Simulating the impact of wild fire emissions on regional chemistry and meteorology using a coupled atmosphere-fire-chemistry model WRF-SFIRE-Chem

Wildland fires are a major source of atmospheric pollutants that can degrade air quality. Volatile organic compounds (VOCs) and nitrogen oxides (NO_x) emitted from fires can contribute to ozone (O₃) formation. Fires emissions also include both organic carbon (OC) and black carbon (BC) aerosols, as well as gaseous emissions which contribute to the formation of secondary organic aerosol (SOA). In addition to air quality degradation, aerosols emitted from fires affect the climate directly by scattering and absorbing radiation and indirectly by changing cloud processes. The vertical profile of emissions in fire plumes has a strong influence on the impacts of a fire on regional air quality and meteorology, because plume dynamics affect the spatial and vertical distributions of pollutants near and downwind of the fires. While coupled meteorology-chemistry models have been used to study the impact of fire emissions on regional-scale meteorology, these meso-scale modeling studies do not account for finer-scale fire-atmosphere dynamics, thus ignoring or diluting the impact of vertical updrafts associated with intense heat from fires.

In this study, we demonstrate the use of a coupled atmosphere-fire-chemistry model WRF- SFIRE-Chem to evaluate the impact of wildland fires on regional chemistry and meteorology. WRF-SFIRE is a two-way coupled fire atmosphere model that simulates the fire spread based on the local meteorological conditions, taking into account the feedback between the fire and the atmosphere. Included in WRF-SFIRE is a fuel moisture model that assimilates RAWS fuel moisture data and accounts for the effects of local meteorology simulated by WRF on the fuel flammability. The fire model is also coupled with WRF-Chem so that chemistry, transport, and radiation-cloud-feedbacks of gases and particles emitted from the fire are taken into account. Emissions rates are based on fuel consumption calculated by the fire model and emission factors from FINN. Emissions are inserted into the lowest atmospheric layer and transported vertically according to updrafts associated with the fire heat flux. The use of WRF- SFIRE-Chem is demonstrated for two very large Santa Ana fires in southern California in 2007.