Urban leakage of liquefied petroleum gas and its impact on Mexico City air quality
Alkane hydrocarbons (propane, isobutane, and n-butane) from liquefied petroleum gas (LPG) are present in major quantities throughout Mexico City air because of leakage of the unburned gas from numerous urban sources. These hydrocarbons, together with olefinic minor LPG components, furnish substantial amounts of hydroxyl radical reactivity, a major precursor to formation of the ozone component of urban smog. The combined processes of unburned leakage and incomplete combustion of LPG play a significant role in causing the excessive ozone characteristic of Mexico City. Reductions in ozone levels should be possible through changes in LPG composition and lowered rates of leakage.
Mexico City lies in a high valley surrounded by mountains capable of holding in pollutants released by its more than 15 million inhabitants and has suffered during the past two decades from increasingly severe smog events with very high ozone ([O.sub.3]) levels (1)(2). The Mexican 1-hour criterion for [O.sub.3] is 0.11 parts per million (ppm; US Environmental Protection Agency, 0.12 ppm; World Health Organization, 0.10 ppm) and was exceeded in Mexico City on 71% of the days in 1986 and 98% in 1992, with values as high as 0.48 ppm on 16 March 1992 (3). Previous investigations of [O.sub.3] formation in Mexico City have primarily emphasized the release of hydrocarbons (HCs) from automobiles and trucks, and secondarily in industrial plants (4). Current control mechanisms during periods of high [O.sub.3] call for reductions in the use of automobiles and in levels of industrial operation. However, our detailed chemical analysis of the specific HC composition of Mexico City air (5)(6) has consistently shown very high concentrations of the [C.sub.3] and [C.sub.4] alkanes not usually found as more than very minor components in vehicular or industrial emissions. In most of our air samples, these three alkanes were the dominant nonmethane HCs (NMHC), with concentrations exceeding those of well-known signature compounds for fossil fuel combustion, such as ethylene and acetylene. The source of these alkanes lies in LPG--the major energy source for cooking and heating in urban households in Mexico City--leaking in unburned form into the atmosphere on a massive scale. These reactive HCs, together with the olefinic minor components in LPG, plus olefinic and acetylenic products from their incomplete combustion, play a major (perhaps the dominant) role in [O.sub.3] production in the Valley of Mexico.
Previous studies have recognized the very high NMHC concentration in Mexico City air (7), and that standard emission inventories have fallen short by a factor of 4 in quantitative explanation of their presence (8). No controls on LPG emissions are currently specified in Mexico City, nor have any been specifically proposed (9). Substantial progress toward reduction of urban [O.sub.3] formation seems possible if attention is directed toward the problems associated with LPG usage. Although control of [O.sub.3] formation by reduction in HC emissions is less effective in areas such as Mexico City with a high ratio of NMHC to nitrogen oxides (N[O.sub.x]), the overall HC reactivity of LPG is very dependent on its particular composition, especially in olefinic content...
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