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. The effects of aerosols on visibility through scattering and absorption oflight and on climate through altering cloud droplet concentration areclosely associated with their hygroscopic properties. Here, based on fieldcampaigns in winter and summer in Beijing, we compare the size-resolvedhygroscopic 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) measurementsusing a simple rule with the hypothesis of uniform internal mixing ofaerosol particles. We mainly focus on contrasting the disparity of  κgf  and  κchem  between summer and winter to reveal the impactof atmospheric processes/emission sources on aerosol hygroscopicity and toevaluate the uncertainty in estimating particle hygroscopicity with thehypothesis. 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 observedaround noontime when aerosols experience rapid photochemical aging. Inwinter, no apparent disparity between  κchem  and  κgf  is observed for those  u003e100  nm particles aroundnoontime, but the  κchem  is much higher than  κgf in the late afternoon when ambient aerosols are greatly influenced by localtraffic and cooking sources. By comparing with the observation from the other two sites (Xingtai, Hebei and Xinzhou, Shanxi) of north China, we verifythat atmospheric photochemical aging of aerosols enhances theirhygroscopicity and leads to 10 %–20 % underestimation in  κchem  if using the uniform internal mixing assumption. The effect isfound more significant for these  u003e100  nm particles observed inremote or clean regions. The lower  κchem  likely resultedfrom multiple impacts of inappropriate application of the density andhygroscopic parameter of organic aerosols in the calculation, as well asinfluences from chemical interaction between organic and inorganic compoundson the overall hygroscopicity of mixed particles. We also find thatlocal/regional primary emissions, which result in a large number ofexternally mixed BC (black carbon) and POA (primary organic aerosol) in urban Beijingduring traffic rush hour time, cause a 20 %–40 % overestimation of thehygroscopic parameter. This is largely due to an inappropriate use ofdensity of the BC particles that is closely associated with its morphologyor the degree of its aging. The results show that the calculation can beimproved 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 secondaryconversion/aging processes and local sources, so as to accuratelyparameterize the effect of BC aging on particle hygroscopicity.

atmospheric chemistry and physics

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