南京北郊大气气溶胶的吸湿性观测研究

大气气溶胶的吸湿性不仅影响气溶胶的光学特性进而影响大气能见度,并且影响气溶胶的直接和间接气候效应.本文利用加湿串联差分迁移分析仪H-TDMA,于2012年5~7月在南京地区,对40~200nm大气气溶胶粒子在不同相对湿度下的吸湿增长因子进行了观测研究.统计结果表明:在90%相对湿度下,颗粒物的吸湿增长因子GF多为双峰分布,分为GF1.15的强吸湿组分.弱吸湿组分的吸湿增长因子(GFLH)在1.10~1.14之间;对应的强吸湿组分增长因子(GFMH )范围在1.47~1.58之间变化.相同粒径下的离散程度(σ)强吸湿组大于弱吸湿组,说明强吸湿组的粒子化学成分更复杂,异质性更强.相对湿度的变化对粒子吸湿增长的影响与粒子大小及化学组分有关,爱根核模态和积聚模态粒子在相同的相对湿度下潮解点不同,硝酸铵和硫酸铵是颗粒物中的主要吸湿成分.对不同天气条件下的气溶胶吸湿性分析,发现污染期间的吸湿增长因子(GF)和强吸湿组的数目比例(NFMH)都高于清洁期间,这与当时的气象条件以及粒子的内外部混合状态相关.观测还发现气溶胶粒子的吸湿性有明显的日变化特征,白天光照所引发的光化学反应以及混合层演变而造成粒子的吸湿性较强.机动车尾气排放的黑碳等不吸湿或弱吸湿颗粒物也会因为影响颗粒物的化学成分并进而对气溶胶吸湿性产生影响.

An observational study of the hygroscopic properties of aerosols in north suburb of Nanjing

Aerosol hygroscopicity influences the aerosol optical properties as well as the atmospheric visibility, and hence affects the aerosol direct and indirect climatic effects. In this study, the hygroscopicity of submicron particles in urban Nanjing was studied from May to July 2012. A Humidity Tandem Differential Mobility Analyser (H-TDMA) instrument was applied to measure the hygroscopic growth factor (GF) at 90% relative humidity (RH) for particles with dry diameter between 40~200nm. The statistical results show that, the GFs of the aerosol show a distinct bimodal distribution, and classified into less-hydrophobic (LH) group (GF1.15). For less-hydrophobic group, the GFLH was 1.10~1.14, while the GFMH varied between 1.47~1.58for more-hygroscopic group. The spread of GF-PDF (σ) are higher in more-hygroscopic group than in less-hydrophobic group with the same size. It means that the compositions of MH group are more complex. The change of relative humidity has an effect on particle hygroscopic growth which is associated with particle size and chemical compositions. Aitken nuclei and condensation nuclei have different deliquescence point under the same relative humidity. The main moisture absorption components are ammonium nitrate and ammonium sulfate. The result of the measurement under different weather condition shows that GF and NFMH are bigger in polluted periods than in clean periods. Obvious diurnal variation characteristics were also observed. The hygroscopicity of particles become stronger mainly caused by photochemical reaction and the evolution of the atmospheric mixing layer. The black carbon from automobile exhausts effect the hygroscopicity of aerosol particles by changing the chemical components of aerosol particle.