华南背景山区云中草酸的形成机制及影响因素

草酸(C₂)是大气颗粒物中有机物的重要组成部分，现有研究推测草酸主要来源于云中液相反应，然而，关于其云中形成机制的研究较少. 本文系统分析对比了华南背景山区的云水和云间隙颗粒物中二羧酸类物质，包括直链饱和二羧酸(C₂~C₉)、支链饱和二羧酸(iC₄~iC₆)、不饱和二羧酸〔马来酸(M)、富马酸(F)、柠康酸(mM)〕以及多官能团羧酸〔苹果酸(hC₄)、丙酮酸(Pyr)和乙醛酸(ωC₂)〕的浓度分布. 利用随机森林和多元线性回归方法进一步定量评估了草酸前体物、温度及云水性质(云水中液态水含量、pH、化学组成)对云水中草酸形成的影响. 结果表明:①云水和云间隙颗粒物中草酸的平均浓度分别为431 μg/L和27.28 ng/m3，分别占直链饱和二羧酸浓度的78.9%和70.0%，占水溶性有机碳浓度的2.4%和1.1%. ②云水中C₂/总二羧酸类物质(浓度比)与二羧酸类物质浓度比〔如C₂/(C₃~C₉)、C₂/(iC₄~iC₆)、C₂/(hC₄+Pyr+ωC₂)和C₂/(M+F+mM)〕均呈显著正相关(R2为0.47~0.76, P均小于0.01)，表明C₃~C₉、iC₄~iC₆、hC₄+Pyr+ωC₂和M+F+mM可能是云水中草酸形成的重要前体物. ③前体物对云水中草酸浓度变化贡献最大，贡献率为79%，其中hC₄+Pyr+ωC₂是最重要的前体物；其次是云水性质，贡献率为20%；温度的贡献率为1%. 研究显示，云中过程是草酸形成的重要途径，其形成过程受前体物、云水性质和温度等因素的影响. 

Formation Mechanism of Oxalic Acid in Cloud Water and Influencing Factors in Background Mountainous Area of Southern China

As an important component of organic matter in the atmospheric particles, oxalic acid is considered to be formed from the in-cloud aqueous-phase reactions. However, there is no convincing evidence for the main factors affecting its formation. This paper systematically analyzed the dicarboxylic acids in cloud water and cloud interstitial particles in a mountainous area of southern China, including linear saturated dicarboxylic acids (C₂-C₉), branched dicarboxylic acids (iC₄-iC₆), unsaturated dicarboxylic acids (maleic acid (M), fumaric acid (F), citraconic acid (mM)), and multifunctional carboxylic acids (malic acid (hC₄), pyruvic acid (Pyr) and glyoxylic acid (ωC₂)). The results show that: (1) The concentrations of oxalic acid in cloud water and cloud interstitial particles are 431 μg/L and 27.28 ng/m3, respectively, accounting for 78.9% and 70.0% of the total linear saturated dicarboxylic acids, and 2.4% and 1.1% of WSOC (water soluble organic carbon), respectively. (2) The percentage of oxalic acid in the total dicarboxylic substances in cloud water is significantly positively correlated with C₂/(C₃-C₉), C₂/(iC₄-iC₆), C₂/(hC₄+Pyr+ωC₂) and C₂/(M+F+mM) (R2 = 0.47-0.76, P <0.01), indicating that C₃-C₉, iC₄-iC₆, hC₄+Pyr+ωC₂ and M+F+mM are important precursors of oxalic acid in cloud water. (3) The influence of the precursors, temperature, and the properties of cloud water (liquid water content of cloud water, pH, chemical composition) on the variation of oxalic acid is assessed by random forest and multiple linear regression analysis. The results show that the precursors contribute the most (79%) to the variation of oxalic acid, among which hC₄+Pyr+ωC₂ are the major precursors. The second is the properties of cloud water, accounting for 20%, while the influence of temperature is 1%. The results show that the in-cloud process is an important pathway for the formation of oxalic acid, and its formation process is affected by factors such as precursors, cloud water properties and temperature.