The evolution of biomass-burning aerosol size distributions due to coagulation: dependence on fire and meteorological details and parameterization

Abstract

Abstract. Biomass-burning aerosols have a significant effect on global and regional aerosol climate forcings. To model the magnitude of these effects accurately requires knowledge of the size distribution of the emitted and evolving aerosol particles. Current biomass-burning inventories do not include size distributions, and global and regional models generally assume a fixed size distribution from all biomass-burning emissions. However, biomass-burning size distributions evolve in the plume due to coagulation and net organic aerosol (OA) evaporation or formation, and the plume processes occur on spacial scales smaller than global/regional-model grid boxes. The extent of this size-distribution evolution is dependent on a variety of factors relating to the emission source and atmospheric conditions. Therefore, accurately accounting for biomass-burning aerosol size in global models requires an effective aerosol size distribution that accounts for this sub-grid evolution and can be derived from available emission-inventory and meteorological parameters. In this paper, we perform a detailed investigation of the effects of coagulation on the aerosol size distribution in biomass-burning plumes. We compare the effect of coagulation to that of OA evaporation and formation. We develop coagulation-only parameterizations for effective biomass-burning size distributions using the SAM-TOMAS large-eddy simulation plume model. For the most-sophisticated parameterization, we use the Gaussian Emulation Machine for Sensitivity Analysis (GEM-SA) to build a parameterization of the aged size distribution based on the SAM-TOMAS output and seven inputs: emission median dry diameter, emission distribution modal width, mass emissions flux, fire area, mean boundary-layer wind speed, plume mixing depth, and time/distance since emission. This parameterization was tested against an independent set of SAM-TOMAS simulations and yields R2 values of 0.83 and 0.89 for Dpm and modal width, respectively. The size distribution is particularly sensitive to the mass emissions flux, fire area, wind speed, and time, and we provide simplified fits of the aged size distribution to just these input variables. The simplified fits were tested against 11 aged biomass-burning size distributions observed at the Mt. Bachelor Observatory in August 2015. The simple fits captured over half of the variability in observed Dpm and modal width even though the freshly emitted Dpm and modal widths were unknown. These fits may be used in global and regional aerosol models. Finally, we show that coagulation generally leads to greater changes in the particle size distribution than OA evaporation/formation does, using estimates of OA production/loss from the literature.

More Than Emissions and Chemistry: Fire Size, Dilution, and Background Aerosol Also Greatly Influence Near‐Field Biomass Burning Aerosol Aging

“…Coagulation also shifts D g to larger values (through coagulational growth) and narrows the size distribution, decreasing σ g , by removing smaller particles (Sakamoto et al, ). Figure shows that for CoagOn cases, this coagulational increase in D g is most evident in larger (1 km 2 and greater) fire sizes, with increases up to a D g of 200 nm from coagulation alone.…”

“…Figure shows that for CoagOn cases, this coagulational increase in D g is most evident in larger (1 km 2 and greater) fire sizes, with increases up to a D g of 200 nm from coagulation alone. As discussed in section , coagulation rates are proportional to the square of the number concentration, and thus, the coagulation‐caused increase in D g amplifies with increasing fire sizes that take longer to dilute as well as for fires with higher emission fluxes leading to higher initial concentrations (Sakamoto et al, ). Additionally, decreasing dilution rates through a more‐stable atmosphere for a given smoke emission flux and size will also amplify the coagulational effect on D g (section S5 and Figures S20 and S22).…”

Larger Submicron Particles for Emissions With Residential Burning in Wintertime San Joaquin Valley (Fresno) than for Vehicle Combustion in Summertime South Coast Air Basin (Fontana)

“…The D VED_mode of rBC distributions were approximately 180 nm in the evening and at night on low‐fog days and approximately 140 nm on high‐fog days. On high‐fog days the mean temperatures (12 °C) are higher than low‐fog days (6 °C), suggesting that residential wood burning activity might have been lower resulting in lower mass flux and smaller D VED_mode on high‐fog days because the fractional contribution of wood burning‐derived rBC mass was lower (Sakamoto et al, ). In the morning and afternoon of low‐fog days, rBC mass distributions have broader peaks and D VED_mode at 140 nm, while the D VED_mode of rBC mass distribution on high‐fog days appears to be below the detection range of the SP2 (80 nm diameter).…”

Connecting smoke plumes to sources using Hazard Mapping System (HMS) smoke and fire location data over North America

“…This methodology does not provide information about smoke concentration (µg m −3 ), and we have not placed additional altitude constraints on the trajectories. A further weakness of smoke hours is that they have no way of accounting for secondary organic aerosol particle formation, which previous studies have shown can be significant (e.g., Janhäll et al, 2010;Sakamoto et al, 2015Sakamoto et al, , 2016. However, these processes remain poorly understood and represented in chemical transport models, so an appreciation for the fact that these processes exist, but not attempting to account for them, is sufficient for understanding the results presented in this work.…”