The stable carbon isotope composition of PM_(2.5) and PM_(10) in Mexico City Metropolitan Area air

摘要

The sources and distribution of carbon in ambient suspended particles (PM_(2.5) and PM_(10)) of Mexico City Metropolitan Area (MCMA) air were traced using stable carbon isotopes (~(13)C/~(12)C).Tested potential sources included rural and agricultural soils, gasoline and diesel, liquefied-petroleum gas, volcanic ash, and street dust. The complete combustion of LP gas, diesel and gasoline yielded the lightest δ~(13)C values (-27 to -29‰, vs. PDB), while street dust (PM_(10)) represented the isotopically heaviest endmember (-17‰). The δ~(13)C values of rural soils from four geographically separated sites were similar (-20.7 ± 1.5‰). δ~(13)C values of particles and soot from diesel and gasoline vehicle emissions and agricultural soils varied between -23 and -26‰. Ambient PM samples collected in November of 2000, and March and December of 2001 at three representative receptor sites of industrial, commercial and residential activities had a δ~(13)C value centered around -25.1‰ in both fractions, resulting from common carbon sources. The predominant carbon sources to MCMA atmospheric particles were hydrocarbon combustion (diesel and/or gasoline) and particles of geological origin. The significantly depleted δ~(13)C values from the industrial site reflect the input of diesel combustion by mobile and point source emissions. Based on stable carbon isotope mass balance, the carbon contribution of geological sources at the commercial and residential sites was approximately 73% for the PM_(10) fraction and 54% for PM_(2.5). Although not measured in this study, biomass-burning emissions from nearby forests are an important carbon source characterized by isotopically lighter values (-29‰), and can become a significant contributor (67%) of particulate carbon to MCMA air under the prevalence of southwesterly winds. Alternative sources of these ~(13)C-depleted particles, such as cooking fires and municipal waste incineration, need to be assessed. Results show that stable carbon isotope measurements are useful for distinguishing between some carbon sources in suspended particles to MCMA air, and that wind direction has an impact on the distribution of carbon sources in this basin.