Brown Carbon Fuel and Emission Source Attributions to Global Snow
Darkening Effect
Abstract
Snow and ice albedo reduction due to deposition of absorbing particles
(i.e., snow darkening effect (SDE)) warms the Earth system and is
largely attributed to black carbon (BC) and dust. Absorbing organic
aerosol (BrC) also contributes to SDE but has received less attention
due to uncertainty and challenges in model representation. This work
incorporates the SDE of absorbing organic aerosol (BrC) from biomass
burning and biofuel sources into the Snow Ice and Aerosol Radiative
(SNICAR) model within a variant of the Community Earth System Model
(CESM). Additionally, 12 different emission regions of BrC and BC from
biomass burning and biofuel sources are tagged to quantify the relative
contribution to global and regional SDE. BrC global SDE (0.021–0.056
Wm-2) is larger than other model estimates,
corresponding to 37%–98% of the SDE from BC. When compared to
observations, BrC simulations have a range in median bias
(-2.5%–+21%), with better agreement in the simulations that include
BrC photochemical bleaching. The largest relative contributions to
global BrC SDE are traced to Northern Asia (23%–31%), Southeast Asia
(16%–21%), and South Africa (13%–17%). Transport from Southeast
Asia contributes nearly half of the regional BrC SDE in Antarctica
(0.084–0.3 Wm-2), which is the largest regional input
to global BrC SDE. Lower latitude BrC SDE is correlated with snowmelt,
in-snow BrC concentrations, and snow cover fraction, while polar BrC SDE
is correlated with surface insolation and snowmelt. This indicates the
importance of in-snow processes and snow feedbacks on modeled BrC SDE.