Type-based assessment of aerosol direct radiative effects: A proof-of-concept using GEOS-Chem and CATCH

The radiative perturbation of the Earth's energy balance caused by all aerosols, the direct radiative effect (DRE), and anthropogenic aerosols, the direct radiative forcing (DRF), remain major sources of uncertainty in climate projections. Here we propose a method for determining DRE and DRF that makes use of the High Spectral Resolution Lidar (HSRL)-retrieved aerosol loading and derived aerosol types (i.e. dust, marine, urban, smoke, etc.) in combination with aerosol-type specific optical properties. As the global spatiotemporal distributions of HSRL-derived aerosol types are not currently available, the methodology is tested here using a global 3-D model of atmospheric chemistry (GEOS-Chem) along with Creating Aerosols from CHemistry (CATCH) algorithm-generated aerosol types analogous to ones derived by HSRL. In this method, the Rapid Radiative Transfer Model for General Circulation Models (RRTMG) is used to perform radiative transfer calculations with the single scattering albedo (SSA) and asymmetry parameter of atmospheric particles assigned based on the aerosol type in each grid box. Average GEOS-Chem/CATCH-derived all-sky DRE and DRF across the North American domain are estimated to be −1.98 W/m 2 and − 0.77 W/m 2 , respectively between mid-January and early February 2013 and − 4.20 W/m 2 and − 1.41 W/m 2 respectively between mid-July and early August 2014. Sensitivity studies revealed that the scheme may produce up to about ±0.42 W/m 2 and ± 0.21 W/m 2 uncertainty in DRE and DRF, respectively, related to variability in aerosol type-specific optical properties. This study presents a new way of determining DRE and DRF estimates once global retrievals of aerosol intensive parameters by HSRL become available. • Aerosol type-specific optical properties were used for the characterization of the direct radiative effect (DRE) and the forcing (DRF). • Derived North American winter (summer) all-sky DRE and DRF across are estimated to be −4.20 (−1.98) W/m 2 and − 1.41 (−0.77) W/m 2 . • Using aerosol type-specific optical properties introduced ±0.42 W/m 2 and ± 0.21 W/m 2 uncertainty in DRE and DRF calculations.