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

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 (<span classCombining double low line"inline-formula">κgf</span>) of ambient fine particles derived by an HTDMA (hygroscopic tandem differential mobility analyzer) to that (denoted as <span classCombining double low line"inline-formula">κchem</span>) 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 <span classCombining double low line"inline-formula">κgf</span> and <span classCombining double low line"inline-formula">κchem</span> 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 <span classCombining double low line"inline-formula">κchem</span> for 110, 150, and 200&thinsp;nm particles was on average <span classCombining double low line"inline-formula">ĝ1/410</span>&thinsp;%-12&thinsp;% lower than <span classCombining double low line"inline-formula">κgf</span>, with the greatest difference between the values observed around noontime when aerosols experience rapid photochemical aging. In winter, no apparent disparity between <span classCombining double low line"inline-formula">κchem</span> and <span classCombining double low line"inline-formula">κgf</span> is observed for those <span classCombining double low line"inline-formula">&gt;100</span>&thinsp;nm particles around noontime, but the <span classCombining double low line"inline-formula">κchem</span> is much higher than <span classCombining double low line"inline-formula">κgf</span> 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&thinsp;%-20&thinsp;% underestimation in <span classCombining double low line"inline-formula">κchem</span> if using the uniform internal mixing assumption. The effect is found more significant for these <span classCombining double low line"inline-formula">&gt;100</span>&thinsp;nm particles observed in remote or clean regions. The lower <span classCombining double low line"inline-formula">κchem</span> 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&thinsp;%-40&thinsp;% 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&thinsp;g&thinsp;cm<span classCombining double low line"inline-formula">-3</span>) in the mixing<span idCombining double low line"page916"/> 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.

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