基于稳定碳同位素的济南市二元羧酸类SOA的污染特征与形成机制

Pollution characteristics and formation mechanisms of dicarboxylic acids and related SOA in Jinan based on stable carbon isotope

为研究济南市冬季PM_(2.5)中二元羧酸类二次有机气溶胶(SOA)的来源、液相形成机制及影响因素,于2016年1—2月昼夜共采集46个PM_(2.5)样品,并对二元羧酸类SOA(包括二元羧酸、酮羧酸与α-二羰基化合物)与左旋葡聚糖的昼夜变化特征进行分析.研究结果表明,二元羧酸、α-二羰基化合物与左旋葡聚糖均呈昼低夜高的变化特征,而酮羧酸的昼夜变化特征与之相反.二元羧酸类SOA的分子组成特征以草酸(C_(2))的浓度最高,其次是丁二酸(C_(4))和丙二酸(C_(3)),与受生物质燃烧较显著的二元羧酸类SOA的分子组成是相同的.C_(2)/总二元羧酸的浓度(TDACs)、甲基乙二醛(mGly)/乙二醛(Gly)、C_(2)/SO^(2-)_(4)比值的昼夜变化特征与稳定碳同位素(δ^(13)C)的组成特征均表明济南市冬季夜晚气溶胶的氧化程度比白天深.C_(2)及其前体物(Gly、mGly)与SO^(2-)_(4)、相对湿度(RH)、液相水含量(LWC)与气溶胶的实际酸度(pH_(is))的相关性均较强,表明C_(2)及其前体物是在液相中经酸催化氧化反应产生的.夜晚二元羧酸类化合物的δ^(13)C值高于白天,且随着含碳量的升高二元羧酸的δ^(13)C值随之降低.C_(2)的δ^(13)C值随气溶胶的老化而偏正,这是由于同位素动力学效应(KIEs)导致的.

To investigate the sources, formation mechanisms and influencing factors of dicarboxylic acids and related secondary organic aerosol(SOA) in PM_(2.5) from Jinan City during winter, a total of 46 PM_(2.5) samples were collected from January to February of 2017 on a day/night basis, and the diurnal variations of dicarboxylic acids, ketocarboxylic acids, α-dicarbonyls and levoglucosan in PM_(2.5) were analyzed. The results showed that the concentrations of dicarboxylic acids, α-dicarbonyls and levoglucosan were higher in nighttime than those in daytime, but ketocarboxylic acids exhibited higher concentration during daytime than that during nighttime. Oxalic acid(C_(2)) is the most abundant species in the whole sampling period, followed by succinic acid(C_(4)) and malonic acid(C_(3)), which is similar to the molecular pattern observed biomass burning plumes. The diurnal variations of the mass ratios of C_(2)/TDCAs, mGly/Gly, C_(2)/SO^(2-)_(4) and stable carbon isotopic compositions(δ^(13)C) of major dicarboxylic acids and related SOA suggested that the nighttime aerosols were more aged than those in daytime. C_(2) and the precursors presented strong correlations with SO^(2-)_(4), relative humidity(RH), liquid water content(LWC) and particle in-situ pH(pH_(is)), indicating that these SOA were mainly originated from the aqueous-phase oxidation driven by acid-catalyzed oxidation. δ^(13)C of major dicarboxylic acids and related SOA were higher in nighttime than those in daytime, and the longer-chain dicarboxylic acids were more depleted in 13C than the shorter-chain dicarboxylic acids. An enrichment of δ^(13)C of C_(2) was found during the aging process of aerosol due to the kinetic isotope effects(KIEs).