大气中有机酸是挥发性有机物的重要氧化产物，并在有机气溶胶二次生成、新粒子生成和降雨与云雾的酸性扮演重要的角色。然而，大气中有机酸的来源和二次生成过程尚不清楚，不同大气环境下模型模拟的有机酸浓度均远远低于实测浓度，暗示大气化学机理对一些关键化学过程的描述存在“缺失”。申请人也发现现有大气化学机理仅能解释美国城市地区甲酸测量浓度的50%。本项目针对这一问题，计划在我国典型污染地区开展10余种有机酸类物种气态和颗粒态在线测量，分析各有机酸的浓度变化规律，基于非线性模型解析有机酸的来源，研究气粒分配和颗粒相反应对有机酸生成的贡献，建立基于观测的模型研究不同反应过程对有机酸生成的贡献，并评估现有大气化学机理对不同有机酸的模拟能力和弥合观测与模拟差异的可能途径。通过本项目的研究，将为识别大气关键化学过程的“缺失”提供重要线索，帮助理解我国大气复合污染的形成机制，从而为空气质量持续改善提供关键科学信息。

Organic acids in the atmosphere are important oxidation products of volatile organic compounds (VOCs), and they play important roles in secondary organic aerosol formation, new particle formation, and acidity of precipitation, cloud and fog. However, the sources and secondary formation of organic acids in the atmosphere remain unclear, with modelled concentrations significantly lower than measured concentrations. This implies that some central aspect of the atmospheric VOCs oxidation chain is not currently understood. The applicant also found current chemical mechanism can only explain less than 50% formic acid concentrations in an urban environment in U.S. This project will plan to conduct online measurements of more than ten organic acids in both gas phase and aerosol in typical polluted environments of China. The variability of concentrations of different organic acids will be analyzed. We will apportion the sources of organic acids using a non-linear regression model. The contributions of gas-particle partitioning and particle-phase reactions to formation of various organic acids will be quantified. We will use an observation-based model to quantify the contributions of various gas-phase reactions to secondary formation of organic acids. Using results from the model, the ability in modelling different organic acids will be systematically evaluated, and the possible routes to close the gaps between measurements and modelling will be explored. The results from this proposed project would provide important clue on the missing chemical reactions of organic compounds in the atmosphere, which can help to understand the formation mechanism of air pollution complex in China and afford key scientific information on continuously improving air quality in China.