Ambient VOC measurements in Mexico City

The MCMA 2003 field campaign was a comprehensive investigation of photochemical gas and aerosol air quality in Mexico City conducted during April, 2003. A preliminary study was also conducted during February, 2002. As part of this study, ambient samples of organic volatile compounds (VOC) were analyzed from airshed boundary sites, central urban core sites and downwind urban receptor sites, as well as during mobile vehicle chase operations. This work presents results from VOC concentrations measured by a combination of methods: canister sampling and GC/FID analysis, continuous real-time olefin detection with a Fast Isoprene Sensor (FIS), real-time and continuous measurements using two Proton Transfer Reaction Mass Spectrometers (PTR-MS)one onboard a mobile laboratory and the second on a tall urban tower. The objectives of this work were to investigate the distribution of VOC species, their magnitudes and diurnal patterns, to characterize different sites using ratios of individual VOC species, to investigate the relative reactivity of different species, to compare different methods to measure VOC and to evaluate emission inventories. The measured concentrations were consistent with previous studies and showed significantly higher levels of light alkanes compared to US cities. The highest concentrations occurred for propane, which was almost 15 times higher than in U.S. cities. However, the m.p-xylenes were the most important VOC according to their reactivity with OH. The elevated levels of propane and butane are sufficient to rank these alkenes in the top 5 in reactivity in Mexico.