Abstract. Observations at a mountain-top site within the Mexico City basin are used to characterize ozone production and destruction, nitrogen oxide speciation and chemistry, and the radical budget, with an emphasis on a stagnant air mass observed on one afternoon. The observations compare well with the results of recent photochemical models. An ozone production rate of ~50 ppbv/h was observed in a stagnant air mass during the afternoon of 12 March 2006, which is among the highest observed anywhere in the world. Approximately half of the ozone destruction was due to the oxidation of NO₂. During this time period ozone production was VOC-limited, deduced by a comparison of the radical production rates and the formation rate of NO_x oxidation products (NO_z). For [NO_x]/[NO_y] values between 0.2 and 0.8, gas-phase HNO₃ typically accounted for less than 10% of NO_z and accumulation-mode particulate nitrate (NO_3(PM1)⁻) accounted for 20%–70% of NO_z, consistent with high ambient NH₃ concentrations. The fraction of NO_z accounted for by the sum of HNO_3(g) and NO_3(PM1)⁻ decreased with photochemical processing. This decrease is apparent even when dry deposition of HNO₃ is accounted for, and indicates that HNO₃ formation decreased relative to other NO_x "sink" processes during the first 12 h of photochemistry and/or a significant fraction of the nitrate was associated with the coarse aerosol size mode. The ozone production efficiency of NO_x on 11 and 12 March 2006 was approximately 7 on a time scale of one day. A new metric for ozone production efficiency that relates the dilution-adjusted ozone mixing ratio to cumulative OH exposure is proposed.