Mexico City is one of the largest cities in the world and also one of the most polluted. It routinely exceeds standards for tropospheric ozone (O₃). O₃ is created by the oxidation of hydrocarbons in the presence of nitrogen oxides NOx (= NO + NO₂) and ultraviolet (UV) radiation. Mexico City’s loc... Show moreMexico City is one of the largest cities in the world and also one of the most polluted. It routinely exceeds standards for tropospheric ozone (O₃). O₃ is created by the oxidation of hydrocarbons in the presence of nitrogen oxides NOx (= NO + NO₂) and ultraviolet (UV) radiation. Mexico City’s location, at an altitude of 2230 m in the tropics, results in strong UV irradiation while the surrounding mountains trap pollutants. The hypothesis that ozone formation in Mexico City is controlled by NOx availability (NOx limited) is highly controversial. We explored this hypothesis using atmospheric data. We used the dataset of RAMA (Red Automática de Monitoreo Atmosférica) containing hourly measurements of O₃, NOx, and NO₂ concentrations over 13 years taken from 32 sites around Mexico City. We organized and analyzed the data using Fortran programs. Monthly plots of daytime O₃, NOx, and CO averaged over all stations indicated that levels of these species decreased from the years 1986-1994 to years 1995-1999. This observation reflects the air pollution control strategies that Mexico City implemented between those two periods. Correlation plots of station-averaged Ox versus NOx showed that as NOx increased so did Ox, indicating a NOx limited regime. Both time periods showed positive correlation slopes for each month indicating a NOx situation year round (except for July 1986-1994). Correlation intercepts, possibly related to background regional ozone, decreased between the two time periods. Ox-NOx correlations support the NOx limited hypothesis, suggesting that decreasing NOx emissions may help to reduce O₃ concentrations in the atmosphere of Mexico City. Show less
