Enhancement of the hygroscopicity parameter kappa of rural aerosols in northern Taiwan by anthropogenic emissions

Abstract

The hygroscopicity of aerosols in a rural site of northern Taiwan, HuaLin, for August 2011 was derived to investigate the impact of anthropogenic emission on the physico-chemical property of aerosols using the measured particle number size distribution, condensation nuclei (CN) concentration and the number of particles activated at supersaturation (SS) of 0.089–0.91%. Based on the concentration of anthropogenic volatile organic compounds (VOCs) and the back trajectory information, this campaign was divided into two periods: with and without significant anthropogenic contributions as 8/12–8/19/2011(Period_1) and 8/19–8/22/2011(Period_2), respectively. The refractive indices without absorption calculated using the measured scattering coefficients and number size distribution of aerosol particles were 1.43 ± 0.03 for 8/13–8/17 and 1.49 ± 0.03 for 8/18–8/21, having a consistent trend with back trajectory and the selected VOC concentrations. The new particles formation happened more significant for Period_1 and likely to be associated with the higher concentration of anthropogenic emission such as SO2 or VOCs which went through the oxidation or photo-oxidation to form more condensable species for nucleation. The derived single hygroscopicity parameter increases with particle size and was higher for Period_1 (0.1 at 50 nm– 0.35 at 165 nm) than Period_2 (0.04 at 70 nm– 0.28 at 175 nm) possibly due to the addition of aged composition as the air parcel passed through cities and went through photo-oxidation processes before arriving HuaLin. However, the hygroscopicity of rural aerosols tends to be higher than the urban aerosol in this study possibly due to the aging level. This study suggests the important role of anthropogenic emission on the secondary aerosol formation and also the physical properties of aerosols over rural area.