The role of aerosol absorption in driving clear‐sky solar dimming over East Asia

Abstract

Surface-based observations indicate a significant decreasing trend in clear-sky downward surface solar radiation (SSR) over East Asia since the 1960s. This "dimming" is thought to be driven by the region's long-term increase in aerosol emissions, but little work has been done to quantify the underlying physical mechanisms or the contribution from aerosol absorption within the atmospheric column. Given the distinct climate impacts that absorption-driven dimming may produce, this constitutes an important, but thus far rather neglected, line of inquiry. We examine experiments conducted in the Geophysical Fluid Dynamics Laboratory's atmospheric general circulation models, AM2.1 and AM3, in order to analyze the model-simulated East Asian clear-sky SSR trends. We also use the models' stand-alone radiation module to examine the contribution from various aerosol characteristics in the two models (such as burden, mixing state, hygroscopicity, and seasonal distribution) to the trends. Both models produce trends in clear-sky SSR that are comparable to that observed but via disparate mechanisms. Despite their different aerosol characteristics, the models produce nearly identical increases in aerosol absorption since the 1960s, constituting as much as half of the modeled clear-sky dimming. This is due to a compensation between the differences in aerosol column burden and mixing state assumed in the two models, i.e., plausible clear-sky SSR simulations can be achieved via drastically different aerosol parameterizations. Our novel results indicate that trends in aerosol absorption drive a large portion of East Asian clear-sky solar dimming in the models presented here and for the time periods analyzed and that mechanistic analysis of the factors involved in aerosol absorption is an important diagnostic in evaluating modeled clear-sky solar dimming trends.

Comparing multiple model-derived aerosol optical properties to spatially collocated ground-based and satellite measurements

“…However, AERONET does show a peak in absorption AOD in March, with high year-to-year variation (large error bar) as well as a drop in both SSA and α. This is consistent with conclusions by Pietruczuk and Chaikovsky (2012) that dust is transported to Europe from the Saharan desert during spring. As shown before on a regional scale, CM3 improved the magnitudes considerably, but the seasonality deteriorated.…”

“…Analyses of back trajectories computed using the NOAA HYSPLIT (Hybrid Single Particle Lagrangian Integrated Trajectory Model) model and fire maps show that these peaks coincide with seasonal biomass burning in Eastern and Southern Europe (Jaroslawski and Pietruczuk, 2010). Another study uses lidar measurements and model results to suggest that transport of Saharan dust also influences springtime AOD in Belsk (Pietruczuk and Chaikovsky, 2012). This is consistent with CM2.1 (and not CM3), which shows a maximum absorption AOD, minimum SSA, and minimum Ångström exponent during March-April-May, as well as a peak in the dust AOD in May.…”

“…Belsk, Poland, is a village 28 miles south of Warsaw with a population of less than 10 000. It is influenced by biomass burning in Eastern and Southern Europe (Jaroslawski and Pietruczuk, 2010) and Saharan dust (Pietruczuk and Chaikovsky, 2012). Poland's shift to a market economy in 1990 and the economic crash of 2008 have reduced pollution over the years (The World Bank, 2011).…”

Disentangling fast and slow responses of the East Asian summer monsoon to reflecting and absorbing aerosol forcings

“…Several studies attempted to contrast the SO 4 and BC responses and indicated that scattering and absorbing aerosols would have markedly different effects on regional temperature, atmospheric circulation, and precipitation over East Asia (e.g., Guo et al, 2013;Jiang et al, 2013;Persad et al, 2014). However, these studies all only considered the fast adjustments of atmosphere and land surface to aerosol forcings, without considering the response of oceans.…”

Analysis of Aerosol Properties Coupled with Meteorological Parameters over East Asia

“…They affect the amount of short wave radiation that reaches the ground through scattering and absorption, and also modify cloud properties, often described as having direct and indirect effects on climate (Tanré et al 1984;Charlson et al 1992;Kaufman and Nakajima 1993;Huang et al 2006;Persad et al 2014). Understanding the aerosol feedback on climate is difficult since aerosol composition, shape and size vary greatly with space and time .…”