霾天气下城市气溶胶吸湿性的观测

Urban Aerosol Hygroscopicity During Haze Weather

气溶胶吸湿性对大气能见度有重要影响,是形成霾污染的主要因素之一.利用南京信息工程大学观测获得的2014年4月17日至5月21日气溶胶吸湿增长因子(GF)、OC/EC以及水溶性无机离子资料,对南京霾日气溶胶吸湿性及主要化学组分进行分析.结果表明,随着粒径的增加,平均吸湿增长因子(GF_(mean))数值变化较小,吸湿性标准差(σ)逐渐增大,化学组分的外混合程度逐渐增强;白天气溶胶粒子的吸湿性强于夜间,但是外混合程度弱于夜晚;非霾日气溶胶吸湿性强,外混合程度高,霾日正好与之相反,且随着霾等级的增加,吸湿性和外混合程度都进一步减小;非霾日气溶胶主要水溶性无机离子为NH_4^+、NO_3^-和SO_4^(2-),而霾日OC/EC的含量占比较高;含量相对丰富的OC/EC是造成低湿背景霾天气下小尺度气溶胶吸湿性变弱的主要原因,霾日所处环境相对湿度的高低也是影响气溶胶吸湿能力的重要因素;整个观测期间,南京市气溶胶化学组分中(NH_4)_2SO_4和OC等不可溶物质含量最多,NH_4NO_3含量次之;利用化学组分计算得到的平均吸湿性参数κ_(chem)和利用H-TDMA仪器实际观测计算得出的κ_(mean)存在较好的一致性,两者的相关性在霾天气下进一步增强,因此可以利用气溶胶主要化学组分来预报气溶胶吸湿性.

The hygroscopicity of aerosols has an important influence on atmospheric visibility and is one of the main causes of haze pollution. Based on observations of the aerosol hygroscopic growth factor(GF),water soluble inorganic ions,and organic carbon/elemental carbon(OC/EC) data during haze weather from April 17 to May 21,in 2014,the hygroscopic properties of aerosols and corresponding effects on haze in Nanjing were analyzed. The results showed that the distribution of GF was bimodal and varied from 1. 12 to 1. 64. With the increase of particle size,the average hygroscopic growth factor(GFmean) changed less and the standard deviation of wettability(σ) increased gradually;meanwhile,the degree of external mixing of chemical components increased gradually. The hygroscopicity of aerosol particles in the day was better than that at night,but the mixing degree was weaker than that at night;in non-haze weather,the hygroscopicity of aerosol particles was stronger and the degree of external mixing was higher,while the hygroscopicity and mixing degree of haze particles showed opposite trends. With the increase of haze levels,the hygroscopicity of aerosol particles grew weaker and the degree of external mixing decreased further. Relative humidity can have a significant impact on the chemical components of aerosols and their hygroscopic capacity. Under a low humidity background,the main chemical components of aerosols included NH4^+,NO3^-,SO4^2-,OC,and EC,and the content of OC/EC in aerosols during haze days was more abundant;in haze weather with low relative humidity,abundant organic matter was the main reason for the decrease of the moisture absorption capacity of small-scale aerosols. The level of relative humidity in the haze weather was also an important factor affecting the hygroscopic capacity of aerosols. The contents of(NH4)2SO4,OC,and insoluble substances in aerosols were the highest,followed by NH4NO3.The contents of these chemical components showed obvious diurnal variation characteristics,which resulted in significant diurnal variation of the hygroscopicity of the aerosols.κchemcalculated by the chemical composition and κmeanacquired by observations using HTDMA showed good consistency,and the correlation coefficient was 0. 890 3. In haze weather,the correlation between them was further enhanced. Therefore,the major chemical components of aerosols could be used to predict the hygroscopic properties of aerosols.