An evaluation of biomass burning aerosol mass, extinction, and size distribution in GEOS using observations from CAMP2Ex

Abstract. Biomass burning aerosol impacts aspects of the atmosphere and
Earth system through direct and semi-direct effects, as well as
influencing air quality. Despite its importance, the representation of
biomass burning aerosol is not always accurate in numerical weather
prediction and climate models or reanalysis products. Using observations
collected as part of the Cloud, Aerosol and Monsoon Processes Philippines
Experiment (CAMP2Ex) in August through October of 2019, aerosol
concentration and optical properties are evaluated within the Goddard Earth
Observing System (GEOS) and its underlying aerosol module, GOCART. In the
operational configuration, GEOS assimilates aerosol optical depth
observations at 550 nm from AERONET and MODIS to constrain aerosol fields.
Particularly for biomass burning aerosol, without the assimilation of
aerosol optical depth, aerosol extinction is underestimated compared to
observations collected in the Philippines region during the CAMP2Ex
campaign. The assimilation process adds excessive amounts of carbon to
account for the underestimated extinction, resulting in positive biases in
the mass of black and organic carbon, especially within the boundary layer,
relative to in situ observations from the Langley Aerosol Research Group
Experiment. Counteracting this, GEOS is deficient in sulfate and nitrate
aerosol just above the boundary layer. Aerosol extinction within GEOS is a
function of the mass of different aerosol species, the ambient relative
humidity, the assumed spectral optical properties, and particle size
distribution per species. The relationship between dry and ambient
extinction in GEOS reveals that hygroscopic growth is too high within the
model for biomass burning aerosol. An additional concern lies in the assumed particle size distribution for GEOS, which has a single mode radius that is too small for organic carbon. Variability in the observed particle size distribution for biomass burning aerosol within a single flight also
illuminates the fact that a single assumed particle size distribution is not sufficient and that for a proper representation, a more advanced aerosol module within GEOS may be necessary.
