252 Insights to the Primary and Secondary Sources of Organic Aerosol in Mexico City During the MILAGRO Experiment 2006

Thursday, November 5, 2009: 12:20 PM
Kohlberg (Camino Real Hotel)
Elizabeth A. Stone, PhD , Carlsbad Environmental Monitoring and Research Center, New Mexico State Unviersity, Carlsbad, NM
James J. Schauer, PhD , Environmental Chemistry and Technology, Unviersity of Wisconsin-Madison, Madison, WI
Atmospheric aerosols impact the earth’s radiative balance, affect climate, and negatively impact human health. Organic carbon (OC) comprised a large fraction of fine aerosol mass in Mexico City in March 2006 and is the focus of source reconciliation. Samples were collected every 24-hours and analyzed for OC, elemental carbon (EC), and water-soluble organic carbon (WSOC). Organic compounds, particularly molecular markers, were measured by solvent extraction, chemical derivitization, and gas chromatography mass spectrometry (GCMS).  Chemical mass balance modeling (CMB) was used to quantify major OC sources. Motor vehicles consistently accounted for 49% of OC in the urban area and 32% on the periphery. The daily contribution of biomass burning was highly variable, and ranged from 5–26% at the urban site and 7–39% at the peripheral site. The OC unapportioned to primary sources correlated strongly secondary organic aerosol (SOA). Using precursor-specific tracers, anthropogenic SOA was found to contribute a large fraction of OC at both sites.  Biogenic SOA was less prominent, but was twice as important to OC at the peripheral site compared to the urban site.  The OC that was not attributed to primary sources or to known secondary sources showed temporal consistency with biomass burning events, suggesting the importance of secondary processing of biomass-burning.  Maximum biomass-burning SOA contributions to OC were in the range of 20-30% during peak biomass events.   This work provides quantitative understanding of the important primary and secondary sources of OC and insights to under-characterized aerosol sources in Mexico City and the surrounding region.