized in Xi'an and Shanghai cities, China, through the measurement of water-soluble organic carbon (WSOC) by using an on-line gas/particle phases of WSOC analyzer. Gas/particle partitioning mechanism of WSOC will be discussed based on the high time resolution of measurements for aerosol liquid water content, aerosol acidity, EC, OC, gas- and solid-phase of dicarboxylic acids, keto-acids and dicarbonyls, and gaseous pollutants such as SO2, NOx, O3 and CO. Filter-based organic biomarkers will be determined with a hourly time resolution and used to make a source apportionment for the gas and solid phases of water-soluble organic compounds.

大气气态水溶性有机物在气溶胶液相上的气固分配是二次有机气溶胶 （S0A）形成的重要途径，但有关气固分配机制目前还不清楚，使得当前模式难以准确模拟SOA时空分布。我国城市大气挥发性有机物污染严重，作为其氧化产物的水溶性有机物浓度高。当前国内关于大气水溶性有机物工作集中在颗粒相，气固两相同步观测工作鲜有报道。针对上述现状，本项目以南北方典型城市西安和上海大气为对象，通过气固两相水溶性有机碳WSOC的高时间分 辨率（1小时）强化观测，并结合小时分辨率的气固相典型二次水溶性有机物、颗粒相示踪物以及气态污染物的物质组成与浓度分析，研究上述城市冬夏两季气固两相大气水溶性有机物时空分布特征，解析其来源，运用多元统计等手段研究不同大气环境下（清洁期与污染期）气象参数和颗粒相理化特征与气固分配常数之间的函数关系，阐明WSOC的气固分配机制和主控因子，为我国灰霾治理提供新思路，为SOA形成机制提供新科学认知。

水溶性有机物的气固分配是大气二次有机气溶胶形成的主要途径，但相关分配机制及其动力学特征目前还不很清楚。当前国内外关于大气水溶性有机物工作大多集中在颗粒相，而气固两相同步观测工作鲜有报道。针对上述现状, 本项目以西安和上海等典型南北方城市大气为对象，通过气固两相水溶性有机碳WSOC的高时间分辨率强化观测，并结合大流量滤膜离线分析以及气态污染物化学组成与浓度变化分析，研究我国南北方典型城市冬夏两季气固两相大气水溶性有机物季节变化特征，解析其来源，阐明WSOC的气固分配机制和主控因子。项目成功研发了一套可同步测量气固两相WSOC和小分子有机酸的浓度与组成以及颗粒相WSOC光学特性的高时间分辨率在线半连续分析系统。运用该系统并结合3-12小时滤膜采样和在线挥发性有机物VOC分析，开展了冬夏季大气中气固两相酸碱物种和WSOC季节变化、来源解析和气固分配机制研究。揭示了上海和西安大气中WSOC组成与浓度的季节变化特征；发现了控制WSOC气固分配的关键过程，在整体分析 (bulk analysis) 和分子组成分析两个维度上，阐明了影响WSOC亨利常数变化的主控因子和动力学机制。定量解析了颗粒相WSOC的来源，获得了上述站点大气气溶胶的酸度、含水量、离子强度等物理参数，阐明了我国典型城市大气中WSOC相较欧美等国气固分配系数高的原因。发现：低相对湿度状况下WSOC气固分配主要是凝结过程, 气态WSOC分配到颗粒态有机相中, 而在高相对湿度状况下, 气态WSOC的气固分配是液相反应过程，气态WSOC分配到气溶胶水相中, 随后发生酸碱中和以及美拉德反应等液相反应。这样两种气固转化机制中, 前者是一个可逆过程, 分配过程受温度控制, 与温度呈线性负相关；而后者则是不可逆过程, 分配过程受气溶胶液态水含量、酸碱度和氨气浓度三种因子控制, 与这三种因子之间呈线性正相关。该机制的发现为数值模式相关参数化方案的建立提供了新视角。