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

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June 27, 2014
Zitely Tzompa
Hosted by Sonia Kreidenweis (advisor), Emily Fischer, Amy Sullivan (affiliate), John Volckens (Environmental and Radiological Sciences)

Abstract

The Mexico City Metropolitan Area (MCMA) is one of the largest megacities in the world with a population of over 20 million people. The city lies in an elevated semi-enclosed air basin where the air quality is strongly influenced by several factors including 1) meteorology, 2) geographical location that favors photochemical reactions, 3) abrupt topography where thermal inversions are common, 4) intense economic and industrial activity, 5) a fleet of over 3 million vehicles, 6) emissions from an active volcano, 7) agricultural activities inside and outside the air basin; 8) and pine and fir forests covering the surrounding mountain ranges, which contribute biogenic emissions as well as combustion emissions when wildfires strike the area. Anthropogenic emissions in MCMA have been controlled in past decades, resulting in decreasing tendencies of many air pollutants due to implementation of rigorous air quality policies. However, emissions transported from outside the basin, such as wildfires and agricultural burning, represent a potentially large contribution to air quality degradation especially during the dry-warm season. This study analyzed PM10 filter samples from six different stations located across the MCMA from May, 2013, which represented the month with the most reported fire counts in the region over the last 11 years (2002- 2013). Two meteorological regimes were established considering the number of satellite derived fire counts, changes in predominant wind direction, ambient concentrations of CO (ppm), PM10 (µg/m3) and PM2.5 (µg/m3), and precipitation patterns inside MCMA during May 2013. The first two sampling dates corresponded to the first regime, which had predominant winds coming from the west, no precipitation and higher fire counts. The second regime had winds mostly coming from the north, lower fire counts and a mean precipitation rate of 3.6 mm/day. The PM10 Hi-volume filter samples were analyzed for biomass burning tracers including levoglucosan (LEV) by high-performance anion-exchange chromatography with pulsed amperometric detection; for seven ions including water-soluble potassium (WSK+) using a Dionex DX-500 series ion chromatograph; and for water- soluble organic carbon (WSOC) measured in the liquid extract using a Portable Total Organic Carbon Analyzer (Sievers Turbo). Results of these analyses show that LEV concentrations correlated positively with ambient concentrations of PM2.5 and PM10 (R2=0.61 and R2=0.46, respectively). Strong correlations were also found between WSOC and LEV (R2=0.94) and between WSK+ and LEV (R2=0.75). An average LEV/WSOC ratio of 0.0147 was estimated for the first meteorological regime and 0.0062 for the second. Our LEV concentrations and LEV/WSOC ratios are consistent with results found during the MILAGRO campaign (March, 2006). To the best of our knowledge, only total potassium concentrations have been measured in aerosol samples from MCMA. Therefore, this is the first study in MCMA to measure ambient concentrations of WSK+. Analysis of gravimetric mass concentrations showed that during the sampling period, PM2.5 accounted for 60% of the PM10 mass concentration with an estimated PM10/PM2.5 ratio of 1.68. Estimates from our laboratory filter sample characterization indicated that we measured 37% of the total PM10 mass concentration. The missing mass is most likely crustal material (soil or dust) and carbonaceous aerosols that were not segregated into WSOC fraction. Assuming that LEV is inert in the atmosphere, The estimated biomass burning contributions to WSOC ranged from 7-23%. When assuming a LEV lifetime of 1.1 to 5 days, the estimated contributions increased on average 80%. Thus, we conclude that biomass burning sources had a large impact on WSOC and PM2.5 during May 2013, potentially explaining up to half of the measured WSOC. However, this finding depends on the source profile used in the apportionment. Source profiles specific to the regions in Mexico in which fires occur in the springtime are needed to increase confidence in this estimate. This work contributes to the limited number of estimates of the relative contributions of primary emissions from biomass burning to carbonaceous aerosols in MCMA during an active fire season. Our results indicate that these sources represent significant contributions to ambient PM. Further, recent published studies show that biomass burning emissions are reactive in many instances and may lead to formation of secondary organic aerosol. Hence, future studies are needed to improve the emission inventories that are commonly used by decision makers in the MCMA to design air quality policies and emission source controls.