Wildfire Smoke Particle Properties and Evolution, From Space-Based Multi-Angle Imaging: The Williams Flats Fire During the FIREX-AQ Campaign

Abstract:

Although the characteristics of biomass burning events and the ambient ecosystem determine emitted smoke composition, we do not adequately understand the conditions that mediate the partitioning of black and brown carbon formation, nor the spatial or temporal frequency of factors driving particle evolution, such as hydration, coagulation, and oxidation, all of which impact smoke radiative forcing. In situ data from surface observation sites and aircraft field campaigns have offered significant insight into the optical, chemical, and microphysical traits of biomass burning (BB) smoke aerosols, such as single scattering albedo (SSA) and size distribution, but cannot by themselves provide robust statistical characterization of both emitted and evolved particles. Data from the NASA Earth Observing System’s Multi-angle Imaging SpectroRadiometer (MISR) instrument can be used to complement in situ observations, providing at least a partial picture of BB particle properties and their evolution downwind if properly validated. Here we use in situ data from the joint NOAA/NASA 2019 FIREX-AQ field campaign to assess the strengths and limitations of particle property retrievals from the MISR Research Aerosol (RA) algorithm, which provides constraints on particle size, shape, light-absorption, and its spectral slope. The satellite and in situ measurements are substantially similar in their characterization of particle property evolution as a function of smoke age for the 06 August Williams Flats Fire, and most of the key differences in particle size and absorption can be attributed to differences in sampling and changes in the plume geometry between sampling times. Whereas the aircraft data provide validation for the MISR retrievals, the satellite data offer a continuous mapping of particle properties over the plume, which helps identify trends in particle downwind evolution that are ambiguous in the sparsely sampled aircraft transects. As such, the combination of satellite and aircraft datasets allows us to deduce the relevant aging mechanisms at various points throughout the plume. In future steps, we will apply the method to different types of plumes that are progressively less well-constrained by in situ observations, with the goal of creating a global climatology of BB particles based on ecosystem and season.