© Author(s) 2008. This work is distributed under the Creative Commons Attribution 3.0 License. Atmospheric Chemistry and Physics

Abstract. The NO-NO2 system was analyzed in different chemical regimes/air masses based on observations of reac-tive nitrogen species and peroxy radicals made during the in-tensive field campaign MIRAGE-Mex (4 to 29 March 2006). The air masses were categorized into 5 groups based on com-binations of macroscopic observations, geographical loca-tion, meteorological parameters, models, and observations of trace gases: boundary layer (labeled as “BL”), biomass burning (“BB”), free troposphere (continental, “FTCO ” and marine, “FTMA”), and Tula industrial complex (“TIC”). In general, NO2/NO ratios in different air masses are near pho-tostationary state. Analysis of this ratio can be useful for testing current understanding of tropospheric chemistry. The ozone production efficiency (OPE) for the 5 air mass cate-gories ranged from 4.5 (TIC) to 8.5 (FTMA), consistent with photochemical aging of air masses exiting the Mexico City Metropolitan Area