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

• Published in: Atmospheric chemistry and physics 2020-01, Vol.20 (2), p.915-929
• Main Authors: Fan, Xinxin, Liu, Jieyao, Zhang, Fang, Chen, Lu, Collins, Don, Xu, Weiqi, Jin, Xiaoai, Ren, Jingye, Wang, Yuying, Wu, Hao, Li, Shangze, Sun, Yele, Li, Zhanqing
• Format: Article
• Language: English
• Subjects: Aerosol effects; Aerosol particles; Aerosols; Ageing; Aging; Atmospheric processes; Black carbon; Cloud droplet concentration; Clouds (Meteorology); Cooking; Density; Electromagnetic absorption; Emission analysis; Emissions; Hygroscopicity; Hypotheses; Inorganic compounds; Light absorption; Mass spectrometry; Mathematical analysis; Morphology; Organic chemistry; Parameters; Peak hour traffic; Photochemicals; Photochemistry; Regions; Summer; Visibility; Winter
• ISSN: 1680-7324; 1680-7316; 1680-7324
• DOI: 10.5194/acp-20-915-2020

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