Abstract
Abstract. Volatile organic compound (VOC) mixing ratios were measured with two different instruments at the T1 ground site in Mexico City during the Megacity Initiative: Local and Global Research Observations (MILAGRO) campaign in March of 2006. A gas chromatograph with flame ionization detector (GC-FID) quantified 18 light alkanes, alkenes and acetylene while a proton-transfer-reaction ion-trap mass spectrometer (PIT-MS) quantified 12 VOC species including oxygenated VOCs (OVOCs) and aromatics. A GC separation system was used in conjunction with the PIT-MS (GC-PIT-MS) to evaluate PIT-MS measurements and to aid in the identification of unknown VOCs. The VOC measurements are also compared to simultaneous canister samples and to two independent proton-transfer-reaction mass spectrometers (PTR-MS) deployed on a mobile and an airborne platform during MILAGRO. VOC diurnal cycles demonstrate the large influence of vehicle traffic and liquid propane gas (LPG) emissions during the night and photochemical processing during the afternoon. Emission ratios for VOCs and OVOCs relative to CO are derived from early-morning measurements. Average emission ratios for non-oxygenated species relative to CO are on average a factor of ~2 higher than measured for US cities. Emission ratios for OVOCs are estimated and compared to literature values the northeastern US and to tunnel studies in California. Positive matrix factorization analysis (PMF) is used to provide insight into VOC sources and processing. Three PMF factors were distinguished by the analysis including the emissions from vehicles, the use of liquid propane gas and the production of secondary VOCs + long-lived species. Emission ratios to CO calculated from the results of PMF analysis are compared to emission ratios calculated directly from measurements. The total PIT-MS signal is summed to estimate the fraction of identified versus unidentified VOC species.
Highlights
Recent estimates of the global emission of volatile organic compounds (VOCs) into the atmosphere range from about 1200 to 1600 TgC yr−1 (Olivier et al, 2005; Goldstein and Galbally, 2007; Reimann and Lewis, 2007; Williams and Koppmann, 2007; Yokelson et al, 2008)
VOC emissions are dominated by isoprene, which accounts for about 65% of biogenic emissions and 40% of total VOC emissions (Guenther et al, 1995, 2006; Williams and Koppmann, 2007)
Samples were collected from an air intake approximately 6 m above ground level that was shared with the proton-transfer-reaction iontrap mass spectrometer (PIT-MS) system described beat times when morning maxima in VOC mixing ratios occurred
Summary
Recent estimates of the global emission of volatile organic compounds (VOCs) into the atmosphere range from about 1200 to 1600 TgC yr−1 (Olivier et al, 2005; Goldstein and Galbally, 2007; Reimann and Lewis, 2007; Williams and Koppmann, 2007; Yokelson et al, 2008). VOC measurements were made by two different groundbased instruments at the sub-urban T1 site (Fast et al, 2007): an in-situ gas chromatograph with flame ionization detection (GC-FID) was used to measure light hydrocarbons and a proton-transfer-reaction ion-trap mass spectrometer (PITMS) was used to measure acetonitrile, aromatics and oxygenated VOCs (de Gouw et al, 2009). These measurements are compared with canister sample analyses and with proton-transfer-reaction mass spectrometry (PTR-MS) measurements made from the Aerodyne mobile laboratory and from the US Department of Energy G1 aircraft. We use the results from the PMF analysis to estimate emission ratios and compare these with emission ratios derived directly from ground-based VOC measurements
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