Abstract. Aerosol mixing state regulates the interactions between water
molecules and particles and thus controls aerosol activation and
hygroscopic growth, which thereby influences visibility degradation,
cloud formation, and its radiative forcing. There are, however, few current studies on the mixing
structure effects on aerosol hygroscopicity.
Here, we investigated the hygroscopicity of ammonium
sulfate / phthalic acid (AS / PA) aerosol particles with
different mass fractions of PA in different mixing states in terms of
initial particle generation. Firstly, the effect of PA coatings
on the hygroscopic behavior of the core-shell-generated mixtures of AS with PA was studied using a coating hygroscopicity tandem
differential mobility analyzer (coating HTDMA). The slow increase in the
hygroscopic growth factor of core-shell-generated particles is observed with
increasing thickness of the coating PA prior to the deliquescence
relative humidity (DRH) of AS. At relative humidity (RH) above 80 %,
a decrease in the hygroscopic growth factor of particles occurs as the thickness
of the PA shell increases, which indicates that the increase of PA mass
fractions leads to a reduction of the overall core-shell-generated particle
hygroscopicity. In addition, the use of the Zdanovskii–Stokes–Robinson (ZSR) relation leads to the
underestimation of the measured growth factors of core-shell-generated
particles without consideration of the morphological effect of
core-shell-generated particles, especially at higher RH. Secondly, in the
case of the AS / PA initially well-mixed particles, a shift of the DRH of AS (∼80 %, Tang and Munkelwitz, 1994) to lower RH is observed due to the presence of
PA in the initially well-mixed particles. The predicted hygroscopic growth
factor using the ZSR relation is consistent with the measured hygroscopic
growth factor of the initially well-mixed particles. Moreover, we compared
and discussed the influence of mixing states on the water uptake of AS / PA
aerosol particles. It is found that the hygroscopic growth factor of the
core-shell-generated particles is slightly higher than that of the initially
well-mixed particles with the same mass fractions of PA at RH above 80 %.
The observation of AS / PA particles may contribute to a growing field of
knowledge regarding the influence of coating properties and mixing structure
on water uptake.
Ambient observations of hygroscopic growth factor and f
(RH) below 1: Case studies from surface and airborne measurements
