The amount of air pollution created by vehicles in cities is almost linearly proportional to the vehicle kilometres travelled (VKT). This has been found to be proportional to the size of the metropolitan area, using data from dozens of megalopolis in the world, such as London, Melbourne, Sydney, Los Angeles and New York (1).
In other words, urban sprawl generates air pollution. For instance, VKT is about 4 x 1010 km/year for a city of 3,000 km2. It follows that the emission rate (in units of grams per square metre per second) is about 5.7 x 10-7 times a special factor for each pollutant. This relationship is appropriate for estimating concentrations of NOx and particulates, which are the main emissions during long trips. This includes lead pollution in countries that still use leaded fuel.
However this linear relationship is less suitable for carbon monoxide and hydrocarbon concentrations, which result especially from start-up and shut-down conditions, important in short trips. So CO and HC pollution is affected little by urban sprawl, but can be reduced by the use of car pools or public transportation.
(1) Lyons, T., J.R. Kenworthy, C. Moy and F. dos Santos 1998. A global urban air pollution model. 2nd Conf. on Urban Environment, Albuquerque, New Mexico (Amer. Meteor. Soc.), preprints p26-9.