南京秋季大气PM2.5中类腐殖质的光学性质与来源分析

类腐殖质(humic-like substances, HULIS)是水溶性有机碳(WSOC)中具有吸光特性的重要组分,对空气质量、气候变化和人体健康均有重要影响.尽管目前对HULIS的研究很多,但不同方法分离机理不同,对于HULIS的分离与测定仍然缺乏统一的标准,针对HULIS分离方法的研究很少.固相萃取法(solid phase extraction, SPE)因其操作简单、分离效果较好而被广泛应用,但对于低浓度样品仍存在检出限较高、回收率较低的问题,且很少有人关注提纯过程中流程空白所包含的含碳组分及其吸光能力.本研究通过调整活化溶液(0.01 mol·L^-1 HCl溶液+甲醇+2%NH₃H₂O/MeOH)与洗脱溶液(2%NH₃H₂O/MeOH)用量的比例对提纯方法进行优化.结果表明,应用优化后的方法对流程空白进行测量时,检出限(MDL)降低到0.035 mg·L^-1以下,精密度RSD <5.41%(n=20),标准品回收率达到95%,在保证回收率的情况下减少了流程空白,提高了样品的精密度,使测定浓度较低的HULIS含量成为可能.为了探究生物质燃烧期间含碳组分的光学特性和来源特征,本研究对2017年10月6日至11月9日南京北郊秋季大气气溶胶样品进行采集.采样期间PM_2.5的浓度为(87.9±43.7)μg·m^-3,WSOC和类腐殖质碳(HULIS-C)的浓度分别为(4.2±2.3)μg·m^-3和(3.6±2.0)μg·m^-3,HULIS-C占WSOC的比例为47.3%,是WSOC中的重要组成部分.本研究还对HULIS在330—400 nm波段的吸光进行测定,使用Angstrom指数(absorption angstrom exponent,AAE)进行表征,得到采样期间AAE的值为2—7,说明HULIS污染主要来自二次转化.后向轨迹结果表明,重污染期间污染物来源为本地生物质燃烧和区域或者长距离气团的输送.

Optical properties and source apportionment of humic-like substances (HULIS) in Nanjing atmospheric PM2.5 in autumn

Humic-like substance(HULIS) is an important component of water-soluble organic carbon(WSOC) with light absorption characteristics, which has important effects on air quality, climate change and human health. Although large amounts of research on HULIS have been conducted, different isolation procedures may refer to a different part of organic fractions, there is still a lack of a standard method for the isolation and quantitation of HULIS, few studies focus on the chemical analysis method of HULIS. The solid phase extraction(SPE) method is most frequently used due to its simplicity of operation and selectivity. However, there are still some problems in the method such as high Method Detection Limit and low recovery, especially in low concentration samples and few types of research pay attention to the carbon components in the blank in the purification and their optical properties. In this study, the purification method was optimized by adjusting the proportion of dosage of activated solution(0.01 mol·L^-1 HCl solution + methanol + 2% NH₃H₂O/MeOH) and elution solution(2% NH₃H₂O/MeOH). The results showed that after the optimization method used to measure process blank, Method Detection Limit(MDL) decreased to 0.035 mg·L^-1. The Relative Standard Deviation(RSD) was less than 5.41%(n = 20) and the recovery of standard substance reached 95%, increasing the precision of the experiment, at the same time guarantee the recovery of HULIS and making the determination of low concentration HULIS becomes possible. To explore the optical properties and sources of carbon components during the biomass burning period, we collected atmospheric aerosol samples at the northern suburbs of Nanjing in autumn from October 6, 2017 to November 9, 2017. During the sampling period, PM_2.5 concentration was(87.9±43.7) μg·m^-3, the WSOC and Humic-like Carbon(HULIS-C) concentrations were(4.2±2.3) and(3.6±2.0) μg·m^-3, respectively. HULIS-C accounted for 47.3% of WSOC and contributed significantly to WSOC. In this study, the Absorption of HULIS at 330—400 nm was also measured, and the absorption was characterized by the Absorption Angstrom Exponent(AAE). The AAE value during the sampling period was 2—7, indicating that the pollution of HULIS mainly came from secondary transformation. The backward trajectory results indicated that the pollution sources during the heavily polluted period were local biomass burning and regional/long-term air masses transportation.