Makoto Michael Kelp, Matthew Carroll, Tianjia Liu, Robert M. Yantosca, Heath Hockenberry, Loretta Mickley

Smoke from wildfires presents one of the greatest threats to air quality, public health, and ecosystems in the United States, especially in the West. Here we quantify the efficacy of prescribed burning as an intervention for mitigating smoke exposure downwind of wildfires across the West during the 2018 and 2020 fire seasons. Using the adjoint of the GEOS-Chem chemical transport model, we calculate the sensitivities of population‐weighted smoke concentrations in receptor regions, including states and rural environmental justice communities, to fire emissions upwind of the receptors. We find that the population-weighted smoke exposure across the West during the September 2020 fires was 44 ug/m3 but would have been 20-30% greater had these wildfires occurred in October or November. We further simulate a set of prescribed burn scenarios and find that controlled burning interventions in northern California and the Pacific Northwest could have reduced the population-weighted smoke exposure across the western United States by 21 ug/m3 in September 2020, while doing so in all other states would have reduced smoke exposure by only 1.5 ug/m3. Satellite records of large, prescribed burns (>1000 acres, or 4 km2) reveal that northern California and western Oregon conducted only seven such prescribed fires over a 6-year period (2015-2020), even though these regions have a disproportionate impact on smoke exposure for rural environmental justice communities and population centers across the West. Our analysis suggests that land managers should prioritize northern California and the Pacific Northwest for prescribed burns to mitigate future smoke exposure.