Contributors to ozone episodes in three US/Mexico border twin-cities.

The Process Analysis tools of the Community Multiscale Air Quality (CMAQ) modeling system together with back-trajectory analysis were used to assess potential contributors to ozone episodes that occurred during June 1-4, 2006, in three populated U.S.-Mexico border twin cities: San Diego/Tijuana, Imperial/Mexicali and El Paso/Ciudad Juárez. Validation of CMAQ output against surface ozone measurements indicates that the predictions are acceptable with regard to commonly recommended statistical standards and comparable to other reported studies. The mean normalized bias test (MNBT) and mean normalized gross error (MNGE) for hourly ozone fall well within the US EPA suggested range of +/-15% and 35%, respectively, except MNBT for El Paso. The MNBTs for maximum 8-h average ozone are larger than those for hourly ozone, but all the simulated maximum 8-h average ozone are within a factor of 2 of those measured in all three regions. The process and back-trajectory analyses indicate that the main sources of daytime ground-level ozone are the local photochemical production and regional transport. By integrating the effects of each process over the depth of the daytime planetary boundary layer (PBL), it is found that in the San Diego area (SD), chemistry and vertical advection contributed about 36%/48% and 64%/52% for June 2 and 3, respectively. This confirms the previous finding that high-altitude regional transport followed by fumigation contributes significantly to ozone in SD. The back-trajectory analysis shows that this ozone was mostly transported from the coastal area of southern California. For the episodes in Imperial Valley and El Paso, respectively, ozone was transported from the coastal areas of southern California and Mexico and from northern Texas and Oklahoma.

Simulation of smoke plumes from agricultural burns: application to the San Luis/Rio Colorado airshed along the U.S./Mexico border.

Vegetation fires emit a number of air pollutants, thus impacting air quality at local, regional and global scales. One such pollutant is the particulate matter (PM) that is known to trigger adverse health effects. In this study, the CALPUFF/CALMET/MM5 modeling system is employed to simulate PM(10) dispersion (PM with aerodynamic diameter less than 10 microm) from agricultural fires in the Yuma/San Luis area along the U.S./Mexico border, with the aim of investigating local and regional air quality impacts of fires. To the extent possible the data collected from and observations made in the study area were employed to infer inputs to the modeling system, but insufficient information available on burning practices and input parameters, such as the duration of fire, PM(10) emission rate and plume rise, necessitated relying on some previously published research as well as the Fire Emission Production Simulator (FEPS) model to provide necessary inputs. Under the simulated conditions the fire plumes did not disperse much, and thus mostly affected the area near the sources. The PM impact of fires on populated (receptor) areas in Yuma/San Luis was less than 15 microg/m(3), calculated on the basis of EPA-recommended 24-hr averaged PM(10). If the formation of secondary particles is considered, the impacts could have been greater. In order to conduct more realistic fire plume simulations, it is imperative to have accurate fire-activity records such as the firing technique applied, fuel condition, time of burning as well as some model updates. In all, this paper presents a methodology for calculating agricultural-burns introduced PM, while identifying critical improvements that need to be made in future work.