Ventilation of liquefied petroleum gas components from the Valley of Mexico

Abstract

Abstract. The saturated hydrocarbons propane and the butane isomers are both indirect greenhouse gases and key species in liquefied petroleum gas (LPG). Leakage of LPG and its component alkanes/alkenes is now thought to explain a significant fraction of the volatile organic burden and oxidative potential in the basin which confines Mexico City. Propane and the butanes, however, are stable enough to escape from the basin. The gas chromatographic measurements which have drawn attention to their sources within the urban area are used here to estimate rates of ventilation into the free troposphere. The calculations are centered on several well studied February/March pollution episodes. Carbon monoxide observations and emissions data are first exploited to provide a rough time constant for the removal of typical inert pollutant species from the valley. The timescale obtained is validated through an examination of meteorological simulations of three-dimensional flow. Heuristic arguments and transport modeling establish that propane and the butanes are distributed through the basin in a manner analogous to CO despite differing emissions functions. Ventilation rates and mass loadings yield outbound fluxes in a box model type computation. Estimated in this fashion, escape from the Valley of Mexico constitutes of the order of half of 1% of the northern hemispheric inputs for both propane and n-butane. Uncertainties in the calculations are detailed and include factors such as flow into the basin via surface winds and the size of the polluted regime. General quantification of the global propane and butane emissions from large cities will entail studies of this type in a variety of locales. In the discussion section the arguments connecting carbon monoxide and alkane fluxes from the valley are formalized through modeling. Simple one-dimensional simulations of the ventilation process are constructed, and limited application is made of some three-dimensional photochemistry/transport codes at our disposal. We deal qualitatively with the potential for surface winds to import the tracers of interest. Other major uncertainties in the analysis are then listed. Sources of error include our definition of the basin and urban periphery, the average height of the low nocturnal inversion, and alkane distributions. We conclude with some comments on the global ramifications of our findings. By our calculations, Mexico City in and of itself constitutes a significant source of the C3 and C4 alkanes to the terrestrial atmosphere. Recent sampling suggests that many other nondomestic (non-United States) urban areas contribute propane and the butanes through LPG leakage. We note that megacities with pollutant characteristics akin to those of Mexico are developing rapidly worldwide [United Nations, 1992;Kretzschmar, 1993Kretzschmar, , 1994Goldemberg, 1995]. To better quantify their role in the global tropospheric ozone system, measurement of LPG component signatures will be required in many urban settings. We also outline relationships between ...

Impact of the leakage of liquefied petroleum gas (LPG) on Santiago Air Quality

“…As a result, the unburned leakage and incomplete combustion of LPG have played a major role in O3 production in Mexico City [Blake and Rowland, 1995]. Indeed, the ventilation of LPG components from the Valley of Mexico may contribute 0.5% to the northern hemispheric inputs for both propane and n-butane [Elliott et al, 1997]. By contrast, the cumulative O3 productivity for Los Angeles LPG is roughly half that of Mexico City, because of both a lower alkene content and a switch in the C3-C4 composition towards seals.…”

ENERGY ANDMATERIALFLOWTHROUGH THEURBANECOSYSTEM

“…Relatively complete estimates of total atmospheric throughputs are available for Mexico City (68,69,110). Located at 20…”

“…Below this level, daytime heating of the surface drives vertical turbulence, and some rising air parcels interchange with the general circulation. Within the volume defined by the chimney effect, total air mass directly influenced by the city is almost 5 Gt and throughput is around 6 Gt per day (68,70). Budgeting for carbon monoxide and ozone indicates a total turnover (reciprocal residence time) of 1.33 per day in the winter.…”

“…Mexico City is perceived as the archetypal settlement of the next century (33,(66)(67)(68)(69)(70). Table 5 shows that the transportation sector is the largest consumer of energy in the city, totaling 50% of fossil fuels (1,71).…”

Pathways for the oxidation of sarin in urban atmospheres

“…Concentration ranges are set for the city reactants through a sampling of results which have been obtained in two of the world's most populous and polluted areas, the Los Angeles basin and Valley of Mexico. We selected these as baseline locations because they are representative of domestic and developing megacities, respectively, and because we have participated in the evaluation of their vexing but distinct pollution problems (MAFU, 1994;Elliott et al, 1997a and1997b). Nonlinearities in the organic/oxidant/ozone system are outlined (Blake and Rowland, 1995).…”