Ozone sensitivity to its precursor emissions in northeastern Mexico for a summer air pollution episode

A summer episode was modeled to address the expected response of ambient air O-3 to hypothetical emission control scenarios in northeastern Mexico, and in particular in the Monterrey Metropolitan Area (MMA). This region is of interest because the MMA holds one of the worst air quality problems in the country and levels of air pollutants in the rest of northeastern Mexico are starting to be a concern. The MM5-SMOKE-CMAQ platform was used to conduct the numerical experiments. Twenty-four control scenarios were evaluated, combining the level of emission controls of O-3 precursors (NOx and volatile organic compounds [VOCs]) from 0% to 50%. For the MMA, VOC-only controls result in the best option to reduce O-3 concentrations, though the benefit is limited to the urban core. This same strategy results in negligible benefits for the rest of northeastern Mexico. NOx controls result in an increase in O-3 concentration within the MMA of up to 20 ppbv and a decrease at downwind locations of up to 11 ppbv, with respect to the base-case scenario. Indicator ratios were also used to probe for NOx-sensitive and VOC-sensitive areas. Locations with an important influence of NOx point sources (i.e., Monclova and Nava/Acuna) are quite sensitive to changes in NOx emissions. Border cities in the Rio Bravo/Grande Valley tend to be marginally NOx-sensitive. Overall, the MMA seems to be dominated by a VOC-sensitive regime, while the rest of the region would tend to have a NOx-sensitive response. The results obtained serve to expand the current knowledge on the chemical regimes that dominate this region (VOC- or NOx-sensitive), and thus could help guide public policies related to emission regional control strategies. Implications: Updated information on the expected response of ambient air O-3 to emission changes in northeastern Mexico is presented. Results suggest that emission control strategies for the Monterrey Metropolitan Area should focus on VOC reductions or combined VOC-NOx reductions, while the Lower Rio Bravo/Grande Valley border has a negligible response to the emission scenarios tested. Changes in NOx emissions from Carbon II, a 1,400-MW coal-fired electric utility, would have a nonnegligible transboundary effect. Supplemental Materials:Supplemental materials are available for this paper. Go to the publisher's online edition of the Journal of the Air & Waste Management Association.