珠三角典型城市大气环境水溶性有机物吸湿增长特性研究

Water-soluble organic matters (WSOM) are major chemical components of PM2.5 in most urban areas in China. Knowledge on the hygroscopicity of WSOM is of great importance to accurately assess their contributions to haze weather. However, the hygroscopicity of WSOM in urban environment remains lacking until now. To obtain the hygroscopicity of WSOM, the hygroscopic growth curve of PM2.5 (i.e. f(RH)), major chemical components of PM2.5 (e.g. water-soluble inorganic ions (WSII), water-soluble organic carbon, mineral elements and black carbon), gas precursors, and meteorological parameters will be measured for one year period in unban Guangzhou, a typical city of Pearl River Delta Region which frequently experiences high temperature, humidity, and oxidation conditions. Firstly, liquid water contents of PM2.5, WSII and WSOM will be calculated using Mie theory and ISORROPIA Ⅱ model. The hygroscopic parameter (к value) of WSOM will then be calculated by using к-Köhler theory model. An estimated hygroscopic growth curve of PM2.5 will be further calculated by using Mie theory model with size- and chemically-resolved particulate matter as input. The closure between the measured and estimated f(RH) will be conducted to obtain the hygroscopic growth curve of WSOM and its uncertainty range. These results are expected to further improve the IMPROVE formula for applying in Chinese urban atmosphere.

水溶性有机物是我国城市PM2.5的主要化学成分，掌握其吸湿增长特性是准确评估其对霾天气贡献的重要前提。然而，对城市大气环境水溶性有机物吸湿增长特性的研究仍十分缺乏。为此，本研究选择在高温高湿高氧化性的珠三角典型城市-广州开展为期一年的外场观测实验，同步在线获取PM2.5吸湿增长曲线(f(RH))、PM2.5主要化学成分(水溶性无机离子、水溶性有机碳、地壳元素和黑碳)、气态前体物和气象因子等资料。在此基础上，首先利用Mie模型和ISORROPIA Ⅱ模型获取总含水量、无机盐含水量和水溶性有机物含水量，再利用к-Köhler模型获取水溶性有机物吸湿性参数к值，结合化学成分粒径分布资料，对f(RH)的估算值和实测值进行闭合研究，最后获取水溶性有机物吸湿增长曲线及其不确定性范围，以期为我国城市版IMPROVE经验公式的应用提供科学支撑。

随着大气氧化性的逐渐增强，颗粒物中水溶性有机物含量逐渐增加，其对颗粒物吸湿性影响不容忽视。为了掌握广州城区水溶性有机物吸湿增长曲线，于2019-2020年在广州城区开展了为期1年的观测研究。本研究在线测量了颗粒物吸湿增长曲线和PM2.5中主要化学成分包括水溶性离子、水溶性有机碳、总碳、黑碳和元素等，还采集了7个粒径段的样品（0.41-10.2μm）和PM2.5样品。此外，本研究还获取污染气体和主要气象因子等资料。在此基础上研究发现，低相对湿度条件下（<50%），水溶性有机物吸湿不明显，相对湿度较高时（>50%），水溶性有机物才开始逐渐吸湿增长。水溶性有机物含水量占颗粒物总含水量的30%。旱季和湿季水溶性有机物吸湿参数к值分别为0.28±0.21和0.47±0.35。水溶性有机物的к值主要与其生成机制有关，液相生成的水溶性有机物的к值大于光化学过程生成的к值。水溶性有机物吸湿增长曲线f(RH<50%)接近于1.00，无明显增长。当颗粒物潮解后f(RH>50%)才会出现增长，f(RH=70%)可以达到1.16-1.18。综合来讲，低相对湿度条件下，水溶性有机物可能抑制颗粒物的吸湿性，而高相对湿度条件下，水溶性有机物才会增强颗粒物的吸湿性。