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Aerosol size distribution and composition evolution during FIREX activities: closure analyses and climate impacts

The problem: The climate impacts of biomass-burning aerosols depend greatly on their size distribution and composition. These aerosol properties evolve in smoke plumes through physical (e.g. coagulation) and chemical (e.g. organic aerosol evaporation/formation) processes with the fastest evolution in the first 24 hours. Variation in fuels, combustion efficiency, fire size, plume dispersion and meteorology can all affect the aerosol size distribution and composition. However, regional and global aerosol models do not have sufficient horizontal resolution to resolve wildfire plumes (at least initially) and thus cannot explicitly calculate the size distribution evolution in these plumes. Regional and global models thus require �??grid-scaleappropriate�?� aged aerosol emissions size distributions to implicitly account for the near-source changes of biomass-burning aerosol size distributions.

Objectives and approach:

�?� Closure in FIREX lab studies: We will quantify how various processes (e.g. OA formation/evaporation, coagulation, wall losses) contribute to changes in aerosol composition and size throughout FIREX laboratory studies from the Missoula fire lab (late 2016) and from the CU Boulder chamber (2017). This will be done through closure studies using the ASP smoke chemistry/physics model and the laboratory data.

�?� Closure in FIREX field studies: We will quantify how these processes contribute to changes in aerosol composition and size throughout FIREX field studies (2018). This will be done through closure studies using the coupled SAM-ASP plume dispersion and chemistry model and the airborne measurements from the NOAA P-3.

�?� Comparison of lab and field data: We will identify lab experiments where similar fuels were burned as in smoke plumes observed in the field, and we will determine if the evolution of the aerosol size distributions observed in the lab are consistent with the field. We will identify processes contributing to any differences (e.g. wall losses, dilution, concentrations).

�?� Regional and global modelling: Through our closure studies, we will develop a parameterization of sub-grid aging of biomass-burning aerosol. We will test this parameterization in the GEOS-Chem-TOMAS global/regional aerosol microphysics model and quantify climate forcings.

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