Contribution of Biomass Burning to Carbonaceous Aerosols in Mexico City during May 2013

During the springtime fire season, wildfires and agricultural burning represent a potentially large contribution to air quality degradation in the Mexico City Metropolitan Area (MCMA). PM10 filter samples were collected at six different stations in May 2013, the month with the maximum reported regional fire counts from 2002 to 2013. Two regimes were identified considering changes in predominant wind direction and precipitation patterns inside MCMA. The filter samples were analyzed for water-soluble organic carbon (WSOC) and the biomass burning tracers including levoglucosan (LEV) and water-soluble potassium (WSK + ). LEV concentrations correlated positively with ambient concentrations of PM2.5 and PM10 (R 2 = 0.61 and R 2 = 0.46, respectively). Strong correlations were also found between WSOC and LEV (R 2 = 0.94) and between WSK + and LEV (R 2 = 0.75). PM2.5 accounted for 60% of the PM10 mass concentrations. Our speciated measurements accounted for 37% of the total PM10 mass concentration and ~60% of the PM2.5 mass concentrations; the missing mass was attributed to crustal material (soil or dust) and carbonaceous aerosols that were not segregated into the WSOC fraction. Average LEV/WSOC ratios ranged from 0.015 in the first, smokier and drier part of the month, to 0.006 during the rainier end of the measurement period. Using previously reported LEV/WSOC emissions ratios, the estimated biomass burning contributions to WSOC ranged from 7–23% assuming LEV is stable in the atmosphere, and 8–57% when accounting for LEV photochemical degradation in the atmosphere. Thus, our findings indicate that primary emissions from biomass burning sources represent significant contributions to ambient WSOC and PM in MCMA during the springtime fire season.