Observation Study Of Atmospheric Aerosol Scattering Characteristics As A Function Of Relative Humidity

Hygroscopic growth is one of the most important characteristics of aerosols. In the atmosphere, the degree of growth and its corresponding optical changes are mainly affected by aerosols'chemical composition, and are challenging the climate model in estimating of aerosol direct radiative forcing effect. In order to get better understand of the scattering coefficient changes caused by particle hygroscopic growth at different relative humidity, we designed a"aerosol hygroscopic property observing system", and made two field experimental observations in Beijing urban area(at top of the building of Meteorological Science and Technology, CAMS) and Tianjin rural area(Baodi District Xin'an Town, TJBD). In these two campaigns, we got the hygroscopic growth properties and the resultant changes in scattering (referred to as f(RH), measured using paired Nephelometer, one dry and the other fitted with a scanning humidifier), and its relationship with chemical composition. In addition, Correlation between meteorological condition, air mass flow and f(RH) was discussed by means of surface meteorological observation data and back trajectory model.Results show that there are two distinguishing eigenvalues in Beijing urban in Spring, high hygroscopic value is between 1.3 and 1.5, which is related with urban pollution aerosols, low value is about 1.2, which represent the hygroscopic capacity of aerosols in clean condition. In Tianjin rural area field campaign, aerosol particle hygroscopic distribution shows one peak mode, corresponding eignvalue is 1.5. In Beijing, the hygroscopic growth factor f(RH=80%) is 1.49±0.06 in haze weather condition, and f(RH=80%) decrease to 1.23±0.1 in clean period. In floating dust period, aerosols in the air show less-hygroscopic properties, the f(RH=80%) is only 1.1±0.04. In Tianjin, f(RH=80%) =1.6 2.2 in haze period of time. Back-trajectory analysis shows that aerosols'hygroscopic capacity from northern-west-direction air mass is lower than from southern-direction air mass in non-special weather phenomenon day, the hygroscopic growth factor f(RH=80%) are 1.30±0.08 and 1.48±0.04 respectively.The ratio of fine particle(Dp4^2-, NO₃^-, NH₄⁺ ion mass and OMC(Organic Carbon Mass) play very important role on aerosol's hygroscopicity. Especially, the higher ratios of sulphate and nitrate in the PM2.1, the more hygroscopic ability the aerosol particle have. OMC are less-hygroscopic or non-hygroscopic matter commonly, but it also can have moisture absorption characteristics in some circumstances, so their effects on the aerosols'hygroscopic capacity have some uncertainty at a certain extent.