Monterrey, Mexico ozone study: Air quality measurements and photochemical modeling

Abstract

The Monterrey Metropolitan Area is one of the fastest growing areas of Mexico. As a result of this growth, this major urban and industrial center has begun to experience air quality degradation. The Mexican ozone standard (110 ppbv) has been exceeded on 63 days during the period 1993-1995. This study is the first integrated examination of air quality, pollutant emissions, and climate to cover the ozone behavior in Monterrey. Pollutant concentrations and weather conditions were characterized spatially and temporally for all seasons during 1995. Higher ozone concentrations were found in the southwestern region during Spring. Diurnal patterns of ambient concentration ratios and distributions of daily ozone maximum for weekdays and weekends indicate that industrial emissions seem to be a important contributor of ozone formation. Pollutant emissions were estimated from continuous data of air quality network and on-road vehicle emissions. Diurnal patterns of CO and NOx emissions were calculated for January and July 1995 using a flow model based on mass conservation and estimated parameters of lateral advection and chemical loss. To predict how ozone will change in response to prescribed changes in emissions, this emission inventory is consistent and the most reliable. Circulation patterns associated to high-ozone episodes were identified from synoptic meteorological maps. High-pressure systems, migratory anticyclones, elevated mixed layer, cold front passages and mountain-valley winds produced subsidence, flow reversal, and stagnation when elevated ozone affected Monterrey. Hydrocarbon reduction was determined as the most effective strategy for reducing ozone applying a photochemical box model to high-ozone episodes in 1995. Using sensitivity analysis, emissions and meteorological variables were ranked by their relative contributions to ozone formation. The model employs the Carbon Bond Mechanism IV and a hybrid method for solving stiff chemical kinetic equations. The application of this model highlights the major mesoscale and synoptic features of the Monterrey climate. Incursions of air masses of subtropical and midlatitude circulation regimes are highly variable, while high solar radiation occurs all year.