西南喀斯特农业区大气降水化学及硫同位素组成特征

Aquatic Chemistry and Sulfur Isotope Composition of Precipitation in a Karstic Agricultural Area,Southwest China

西南酸雨区属于中国重要的酸雨区,为了理解其中喀斯特农业区酸雨成因及其变化趋势,本研究于2016年5月至2017年9月在中国科学院普定喀斯特生态系统观测研究站进行了大气降水采集(n=147),分析了其水化学成分和硫酸盐硫同位素组成(δ^34S-SO4^2-)。结果表明,研究区内710%的降水为非酸性雨水(pH>56),降水中SO4^2-和Ca^2+为主要离子,分别占总阴、阳离子浓度的781%和566%。与研究区2008年降水中硫酸盐浓度(加权平均浓度为1406μmol/L)相比,本次采样期间降水中SO4^2-浓度(加权平均浓度为989μmol/L)显著降低。在研究期间,降水δ^34S-SO4^2-为-65‰~212‰,加权平均值为13‰±62‰,与贵阳2008~2009年降水δ^34S-SO4^2-(平均值-28‰±14‰)相比有所升高,说明研究区降水硫酸盐中来自工业燃煤排放的贡献降低。研究区降水化学组分和δ^34S-SO4^2-均受到降雨量的影响,随着降水量减小,SO4^2-浓度升高,δ^34S-SO4^2-则下降,工业燃煤对雨水中硫酸盐的贡献也增加。西南喀斯特农业区大气硫酸盐沉降介于城市和森林之间,季节变化明显,受控于工业燃煤排放和生物成因硫释放。该研究表明优化产业结构,降低工业燃煤,可进一步优化大气环境。

The acid rain is common in southwest China.Samples of rainwater were collected in the Puding Karst Ecosystem Observation and Research Station,Chinese Academy of Sciences,during the period of from May 2016 to September 2017 and their chemistry and sulfur isotope compositions were measured in order to understanding the variation tendency and formation mechanism of acid rain in the karst agricultural area of southwest China.The results showed that 71.0%of precipitations in the study area were not acid rain(pH>5.6),SO4^2-and Ca^2+ were predominant ions in rainwater,accounting for 78.1% of total anion and 56.6%of total cation contents,respectively.The weighted mean sulfate concentration(98.9μmol/L)of rainwater samples was significantly lower compared with that(140.6μmol/L)in the year 2008.The sulfur isotopic compositions of sulfate in precipitations ranged from-6.5‰ to 21.2‰ and the weighted mean was 1.3‰±6.2‰ and larger than that in Guiyang during years 2008-2009(-2.8‰±1.4‰).It may indicate that the contribution of coal combustion to sulfate in rainwater has decreased.Sulfur isotopes and chemical compositions of rainwater varied with the amount precipitation,when the amount of precipitation decreased,the contribution of coal combustion to rainwater sulfate increased,the sulfate concentration increased and the δ^34 S-SO4^2-value increased.The amount of atmospheric sulfate deposition in the karstic agricultural area,which controlled by coal burning and biogenic sulfur emissions,changed seasonally and fell in the S deposition range of from cities and forests.This study showed that upgrading of industrial structure and reduction of industrial coal burning can improve the air quality in Southwest China.