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The production of photochemical atmospheres under controlled conditions in an irradiation chamber permits the manipulation of parameters that influence the resulting air pollutant chemistry and potential biological effects. To date, no studies have examined how contrasting atmospheres with a similar Air Quality Health Index (AQHI), but with differing ratios of criteria air pollutants, might differentially affect health end points. Here, we produced two atmospheres with similar AQHIs based on the final concentrations of ozone, nitrogen dioxide, and particulate matter (PM2.5). One simulated atmosphere (SA-PM) generated from irradiation of similar to 23 ppmC gasoline, 5 ppmC alpha-pinene, 529 ppb NO, and 3 mu g m(-3) (NH4)(2)SO4 as a seed resulted in an average of 976 pig m(-3) PM2.5, 326 ppb NO2, and 141 ppb O-3 (AQHI 97.7). The other atmosphere (SA-O-3) generated from 8 ppmC gasoline, 5 ppmC isoprene, 874 ppb NO, and 2 mu g m(-3) (NH4)(2)SO4 resulted in an average of SS mu g m-3 PM2.5, 643 ppb NO2, and 430 ppb O-3 (AQHI of 99.8). Chemical speciation by gas chromatography showed that photo-oxidation degraded the organic precursors and promoted the de novo formation of secondary reaction products such as formaldehyde and acrolein. Further work in accompanying papers describe toxicological outcomes from the two distinct photochemical atmospheres.