VENTILATION OF LIQUEFIED PETROLEUM GAS COMPONENTS FROM THE VALLEY OF MEXICO

The saturated hydrocarbons propane and the butane isomers are both ind irect 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 oxid ative potential in the basin which confines Mexico City. Propane and t he butanes, however, are stable enough to escape from the basin. The g as chromatographic measurements which have drawn attention to their so urces within the urban area are used here to estimate rates of ventila tion into the free troposphere. The calculations are centered on sever al well studied February/March pollution episodes, Carbon monoxide obs ervations and emissions data are first exploited to provide a rough ti me constant for the removal of typical inert pollutant species from th e valley. The timescale obtained is validated through an examination o f meteorological simulations of three-dimensional flow. Heuristic argu ments and transport modeling establish that propane and the butanes ar e distributed through the basin in a manner analogous to CO despite di ffering emissions functions. Ventilation rates and mass loadings yield outbound fluxes in a box model type computation. Estimated in this fa shion, escape from the Valley of Mexico constitutes of the order of ha lf of 1% of the northern hemispheric inputs for both propane and n-but ane. Uncertainties in the calculations are detailed and include factor s such as flow into the basin via surface winds and the size of the po lluted regime. General quantification of the global propane and butane emissions from large cities will entail studies of this type in a var iety of locales.