The NO-NO<sub>2</sub> system was analyzed in different chemical regimes/air masses based on observations of reactive nitrogen species and peroxy radicals made during the intensive field campaign MIRAGE-Mex (4 to 29 March 2006). In general, NO<sub>2</sub>/NO ratios, which can be used as an indicator to test current understanding of tropospheric chemistry mechanism, are near photostationary state. The air masses were categorized into 5 groups: boundary layer (labeled as "BL"), free troposphere (continental, "FTCO" and marine, "FTMA"), biomass burning ("BB"), and Tula industrial complex ("TIC"). The time- and air mass-dependent NO<sub>2</sub>/NO ratios ranged from 2.35 (TIC) to 5.18 (BB), while the NO<sub>x</sub>/NO<sub>y</sub> ratios varied from 0.17 (FTCO) to 0.54 (BL). The ozone production efficiency for the 5 air mass categories ranged from 5.0 (TIC) to 10.2 (BL), indicating photochemically young and reactive air masses.