Three dimensional air quality models have established themselves as valuable tools for the development of emission control strategies. They give scientists the ability to investigate the consequences of multiple emission reduction scenarios as well as research optimum sets of pollutant and/or precursor emission reductions under various meteorological conditions. This capability is becoming crucial as advances in atmospheric science in the past decades have revealed numerous links between ground-level air pollution problems such as ozone, acid rain and particulate matter (PM) (Hidy et al., 1998). With the recognition of PM, especially, as a severe human health concern (Samet et al., 2000; Brook et al., 2002), efforts have lately focused on addressing this new issue without compromising any achievement in reducing ozone and acid rain. As demonstrated by (1997), decreasing VOCs, which are ozone precursors, could free nitrogen oxides and results, under certain conditions, in an increase in PM mass. The so-called ‘one atmosphere’ models are therefore desirable to study PM issues.