Contrasting size-resolved hygroscopicity of fine particles derived by HTDMA and HR-ToF-AMS measurements between summer and winter in Beijing: the impacts of aerosol aging and local emissions

Abstract

Abstract. The effects of aerosols on visibility through scattering and absorption of light and on climate through altering cloud droplet concentration are closely associated with their hygroscopic properties. Here, based on field campaigns in winter and summer in Beijing, we compare the size-resolved hygroscopic parameter (κgf) of ambient fine particles derived by an HTDMA (hygroscopic tandem differential mobility analyzer) to that (denoted as κchem) calculated by an HR-ToF-AMS (high-resolution time-of-flight aerosol mass spectrometer) measurements using a simple rule with the hypothesis of uniform internal mixing of aerosol particles. We mainly focus on contrasting the disparity of κgf and κchem between summer and winter to reveal the impact of atmospheric processes/emission sources on aerosol hygroscopicity and to evaluate the uncertainty in estimating particle hygroscopicity with the hypothesis. We show that, in summer, the κchem for 110, 150, and 200 nm particles was on average ∼10 %–12 % lower than κgf, with the greatest difference between the values observed around noontime when aerosols experience rapid photochemical aging. In winter, no apparent disparity between κchem and κgf is observed for those >100 nm particles around noontime, but the κchem is much higher than κgf in the late afternoon when ambient aerosols are greatly influenced by local traffic and cooking sources. By comparing with the observation from the other two sites (Xingtai, Hebei and Xinzhou, Shanxi) of north China, we verify that atmospheric photochemical aging of aerosols enhances their hygroscopicity and leads to 10 %–20 % underestimation in κchem if using the uniform internal mixing assumption. The effect is found more significant for these >100 nm particles observed in remote or clean regions. The lower κchem likely resulted from multiple impacts of inappropriate application of the density and hygroscopic parameter of organic aerosols in the calculation, as well as influences from chemical interaction between organic and inorganic compounds on the overall hygroscopicity of mixed particles. We also find that local/regional primary emissions, which result in a large number of externally mixed BC (black carbon) and POA (primary organic aerosol) in urban Beijing during traffic rush hour time, cause a 20 %–40 % overestimation of the hygroscopic parameter. This is largely due to an inappropriate use of density of the BC particles that is closely associated with its morphology or the degree of its aging. The results show that the calculation can be improved by applying an effective density of fresh BC (0.25–0.45 g cm−3) in the mixing rule assumption. Our study suggests that it is critical to measure the effective density and morphology of ambient BC, in particular in those regions with influences of rapid secondary conversion/aging processes and local sources, so as to accurately parameterize the effect of BC aging on particle hygroscopicity.

Tropospheric aerosol hygroscopicity in China

“…Wang et al, 2018a), Qingdao (K. , Shanghai (Leng et al, 2013(Leng et al, , 2014, Guangzhou (Duan et al, 2017(Duan et al, , 2018, and Mt. Huang (Fang et al, 2016), as well as over marginal seas in China (Zhu et al, 2019;Gao et al, 2020) and the northwestern Pacific (J. Zhu et al, 2019).…”

“…All the aerosol hygroscopic growth measurements (Y. Wang et al, 2017bFan et al, 2020;Jin et al, 2020) were conducted at 90 % RH at ground level.…”

“…Overall, organic materials were estimated to contribute to 30 ± 22 % of ALWC (Jin et al, 2020), highlighting the importance of organics to aerosol hygroscopicity in urban Beijing. Fan et al (2020) further conducted aerosol hygroscopic growth measurements at the IAP site in May-June 2017, and bimodal hygroscopicity distributions were also observed for 40-200 nm aerosols. The summertime measurement in 2017 was compared with the wintertime measurement at the same site in 2016 (Y. , and the size dependence of aerosol hygroscopicity was found to differ for the two seasons (Fan et al, 2020).…”

“…Fan et al (2020) further conducted aerosol hygroscopic growth measurements at the IAP site in May-June 2017, and bimodal hygroscopicity distributions were also observed for 40-200 nm aerosols. The summertime measurement in 2017 was compared with the wintertime measurement at the same site in 2016 (Y. , and the size dependence of aerosol hygroscopicity was found to differ for the two seasons (Fan et al, 2020). The average κ values increased from 0.158 at 40 nm to 0.271 at 110 nm in winter, and a further increase in particle size (to 200 nm) led to slight decrease in κ because mass fractions of secondary inorganic species decreased slightly from 61.8 % at 110 nm to 59.3 % at 200 nm (Fan et al, 2020).…”

“…The summertime measurement in 2017 was compared with the wintertime measurement at the same site in 2016 (Y. , and the size dependence of aerosol hygroscopicity was found to differ for the two seasons (Fan et al, 2020). The average κ values increased from 0.158 at 40 nm to 0.271 at 110 nm in winter, and a further increase in particle size (to 200 nm) led to slight decrease in κ because mass fractions of secondary inorganic species decreased slightly from 61.8 % at 110 nm to 59.3 % at 200 nm (Fan et al, 2020). For comparison, the average κ values increased with particle size in summer, from 0.211 at 40 nm to 0.267 at 200 nm, as mass fractions of secondary inorganic species increased from 56.7 % at 80 nm to 63.0 % at 200 nm (Y.…”