Aerosol Optical Properties of Extreme Global Wildfires and Estimated Radiative Forcing with GCOM-C SGLI

Abstract. Wildfires damage land ecosystems and significantly impact the atmosphere by releasing large amounts of CO2 and aerosols. Aerosols, in particular, have a radiative forcing potential that perturbs the global radiation balance. Therefore, understanding their optical properties is essential to estimate the radiative forcing for predicting the climatic impact due to burning. The Global Change Observation Mission—Climate, or GCOM-C (SHIKISAI), is a polar-orbit satellite launched by JAXA on 23 December 2017. This study analyzed wildfires in the Amazon, Angola, Australia, California, Siberia, and Southeast Asia that occurred after 2018 using data from the GCOM-C satellite obtained with the Second-generation Global Imager (SGLI), a multi-band optical imaging radiometer. We compared the aerosol optical properties of Ångström Exponent (AE) and Single Scattering Albedo (SSA) and found that their distributions are different for each region. In addition, the negative correlations are found in the Amazon, Angola, Siberia, and Southeast Asia when we locally fitted the AE and SSA relationship for each wildfire by a linear function. This is likely due to the effect of hygroscopic growth (SSA becomes larger with water uptake), considering the difference in relative humidity in the six regions, and the significant behavior of the aerosol aging. We also found that the relationship between SSA and relative humidity varies depending on the type of the burned vegetation. For the needle-forest-dominated group, a higher SSA is observed in the range from 40 % to 70 % of relative humidity than the broad forest-dominated group. This global investigation of the aerosol optical properties reveals that their characteristics differ due to regional differences such as relative humidity and vegetation type. Thus, the regional characteristics and the aging effect of biomass-burning aerosols must be considered in model predicting. Moreover, we estimated the net incoming radiation at the top of the atmosphere during the intense fire periods to understand the impact of the direct effect of biomass burning aerosols on radiative forcing. The estimates show a significant negative radiative forcing (i.e., a cooling effect) over the ocean (−78 and −96 Wm−2 in Australia and California, respectively). In contrast, small values are observed over land (∼ −10 Wm−2 for all six regions). This suggests that the radiative forcing depends on the region of the wildfire plumes. Therefore, taking the regional characteristics of the optical property and surface reflectance into account is necessary to estimate the effect on radiative forcing and future impacts of a short-lived climate forcer from wildfires.