Assessment of the Ozone-Nitrogen Oxide-Volatile Organic Compound Sensitivity of Mexico City through an Indicator-Based Approach: Measurements and Numerical Simulations Comparison

摘要

The ozone (O_3) sensitivity to nitrogen oxides (NO_X, or nitric oxide [NO] + nitrogen dioxide [NO_2]) versus volatile organic compounds (VOCs) in the Mexico City metropolitan area (MCMA) is a current issue of scientific controversy. To shed light on this issue, we compared measurements of the indicator species O_3/NO_y (where NO_y represents the sum of NO + NO_2 + nitric acid [HNO_3] + peroxyacetyl nitrate [PAN] + others), NO_y, and the semiempirically derived O_3/NO_(Z surrogate)ate (where NO_(Z surrogate)rnis the derived surrogate NO_Z, and NO_Z represents NO_X reaction products, or NO_y - NO_X) with results of numerical predictions reproducing the transition regimes between NO_X and VOC sensitivities. Ambient air concentrations of O_3, NO_X, and NO_y were measured from April 14 to 25, 2004 in one downwind receptor site of photo-chemically aged air masses within Mexico City. MCMA-derived transition values for an episode day occurring during the same monitoring period were obtained through a series of photochemical simulations using the Multi scale Climate and Chemistry Model (MCCM). The comparison between the measured indicator species and the simulated spatial distribution of the indicators O_3/ NO_y, O_3/NO_(Z surrogate), and NO_y in MCMA suggest that O_3 in this megacity is likely VOC-sensitive. This is in opposition to past studies that, on the basis of the observed morning VOC/NO_X ratios, have concluded that O_3 in Mexico City is NO_x-sensitive. Simulated MCMA-derived sensitive transition values for O_3/NO_y, hydrogen peroxide (H_ZO_Z)/HNO_3, and NO_y were found to be in agreement with threshold criteria proposed for other regions in North America and Europe, although the transition crossover for O_3/NO_Z and O_3/HNO_3 was not consistent with values reported elsewhere. An additional empirical evaluation of weekend/weekday differences in average maximum O_3 concentrations and 6:00- to 9:00-a.m. NO_X and NO levels registered at the same site in April 2004 indirectly confirmed the above results. A preliminary conclusion is that additional reductions in NO_X emissions in MCMA might cause an increase in presently high O_3 levels.