Satellite observations indicate substantial spatiotemporal variability in biomass burning NOx emission factors for South America

摘要

Biomass burning is an important contributor to global total emissions ofNO (NO+NO). Generally bottom-up fire emissions modelscalculate NO emissions by multiplying fuel consumption estimates withstatic biome-specific emission factors, defined in units of grams of NO perkilogram of dry matter consumed. Emission factors are a significant sourceof uncertainty in bottom-up fire emissions modeling because relatively fewobservations are available to characterize the large spatial and temporalvariability of burning conditions. In this paper we use NOtropospheric column observations from the Ozone Monitoring Instrument (OMI)from the year 2005 over South America to calculate monthly NO emissionfactors for four fire types: deforestation, savanna/grassland, woodland, andagricultural waste burning. In general, the spatial patterns in NOemission factors calculated in this work are consistent with emissionfactors derived from in situ measurements from the region but are morevariable than published biome-specific global average emission factorswidely used in bottom-up fire emissions inventories such as the Global FireEmissions Database (GFED). Satellite-based NO emission factors alsoindicate substantial temporal variability in burning conditions. Overall, wefound that deforestation fires have the lowest NO emission factors, onaverage 30% lower than the emission factors used in GFED v3. Agriculturalfire NO emission factors were the highest, on average a factor of 1.8higher than GFED v3 values. For savanna, woodland, and deforestation fires,early dry season NO emission factors were a factor of ~1.5–2higher than late dry season emission factors. A minimum in theNO emission factor seasonal cycle for deforestation fires occurred inAugust, the time period of severe drought in South America in 2005,supporting the hypothesis that prolonged dry spells may lead to an increasein the contribution of smoldering combustion from large-diameter fuels,offsetting the higher combustion efficiency of dryer fine fuels. Weevaluated the OMI-derived NO emission factors with SCIAMACHY NOtropospheric column observations and found improved model performance inregions dominated by fire emissions.