(1) For "stable" air pollutants (i.e., sulfur dioxide, particulate matter and carbon monoxide) , the determination of whether a source will cause or contribute to a violation of a national ambient air quality standard generally should be made on a case-by-case basis as of the proposed new source's startup date using the source's allowable emissions in an atmospheric simulation model (unless a source will clearly impact on a receptor which exceeds a national ambient air quality standard) .

(2) For sources of nitrogen oxides, the initial determination of whether a source would cause or contribute to a violation of the national ambient air quality standard for nitrogen dioxide should be made using an atmospheric simulation model assuming all the nitric oxide emitted is oxidized to nitrogen dioxide by the time the plume reaches ground level. The initial concentration estimates may be adjusted if adequate data are available to account for the expected oxidation rate.