Mixing state and particle hygroscopicity of organic-dominated aerosols over the Pearl River Delta region in China

Abstract

Abstract. Simultaneous measurements of aerosol hygroscopicity and particle-phase chemical composition were performed at a suburban site over the Pearl River Delta region in the late summer of 2016 using a self-assembled hygroscopic tandem differential mobility analyzer (HTDMA) and an Aerodyne quadruple aerosol chemical speciation monitor (ACSM), respectively. The hygroscopic growth factor (HGF) of the Aitken mode (30 nm, 60 nm) and accumulation mode (100 nm, 145 nm) particles were obtained under 90 % relative humidity (RH). An external mixture was observed for particles of every size during this study, with a dominant mode of more-hygroscopic (MH) particles, as aged aerosols dominated due to the anthropogenic influence. The HGF of less-hygroscopic (LH) mode particles increased, while their number fractions decreased during the daytime due to a reduced degree of external mixing that probably resulted from the condensation of gaseous species. These LH mode particles in the early morning or late afternoon could be possibly dominated by carbonaceous material emitted from local automobile exhaust during rush hours. During polluted days with air masses flowing mainly from the coastal areas, the chemical composition of aerosols had a clear diurnal variation and a strong correlation with the mean HGF. Closure analysis was carried out between the HTDMA-measured HGF and the ACSM-derived hygroscopicity using various approximations for the hygroscopic growth factor of organic compounds (HGForg). Considering the assumptions regarding the differences in the mass fraction of each component between PM1 and 145 nm particles, the hygroscopicity-composition closure was achieved using an HGForg of 1.26 for the organic material in the 145 nm particles and a simple linear relationship between the HGForg and the oxidation level inferred from the O:C ratio of the organic material was suggested. Compared with the results from other environments, HGForg obtained from our measurements appeared to be less sensitive to the variation of its oxidation level, which is, however, similar to the observations in the urban atmosphere of other megacities in China. This finding suggests that the anthropogenic precursors or the photooxidation mechanisms might differ significantly between the suburban and urban atmosphere in China and those in other background environments. This may lead to different characteristics of the oxidation products in secondary organic aerosols (SOA) and therefore to a different relationship between the HGForg and its O:C ratio.

Aerosol hygroscopicity and its link to chemical composition in coastal atmosphere of Mace Head: marine and continental air masses

“…The results of the comparison were suggesting that (1) the chemical composition which was used in deriving GF was bulk PM1 data which may differ significantly from Aitken mode particles, but approximating to accumulation mode particles thus affecting poorer comparison of Aitken mode particles and (2) the Kelvin effect, which would become significant at small sizes was neglected in calculation. Contrary to the study of Hong et al 2018, the correlations between GF_AMS and GF_HTDMA of even small sizes were correlating much better than no-correlation found by (Hong et al, 2018).…”

“…Freshly emitted hydrocarbon compounds have relatively low O:C ratio and lower hygroscopicity, therefore, a GForg = 1.1 is likely to cause overestimation. When the O:C increases to 0.6, the reported GForg ranges from 1.15 to 1.4 depending on the air mass (Hong et al 2018).…”

Distinct diurnal variation in organic aerosol hygroscopicity and its relationship with oxygenated organic aerosol

“…The mass concentration and chemical composition of NR-PM species were measured with the Aerodyne ToF-ACSM, which is equipped with a PM 2.5 aerodynamic lens (Williams et al, 2010) and a capture vaporizer (CV; Xu et al, 2017;Hu et al, 2017) to extend the measured particle size to 2.5 µm. Detailed instrument descriptions were given in Fröhlich et al 2013and Xu et al (2017).…”

“…Field measurements indicate that the CE of CV was fairly robust and was roughly equivalent to 1. Therefore, a CE of 1 was applied to all measured species in this study (Hu et al, 2017(Hu et al, , 2018b. Relative ionization efficiencies (RIEs) of 3.06 and 1.09 were used for ammonium and sulfate quantification, respectively, and the default values of 1.1 and 1.4 were used for nitrate and organic aerosol (OA), respectively.…”

A review of experimental techniques for aerosol hygroscopicity studies

“…To further understand hygroscopic properties of ambient aerosol particles, aerosol hygroscopicity closure studies have been widely carried out (Swietlicki et al, 1999;Dick et al, 2000;Gysel et al, 2007;Cerully et al, 2011;Wu et al, 2013;Wu et al, 2016;Schurman et al, 2017;Hong et al, 2018). In such studies, hygroscopic growth measurements using H-TDMA are concurrently performed with aerosol chemical composition measurements, and measured growth factors can then be compared to these calculated based measured chemical composition.…”