How important is organic aerosol hygroscopicity to aerosol indirect forcing?

Abstract

Organics are among the most abundant aerosol components in the atmosphere. However,there are still large uncertainties with emissions of primary organic aerosol (POA) andvolatile organic compounds (VOCs) (precursor gases of secondary organic aerosol, SOA),formation of SOA, and chemical and physical properties (e.g., hygroscopicity) of POA andSOA. All these may have significant impacts on aerosol direct and indirect forcingestimated from global models. In this study a modal aerosol module (MAM) in theNCAR community atmospheric model (CAM) is used to examine sensitivities ofaerosol indirect forcing to hygroscopicity (represented by a single parameter ‘κ’ ) ofPOA and SOA. Our model simulation indicates that in the present-day (PD) condition changing the ‘κ’ value ofPOA from 0 to 0.1 increases the number concentration of cloud condensational nuclei (CCN) at supersaturationS = 0.1% by 40–80% over the POA source regions, while changing the ‘κ’ value ofSOA by ± 50% (from 0.14 to 0.07 and 0.21) changes the CCN concentration within 40%. There aredisproportionally larger changes in CCN concentration in the pre-industrial (PI)condition. Due to the stronger impact of organics hygroscopicity on CCN and clouddroplet number concentration at PI condition, global annual mean anthropogenicaerosol indirect forcing (AIF) between PD and PI conditions reduces with theincrease of the hygroscopicity of organics. Global annual mean AIF varies by0.4 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 itsrepresentation in global models.