Assessing Vertical Allocation of Wildfire Smoke Emissions Using Observational Constraints From Airborne Lidar in the Western U.S.

Wildfire emissions are a key contributor of carbonaceous aerosols and trace gases to the atmosphere. Induced by buoyant lifting, smoke plumes can be injected into the free troposphere and lower stratosphere, which by consequence significantly affects the magnitude and distance of their influences on air quality and radiation budget. However, the vertical allocation of emissions when smoke escapes the planetary boundary layer (PBL) and the mechanism modulating it remain unclear. We present an inverse modeling framework to estimate the wildfire emissions, with their temporal and vertical evolution being constrained by assimilating aerosol extinction profiles observed from the airborne Differential Absorption Lidar-High Spectral Resolution Lidar during the Fire Influence on Regional to Global Environments and Air Quality field campaign. Three fire events in the western U.S., which exhibit free-tropospheric injections are examined. The constrained smoke emissions indicate considerably larger fractions of smoke injected above the PBL (f>PBL, 80%–94%) versus the column total, compared to those estimated by the WRF-Chem model using the default plume rise option (12%–52%). The updated emission profiles yield improvements for the simulated vertical structures of the downwind transported smoke, but limited refinement of regional smoke aerosol optical depth distributions due to the spatiotemporal coverage of flight observations. These results highlight the significance of improving vertical allocation of fire emissions on advancing the modeling and forecasting of the environmental impacts of smoke. Plain Language Summary Wildfires play a key role in air quality and the climate system. Lofted smoke into the free troposphere has been found to extend over long distances imposing regional and global impacts, but the vertical allocation of smoke emissions associated with fire combustion heat release and plume rise has yet to be objectively characterized and understood. Merging models and observations for four cases with free-tropospheric injections detected, we show significantly larger fractions of emissions above the planetary boundary layer versus the column totals than those specified by the plume rise parameterization. The enlarged injection fractions are confirmed independently with observations over the transported smoke plumes. Implications for future improvement of the plume rise model are discussed.