Ambient temperature effect on urban CO air quality

Abstract

Changes in the ambient temperature cause changes in vehicle emissions mainly during the cold-start period of operation. The influence that this effect has on the seasonal variation of urban CO concentrations is examined by means of a simple air quality model. For fixed meteorological conditions, the model predicts that the atmospheric CO concentrations at a center city measuring location will increase by 5–15% for a decrease in temperature from 75° to 25°F when none of the vehicles in that vicinity are in the cold-start mode and will increase by 55–75% when all of the nearby vehicles are in the cold-start mode. An inverse variation of CO concentration with change in temperature is derived from a statistical analysis of CO data reported both for midtown Manhattan (1975–1977) and for downtown Los Angeles (1970–1975). The magnitude of the derived variation is small for both cities and corresponds to the case where none of the vehicles in the vicinity of the measuring site are in the cold-start mode. It therefore follows that the variation of CO vehicle emissions with temperature does not have an important effect on CO air quality at these two urban centers. The highest CO concentrations are observed during the colder months in most American cities and the differences in meteorological conditions between winter and summer that might contribute to this are considered. Some significant factors are that, in the winter, multiday episodes of slowest dilution occur as well as nocturnal inversions of greater stability. From these considerations it is concluded that the meteorological factors play a significant role in the observation of high CO concentrations in urban areas during the colder months.