Particle-nitrate and particle-sulfate emissions from wildfire smoke

Wildfires are an intrinsic and necessary component of Canadian landscapes, yet recent decades have seen a marked increase in the average annual area burned and the persistence of smoke-impacted conditions. Large wildfire events release substantial quantities of trace gases and particulate matter into the atmosphere, including precursors that contribute to the formation of secondary inorganic aerosols such as particle nitrate (NO3⁻) and sulfate (SO4²⁻). Another potential source of particulate nitrate and sulfate are direct primary emissions of these species. These species influence local and regional atmospheric chemistry, radiative effects, and surface-level air quality, yet the relative contributions of emissions (primary particulate matter) versus chemistry (secondary particulate matter) towards their concentrations within wildfire plumes remains uncertain.
Atmospheric chemical transport models (CTMs) are essential tools used to simulate the evolution and impacts of wildfire smoke. In Canada, one of the primary operational CTMs is Environment and Climate Change Canada’s (ECCC) Global Environmental Multiscale – Modelling Air quality and CHemistry (GEM-MACH). Within GEM-MACH, the Canadian Forest Fire Emissions Prediction System (CFFEPS) estimates wildfire emissions from fire activity, plume rise, and chemical speciation. However, assumptions regarding primary particle composition and gas-phase precursors can strongly influence simulated secondary inorganic aerosol formation through gas–particle partitioning and thermodynamic processes.
This research evaluates the sensitivity of particle-phase nitrate and sulfate formation to wildfire emission speciation within GEM-MACH. Using a fire-impacted case study in Lac La Loche, Saskatchewan, Canada, model simulations making use of revised primary particle emissions profiles are compared against surface-based measurements, to assess changes in simulated aerosol composition and overall air quality performance with respect to particle-phase concentrations of particle-nitrate and sulfate. One of our key findings is that both particulate nitrate and sulphate from forest fire combustion are mostly derived from secondary chemistry – the rapid conversion of sulfur dioxide and nitrogen oxides to particulate sulfate and nitrate in the plumes.