庐山2016年冬季三级分档雾水化学特征

Chemical characteristics of the three-stage fog water in the winter of 2016 in Lushan

探究不同尺度雾滴化学特征是深化雾微物理化学研究的重要内容。2016年12月-2017年1月在庐山开展雾综合观测实验,利用主动式三级分档雾水采集器(CASCC 3stage)收集到3次雾过程73个分档雾水样本,雾滴分档粒径:4-16μm (3级),16-22μm(2级)和≥22μm(1级)。定量得到了分档雾水的pH、电导率(EC)及9种水溶性无机离子(Na^+、NH4^+、K^+、Mg^2+、Ca^2+、Cl^-、NO2^-、NO3^-、SO4^2-)浓度(μeq/L)。结果表明,庐山冬季雾水酸化严重,pH为3.96-5.82,pH<5.6的样品占98.6%且直径4-16μm小雾滴的强酸性(pH<4.5)样品最多,占比达54.2%,小雾滴酸性和电导率更强;NH4^+、Ca^2+、NO3^-、SO4^2-是雾水的主要离子组分,[NH4^++Ca^2++NO3^-+SO4^2-]分别在三级分档雾水中占总离子浓度(TIC)的83.8%、88.0%和88.7%;综合3次雾过程,总离子浓度、NH4^+、K^+、NO3^-、SO4^2-在4-16μm小雾滴存在富集,表现出尺度依赖特征;86%雾水样品[SO4^2-]/[NO3^-]介于0.5-3.0,属于硫酸和硝酸混合型酸化,雾水酸化主要原因是碱性缓冲物质与酸性组分不平衡以及可能存在有机酸贡献;同一观测点不同雾过程化学特征存在个例差异,第二次雾过程(12月25日14时-26日21时)(北京时),总离子浓度、NH4^+、Ca^2+、NO3^-、SO4^2-在16-22μm尺度雾滴存在富集,这可能是雾区气溶胶浓度较低、降温、采样间隔和污染气体及气溶胶输送共同作用的结果。后向轨迹聚类和潜在源(PSCF)分析表明,观测期间影响庐山的气团均来自西部,来自湖南北部的局地气团占总轨迹数的68.99%,最为重要;PM2.5、SO2、NO2具有相似潜在源区空间分布,主要位于湖北、湖南、安徽西南部和江西北部等邻近省份地区,以近距离输送为主。

Exploring the chemical characteristics of different scales of droplets is an important issue in further in-depth research of fog microphysics and chemistry. From December 2016 to January 2017, a comprehensive fog observation experiment was carried out in Lushan. The three-stage Caltech Active Strand Cloud Collector(CASCC 3stage) was used for size resolved fog sampling. 50% droplet size cuts at 22, 16 and 4 μm diameter are featured in stages one, two and three, respectively. A total of 73 samples were collected during three fog events. The pH, conductivity(EC) and concentration of nine water-soluble inorganicions(Na+, NH4+, K^+, Mg^2+, Ca^2+, Cl^-, NO2^-, NO3^-, SO4^2-) were quantitatively obtained from the three-stage fog water. The results show that the fog water in Lushan was seriously acidified in the winter, with a pH of 3.96-5.82 and the samples with pH<5.6 account for 98.6% of the total. Moreover, heavy acid samples(pH<4.5) of 4-16 μm small droplets account for up to 54.2% of the total, and the small droplets are more acidic and conductive. NH4^+, Ca^2+, NO3^-, SO4^2- are the main ionic components of the fog water,[NH4++Ca2++NO3-+SO42-] account for 83.8%, 88.0% and 88.7% of the total ion concentration(TIC) in the three-stage fog water, respectively. Combined with three fog processes, TIC, NH4^+, K^+, NO3^- and SO4^2- are enriched in small droplets of 4-16 μm, showing size-dependent characteristics. 86% of the fog water samples [SO42-]/[NO3-] is between 0.5-3.0, which is a mixed acidification of sulfuric acid and nitric acid. The main reasons for the acidification of the fog water are the imbalance between alkaline buffer and acid component and possible contribution of organic acid. There are some differences in the chemical characteristics between different fog processes at the same observation point. In the second event(14:00 BT 25 Dec-21:00 BT 26 Dec), TIC, NH4^+, Ca^2+, NO3^- and SO4^2- are enriched in droplets of 16-22μm.This phenomenon may be the result of a combination of lower aerosol concentration,temperature drop,sampling interval,pollutant gas and aerosol transport in the fog region.Backward trajectory clustering and potential source contribution function(PSCF)show that the air masses affecting Lushan during the observation period all came from the west,and local air mass from northern part of Hunan accounted for 68.99%of the total trajectory.PM2.5,SO2,NO2 have similar spatial distribution of potential source areas,which are mainly located in neighboring provinces such as Hubei,Hunan,southwestern Anhui and northern Jiangxi.Short-distance transport dominated pollutants transport.