Abstract
Abstract. Biomass burning is a significant global source of aerosol number and mass. In fresh biomass burning plumes, aerosol coagulation reduces aerosol number and increases the median size of aerosol size distributions, impacting aerosol radiative effects. Near-source biomass burning aerosol coagulation occurs at spatial scales much smaller than the grid boxes of global and many regional models. To date, these models have ignored sub-grid coagulation and instantly mixed fresh biomass burning emissions into coarse grid boxes. A previous study found that the rate of particle growth by coagulation within an individual smoke plume can be approximated using the aerosol mass emissions rate, initial size distribution median diameter and modal width, plume mixing depth, and wind speed. In this paper, we use this parameterization of sub-grid coagulation in the GEOS-Chem–TOMAS (TwO-Moment Aerosol Sectional) global aerosol microphysics model to quantify the impacts on global aerosol size distributions, the direct radiative effect, and the cloud-albedo aerosol indirect effect. We find that inclusion of biomass burning sub-grid coagulation reduces the biomass burning impact on the number concentration of particles larger than 80 nm (a proxy for CCN-sized particles) by 37 % globally. This cloud condensation nuclei (CCN) reduction causes our estimated global biomass burning cloud-albedo aerosol indirect effect to decrease from −76 to −43 mW m−2. Further, as sub-grid coagulation moves mass to sizes with more efficient scattering, including it increases our estimated biomass burning all-sky direct effect from −224 to −231 mW m−2, with assumed external mixing of black carbon and from −188 to −197 mW m−2 and with assumed internal mixing of black carbon with core-shell morphology. However, due to differences in fire and meteorological conditions across regions, the impact of sub-grid coagulation is not globally uniform. We also test the sensitivity of the impact of sub-grid coagulation to two different biomass burning emission inventories to various assumptions about the fresh biomass burning aerosol size distribution and to two different timescales of sub-grid coagulation. The impacts of sub-grid coagulation are qualitatively the same regardless of these assumptions.
Highlights
Because FINN version 1 used in GEOSChem (FINNv1) has a lower mass of emissions, the increases in organic aerosol (OA) and black carbon (BC) mass over major biomass burning regions and downwind are not as high as they are with Global Fire Emissions Database version 4 (GFED) emissions
We use a global chemical-transport model with aerosol microphysics and a parameterization of sub-grid biomass burning coagulation to estimate the impacts of subgrid coagulation on the ambient size distribution and aerosol radiative effects
In our sensitivity cases testing different initial size distributions, described below, the direct radiative effect (DRE) is more affected by the presence or absence of sub-grid coagulation of biomass burning aerosol, changing as much as 22 %
Summary
Atmospheric aerosol particles, including those from biomass burning, impact the climate system directly by scattering and absorbing radiation and indirectly by influencing cloud properties (Bauer et al, 2010; Bond and Bergstrom, 2006; Bond et al, 2013; Hodshire et al, 2018; Kodros et al, 2015, 2016; Kodros and Pierce, 2017; Pierce et al, 2007; Reid et al, 2005; Twomey, 1974; Weigum et al, 2016). Emissions from biomass burning include organic aerosol (OA), black carbon (BC), and inorganic particles, as well as aerosol precursor vapors such as sulfur dioxide and volatile organic compounds (e.g., Akagi et al, 2011). The largest biomass burning emissions occur over tropical Africa, South America, and Southeast Asia, but substantial emissions . E. Ramnarine et al.: Effects of near-source coagulation of biomass burning aerosols occur in temperate and boreal forests (Bond et al, 2013; van der Werf et al, 2017; Wiedinmyer et al, 2011). Bond et al (2013) estimated that biomass burning makes up 66 % of primary OA mass emissions and 37 % of BC mass emissions, globally Biomass burning smoke concentrations are spatially and temporally heterogeneous throughout most regions (Rodhe et al, 1972; Bond et al, 2013), and biomass burning aerosol may be transported thousands of kilometers downwind, potentially affecting areas far from the emitting fires (e.g., Val Martin et al, 2006). Bond et al (2013) estimated that biomass burning makes up 66 % of primary OA mass emissions and 37 % of BC mass emissions, globally
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