摘要：Organics are among the most abundant aerosol components in the atmosphere. However,
there are still large uncertainties with emissions of primary organic aerosol (POA) and
volatile organic compounds (VOCs) (precursor gases of secondary organic aerosol, SOA),
formation of SOA, and chemical and physical properties (e.g., hygroscopicity) of POA and
SOA. All these may have significant impacts on aerosol direct and indirect forcing
estimated from global models. In this study a modal aerosol module (MAM) in the
NCAR community atmospheric model (CAM) is used to examine sensitivities of
aerosol indirect forcing to hygroscopicity (represented by a single parameter 'κ' ) of
POA and SOA. Our model simulation indicates that in the present-day (PD) condition changing the 'κ' value of
POA from 0 to 0.1 increases the number concentration of cloud condensational nuclei (CCN) at supersaturation
S = 0.1% by 40–80% over the POA source regions, while changing the 'κ' value of
SOA by ± 50% (from 0.14 to 0.07 and 0.21) changes the CCN concentration within 40%. There are
disproportionally larger changes in CCN concentration in the pre-industrial (PI)
condition. Due to the stronger impact of organics hygroscopicity on CCN and cloud
droplet number concentration at PI condition, global annual mean anthropogenic
aerosol indirect forcing (AIF) between PD and PI conditions reduces with the
increase of the hygroscopicity of organics. Global annual mean AIF varies by
0.4 W m − 2 in the sensitivity runs with the control run of − 1.3 W m − 2, highlighting the need for improved understanding of organics hygroscopicity and its
representation in global models.
