Aerosol Indirect Forcing in NCAR CAM5: Sensitivity to Organic Hygroscopicity
XIAOHONG LIU (1), Jian Wang (2)
(1) Pacific Northwest National Laboratory, Richland (2) Brookhaven National Laboratory, Upton
Abstract Number: 352
Preference: Platform Presentation
Last modified: May 10, 2010
Working Group: Aerosols, Clouds, and Climate
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 and yield of secondary organic aerosol, 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 presentation a modal aerosol module (MAM) in the NCAR Community Atmospheric Model (CAM) is used to examine sensitivities of aerosol indirect forcing to hygroscopicity (“B” value, Hudson and Da, J. Geophys. Res. 1996) of POA and SOA. MAM predicts both aerosol mass and number, and internal/external mixing between aerosol components, and includes treatment of emission and aging of POA into the accumulation mode, and the formation of SOA.
Our model simulation indicates that in the present-day condition changing “B” value of POA from 0 to 0.1 increases the number concentration of cloud condensational nuclei (CCN) at supersaturation S=0.1% by 40-60% over the POA source regions, while changing “B” value of SOA by ±50% (from 0.14 to 0.07 and 0.21) changes the CCN in less than 30%. Changes in the in-cloud droplet number concentrations (CDNC) are within 20% in most locations on the globe with the above changes in “B” value of POA and SOA. Anthropogenic aerosol indirect forcing (AIF) between present-day (PD) and pre-industrial (PI) conditions change by 0.37 W m$^(-2) from the control run of -1.26 W m$^(-2). AIF reduces with the increase hygroscopicity of organic aerosol, indicating the important role of natural organic aerosol in buffering the relative change of CDNC from PI to PD.