Ozone production, nitrogen oxides, and radical budgets in Mexico City: observations from Pico de Tres Padres

Abstract

Observations at a mountain-top site within the Mexico City basin are used to characterize ozone production and destruction, the nitrogen oxide budget, and the radical budget during the MILAGRO campaign. 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 NO sub(2). During this time period ozone production was VOC-limited, deduced by a comparison of the radical production rates and the formation rate of NO sub(x) oxidation products (NO sub(z)) For [NO sub(x)]/[NO sub(y)] values between 0.2 and 0.8, gas-phase HNO sub(3) typically accounted for less than 10% of NO sub(z) and accumulation-mode particulate nitrate (NO sub(3) super(− [/superscript ]) sub(()PM) accounted for 20%-70% of NO sub(z), consistent with high ambient NH sub(3) concentrations. The fraction of NO sub(z) accounted for by the sum of HNO sub(3(g)) and NO sub(3) super(− ) sub((PM)) decreased with photochemical processing. This decrease is apparent even when dry deposition of HNO sub(3) is accounted for, and indicates that HNO sub(3) formation decreased relative to other NO sub(x) QUOTATION_MARKsinkQUOTATION_MARK 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 sub(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.