Volatile organic compounds in the downtown area of Mexicali, México during the spring of 2005: analysis of ambient data and source-receptor modeling

The border city of Mexicali has some of the worst air pollution in México, mainly due to the high levels of fine particulate matter (PM) and O3 registered. This also impacts the air quality of communities across the border in California's Imperial Valley. A field campaign was conducted in April of 2005 to obtain ambient concentrations of volatile organic compounds (VOC), and thus obtain insight on the levels and sources of these O3 and fine PM precursors. Six-liter stainless steel canisters were deployed at a site located in the downtown area of the city to obtain ambient air samples that were then analyzed for 54 selected target species. Samples were collected on a daily basis at three different sampling times: 6-9, 13-16, and 20-23 h LT. Average total non-methane organic compounds (TNMOC) concentration was 560 ppbC, with the morning samples having the highest reported average concentration (795) and the afternoon the lowest (257 ppbC). Aromatics contributed around 24% to the TNMOC concentration. Correlation between different species indicates strong influence of local anthropogenic sources (e.g., xylenes-benzene ratio of 2.1). In addition, high correlations between typical products from internal combustion engines (e.g., acetylene-ethene R² = 0.90, ethene-propylene R² = 0.94, benzene-ethene R² = 0.75) across all samples and a consistent contribution of more than 15% of these species (ethene, acetylene, propylene and benzene) indicate major contribution from mobile sources. This is supported by receptor modeling results obtained through the application of the Chemical Mass Balance model to the ambient data. Source apportionment estimates indicate a 56% contribution of gasoline-related mobile source emissions to the VOC that were measured, 18 contribution from LPG emissions, 6 from diesel exhaust, and 5 from consumer products.