Influence of aerosol hygroscopicity and mixing state on aerosol optical properties in the Pearl River Delta region, China

• Published in: The Science of the total environment 2018-06, Vol.627, p.1560-1571
• Main Authors: Liu, Li, Tan, Haobo, Fan, Shaojia, Cai, Mingfu, Xu, Hanbing, Li, Fei, Chan, Pakwai
• Format: Article
• Language: English
• Subjects: Aerosol optical properties; Enhancement factor; Hygroscopicity; Mixing state
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2018.01.199

•The measured light-scattering enhancement factors (f(RH)sp) agreed well with those simulated by Mie model.•The f(RH) of extinction, scattering and backscattering are more sensitive to Gf than PNSD and BC volume fraction.•For core-shell mixture, the f(RH)absp increases with Gf and reaches the highest value when Gf is around 1.3•Aerosol hygroscopicity could result in stronger “Towmey effect” with elevated RH. The hygroscopic enhancement factor of extinction (a), scattering (b), backscattering (c) absorption (d), HBF (e) and SSA (f) calculated from the Mie model for external (black dotes), homogeneously internal (green dotes), and core-shell (red dotes) mixing states. [Display omitted] Both the effects of aerosol hygroscopicity and mixing state on aerosol optical properties were analyzed using ground-based measurements and a Mie model in this study. The sized-resolved particle hygroscopic growth factor at RH = 90% (Gf(90%)) and the enhancement factor for the scattering coefficients (f(RH)sp) were measured by a self-constructed Hygroscopic Tandem Differential Mobility Analyzer (H-TDMA) and two nephelometers in parallel (PNEPs) respectively from 22nd February to 18th March 2014 in the Pearl River Delta, China. In addition, the particle number size distribution (PNSD) and BC mass concentration (MBC) were measured simultaneously. During the observation period, the f(RH)sp increased sharply along with increasing RH (40%–85%) and the value of f(80%)sp was 1.77 ± 0.18. The mean Gf(90%) for all particles are 1.44 (80 nm), 1.48 (110 nm), 1.52 (150 nm) and 1.55 (200 nm), and the mean Gf(90%) for more-hygroscopic particles are 1.58 (80 nm), 1.63 (110 nm), 1.66 (150 nm) and 1.67 (200 nm) respectively. Based on Gf, PNSD and MBC, the enhancement factor of the aerosol optical properties (extinction (f(RH)ep), scattering (f(RH)sp), backscattering (f(RH)hbsp), absorption (f(RH)absp), and hemispheric backscatter fraction (f(RH)hbsp)) were calculated under three aerosol mixing state assumptions. The results show that the calculated f(80%)sp values agreed well with the ones measured by PNEPs, illustrating that the Gf size distribution fittings are reasonable. The f(RH)ep, f(RH)sp and f(RH)hbsp increased along with increasing RH for three mixtures, while f(RH)HBF decreased. The f(RH)absp increased for the homogenously internal mixture, but remained stable for the external mixture. For the core-shell mixture, the f(RH)absp increased from RH = 0 to 75% and then decreased, due to a