Ground-based and aircraft measurements of volatile organic compounds in United States and Mexico City urban atmospheres

This study presents measurements made from ground-based samples collected in 28 cities in the United States and in Mexico City, as well as data from aircraft measurements made of outflow from these urban areas. Urban samples are used to identify and quantify the major volatile organic compounds (VOCs) present in the cities studied, while aircraft samples are used for comparison and to examine both local and transported pollutants.; Between 1999 and 2005 whole air canister samples were collected in 28 United States cities during the summertime (August/September) and in 8 cities during November, February and May. Samples were analyzed for methane, carbon monoxide (CO), NMHCs and halocarbons. Mixing ratios of NMHCs exhibited high inter- and intra-city variability, so ratios of individual NMHCs to CO, a combustion tracer, were examined to facilitate comparison between cities. Most NMHCs were found to have correlation coefficients (r2 ) greater than 0.6, highlighting the influence of vehicular emissions on NMHC mixing ratios. Notable exceptions were the C2-C 4 alkanes, which generally had poor correlations with CO. Mixing ratios of halocarbons regulated by the Montreal Protocol highlighted the success of regulatory efforts in the United States, while measurements of unregulated species identified strong urban sources.; In Mexico City samples were collected at two fixed sites (T0 and T1) and throughout the central urban area. The T0 site was at an urban/industrial location while T1 was a suburban location. All samples were analyzed for methane, CO, NMHCs, halocarbons and alkyl nitrates. The most abundant NMHC at all sites was propane, the major component of liquefied petroleum gas (LPG). For nearly all species, mixing ratios were the greatest at the T0 site, where mean values of many NMHCs were in excess of 10 ppbv and where halocarbons were up to an order of magnitude greater than background values. Simple calculations of OH loss frequencies at each site show ethene and toluene to be the largest NMHC sinks for OH; propane, which has a relatively slow reaction rate but is the most abundant NMHC, was also a significant sink.; Samples collected in 2006 in Mexico City were also compared to samples collected in 1993. Mixing ratios of NMHCs showed an approximately 75% reduction in vehicular emissions and a 50% reduction in emissions of LPG components. Ratios between transportation-related species indicate little change in the composition of vehicular emissions and fuels, while ratios between components of LPG reflect reformulation in 1997 to increase the propane fraction and reduce the fractions of butanes and butenes.; Whole air samples collected during the INTEX-NA, INTEX-B and MILAGRO research campaigns collected within the boundary layer over different cities and source regions were used as confirmation of ground-based measurements in the mid-continent oil field region, the central valley (Fresno) and Mexico City, and in the case of Seattle, where ground measurements were not made, were used to determine the composition of VOCs. Additionally, samples collected downwind provided information about the composition, persistence and chemical transformations of these source plumes.