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Aerodynamic size-resolved composition and cloud condensation nuclei properties of aerosols in a Beijing suburban region

Abstract. The size-resolved physiochemical properties of aerosols
determine their atmospheric lifetime, cloud interactions and the deposition
rate on the human respiratory system; however most atmospheric composition
studies tend to evaluate these properties in bulk. This study investigated
size-resolved constituents of aerosols on mass and number basis, and their
droplet activation properties, by coupling a suite of online measurements
with an aerosol aerodynamic classifier (AAC) based on aerodynamic diameter
(Da) in Pinggu, a suburb of Beijing. While organic matter accounted for
a large fraction of mass, a higher contribution of particulate nitrate at
larger sizes (Da>300 nm) was found under polluted cases. By
considering the mixing state of refractory-black-carbon-containing particles
(rBCc) and composition-dependent densities, aerosols including rBCc were
confirmed to be nearly spherical at Da>300 nm. Importantly, the
number fraction of rBCc was found to increase with Da at all pollution
levels. The number fraction of refractory black carbon (rBC) is found to increase from ∼3 % at ∼90 nm to ∼15 % at ∼1000 nm, and this increasing rBC number fraction may be caused by the
coagulation during atmospheric ageing. The droplet activation diameter at a
water supersaturation of 0.2 % was 112±6 and 193±41 nm
for all particles with Da smaller than 1 µm (PM1) and
rBCc respectively. As high as 52±6 % of rBCc and 50±4 %
of all PM1 particles in number could be activated under heavy pollution
due to enlarged particle size, which could be predicted by applying the
volume mixing of substance hygroscopicity within rBCc. As rBCc contribute
to the quantity of aerosols at larger particle size, these thickly coated
rBCc may contribute to the radiation absorption significantly or act as an
important source of cloud condensation nuclei (CCN). This size regime may
also exert important health effects due to their higher deposition rate.
