10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Effects of Near-Source Coagulation of Biomass Burning Aerosols on Global Predictions of Aerosol Size Distributions and Implications for Aerosol Radiative Effects
EMILY RAMNARINE, Jack Kodros, Jeffrey R. Pierce, Colorado State University
Abstract Number: 99 Working Group: Aerosol Transport and Transformation
Abstract Motivation: Biomass burning is a significant global source of aerosol number and mass. Aerosols scatter and absorb solar radiation along with potentially acting as cloud condensation nuclei, affecting cloud albedo and lifetime. These climatic effects are known as the direct radiative effect (DRE) and the cloud-albedo aerosol indirect effect (AIE), respectively. These effects are influenced by the size distribution of the aerosol. Coagulation of particles is an important process in determining this size distribution. As coagulation occurs, the number of particles is diminished and the distribution is shifted to a larger mean diameter.
Sakamoto et al. (2016) found that the rate of growth by coagulation of a single fire plume can be approximated using the emissions rate of biomass burning aerosol mass, initial size distribution median diameter and modal width, plume mixing depth, and wind speed. This parameterization is physically intuitive as more concentrated emissions or emissions that are harder to dilute (due to larger emissions fluxes, larger fires, smaller wind speed, or smaller mixing depth) lead to the signatures of faster coagulation.
Methods: To explore the effects that coagulation has on global aerosol size distributions, we incorporated this parameterization into GEOS-Chem, a global model, with TwO Moment Aerosol Sectional (TOMAS) microphysics and fire emissions from the Fire INventory from NCAR (FINN). In the default configuration of GEOS-Chem-TOMAS, the fire emissions are given a fixed size distribution regardless of fire characteristics and meteorology. We adjust this emitted size distribution by aging it for one day using the above parameterization while keeping the mass of emissions constant. We use the rapid radiative transfer model for global climate models (RRTMG) on our GEOS-Chem-TOMAS aerosol output to study the effect that coagulation has on the DRE and AIE.
Sensitivities simulations: We test the sensitivity to two major uncertainties in our simulations. First, the initial size distribution of biomass burning emissions is important to the coagulation parameterization, but is not well known. To address this uncertainty, we have tested a range of initial median diameters and lognormal modal widths. Another uncertainty regarding the subgrid smoke coagulation parameterization is how much the smoke plumes overlap while coagulation is occurring. Plumes that do not overlap dilute more quickly than those that overlap and hence have less coagulation. We look at the two extreme cases: all plumes in each gridcell overlap instantly, creating one large plume, or they do not overlap at all.
Results: Our main results are as follows. (1) When there is more coagulation there is generally a smaller number of particles overall, but in some locations there is a high number of particles at small diameters due to a microphysical feedback creating more nucleation. (2) In regions/cases with more subgrid coagulation, there is lower sensitivity to uncertainty in the initial size distribution. (3) Regions and cases with more coagulation have fewer CCN-sized particles, which leads to a decrease in the magnitude of the AIE of biomass burning particles, though the effect remains negative in all cases. (4) DRE due to biomass burning is negative but is not found to be sensitive to our assumed coagulation scheme or initial size distribution because the size distribution changes do not greatly impact the mass extinction efficiencies in our cases.
Recommendation: In order to improve our estimates of biomass burning size distributions and radiative effects, we need (1) estimates of how plumes merge and overlap in different regions and (2) improved estimates of emitted size distributions in different regions in our simulations.
Sakamoto, K. M. et al., Atmos. Chem. Phys., 16, 7709-7724, doi:10.5194/acp-16-7709-2016, 2016.