Measurements of volatile organic compounds at a suburban ground site (T1) in Mexico City during the MILAGRO 2006 campaign: measurement comparison, emission ratios, and source attribution

摘要

Volatile organic compound (VOC) mixing ratios were measured with twodifferent instruments at the T1 ground site in Mexico City during theMegacity Initiative: Local and Global Research Observations (MILAGRO)campaign in March of 2006. A gas chromatograph with flame ionizationdetector (GC-FID) quantified 18 light alkanes, alkenes and acetylene while aproton-transfer-reaction ion-trap mass spectrometer (PIT-MS) quantified 12VOC species including oxygenated VOCs (OVOCs) and aromatics. A GC separationsystem was used in conjunction with the PIT-MS (GC-PIT-MS) to evaluatePIT-MS measurements and to aid in the identification of unknown VOCs. TheVOC measurements are also compared to simultaneous canister samples and totwo independent proton-transfer-reaction mass spectrometers (PTR-MS)deployed on a mobile and an airborne platform during MILAGRO. VOC diurnalcycles demonstrate the large influence of vehicle traffic and liquid propanegas (LPG) emissions during the night and photochemical processing during theafternoon. Emission ratios for VOCs and OVOCs relative to CO are derivedfrom early-morning measurements. Average emission ratios for non-oxygenatedspecies relative to CO are on average a factor of ~2 higher thanmeasured for US cities. Emission ratios for OVOCs are estimated andcompared to literature values the northeastern US and to tunnel studies inCalifornia. Positive matrix factorization analysis (PMF) is used to provideinsight into VOC sources and processing. Three PMF factors weredistinguished by the analysis including the emissions from vehicles, the useof liquid propane gas and the production of secondary VOCs + long-livedspecies. Emission ratios to CO calculated from the results of PMF analysisare compared to emission ratios calculated directly from measurements. Thetotal PIT-MS signal is summed to estimate the fraction of identified versusunidentified VOC species.