Late summer changes in burning conditions in the boreal regions and their implications for NOx and CO emissions from boreal fires

Building emission inventories for the fires in boreal regions remains a challenging task with significant uncertainties in the methods used. In this work, we assess the impact of seasonal trends in fuel consumption and flaming/smoldering ratios on emissions of species dominated by flaming combustion (e.g., NOx) and species dominated by smoldering combustion (e.g., CO). This is accomplished using measurements of CO and NOy at the free tropospheric Pico Mountain observatory in the central North Atlantic during the active boreal fire seasons of 2004 and 2005. ΔNOy/ΔCO enhancement ratios in aged fire plumes had higher values in June‐July (7.3 × 10−3 mol mol−1) relative to the values in August‐September (2.8 × 10−3 mol mol−1), indicating that NOx/CO emission ratios declined significantly as the fire season progressed. This is consistent with our understanding that an increased amount of fuel is consumed via smoldering combustion during late summer, as deeper burning of the drying organic soil layer occurs. A major growth in fuel consumption per unit area is also expected, due to deeper burning. Emissions of CO and NOx from North American boreal fires were estimated using the Boreal Wildland Fire Emissions Model, and their long‐range transport to the sampling site was modeled using FLEXPART. These simulations were generally consistent with the observations, but the modeled seasonal decline in the ΔNOy/ΔCO enhancement ratio was less than observed. Comparisons using alternative fire emission injection height scenarios suggest that plumes with the highest CO levels at the observatory were lofted well above the boundary layer, likely as a result of intense crown fires.