Boreal forest fire CO and CH4 emission factors derived from tower observations in Alaska during the extreme fire season of 2015

Abstract

Abstract. Recent increases in boreal forest burned area, which have\nbeen linked with climate warming, highlight the need to better understand\nthe composition of wildfire emissions and their atmospheric impacts. Here we\nquantified emission factors for CO and CH 4 from a massive regional fire\ncomplex in interior Alaska during the summer of 2015 using continuous\nhigh-resolution trace gas observations from the Carbon in Arctic Reservoirs\nVulnerability Experiment (CRV) tower in Fox, Alaska. Averaged over the 2015\nfire season, the mean CO\u2009 / \u2009CO 2 emission ratio was 0.142\u2009 ± \u20090.051, and\nthe mean CO emission factor was 127\u2009 ± \u200940\u2009g\u2009kg −1 dry biomass\nburned. The CO\u2009 / \u2009CO 2 emission ratio was about 39\u2009% higher than the mean\nof previous estimates derived from aircraft sampling of wildfires from\nboreal North America. The mean CH 4 / \u2009CO 2 emission ratio was 0.010\u2009 ± \u20090.004, and the CH 4 emission factor was 5.3\u2009 ± \u20091.8\u2009g\u2009kg −1 dry biomass burned, which are consistent with the mean of previous\nreports. CO and CH 4 emission ratios varied in synchrony, with higher\nCH 4 emission factors observed during periods with lower modified\ncombustion efficiency (MCE). By coupling a fire emissions inventory with an\natmospheric model, we identified at least 34 individual fires that\ncontributed to trace gas variations measured at the CRV tower, representing\na sample size that is nearly the same as the total number of boreal fires\nmeasured in all previous field campaigns. The model also indicated that\ntypical mean transit times between trace gas emission within a fire\nperimeter and tower measurement were 1–3\u2009d, indicating that the time\nseries sampled combustion across day and night burning phases. The high CO\nemission ratio estimates reported here provide evidence for a prominent role\nof smoldering combustion and illustrate the importance of continuously\nsampling fires across time-varying environmental conditions that are\nrepresentative of a fire season.