Regional air quality impact of northern South America biomass burning emissions

Biomass burning emissions have a substantial impact on regional air quality and climate. The region of Amazonia in South America has long been identified as one of the largest contributors to short lived pollutants globally. However, massive natural wildfires and agricultural burns also occur every year in the grassland plains of Northern South America during the dry season (November to April). The regional-scale air quality impact of these biomass burning emissions has not been studied in depth and is analyzed in this study. We used PM2.5 and PM10 concentrations from three large urban areas: Bogota, Medellin, and Bucaramanga, for the period 2006-2016. Carbon monoxide data was only available for the city of Bogota for the analysis period. These cities are located hundreds of kilometers westward of the emission areas. The spatio-temporal distribution of fires was obtained from MODIS Active Fire Data. The back-trajectories of air masses reaching the receptor sites were computed with two different meteorological datasets. Radiosonde data, available only for Bogota, was used to account for local meteorological factors impacting pollution dispersion. A novel analysis algorithm was developed to combine active fire data with back-trajectory locations to select those active fires in the vicinity of the air masses arriving at each city. This analysis allows the selection of only those upwind fires that can be related to the air quality in the selected locations. We show that anomalously high PM and CO levels occurred when air masses originated from the Orinoco grasslands during the times when the largest number of fires in the region were active. The correlation between number of fires and PM10 concentration was found to decrease with increasing distance from the sources, ranging from 0.6 to 0.25. Our results are insensitive to the meteorological dataset used to generate back-trajectories. For Bogota it was found that mixing height variations can explain an important fraction of the observed seasonal variations in PM10, PM2.5, and CO concentration. The number of upwind fires can explain 11%+/- 5% of the seasonal variability in CO concentrations. Estimates of the seasonal variability of PM10 and PM2.5 explained by fires are 45%+/- 7% and 39%+/- 8% respectively. However, covariance between occurrence of fires and non-combustion local sources of PM imply that the latter estimates are likely an overestimation of the actual contribution. Our findings support the possibility that fires in the Orinoco river basin deteriorate air quality in highly populated urban centers hundreds of kilometers away from the sources.