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
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 
Published: January 2020

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.