Characterization of a spatial gradient of nitrogen dioxide across a United States–Mexico border city during winter

A gradient of ambient nitrogen dioxide (NO2) concentration is demonstrated across metropolitan El Paso, Texas (USA), a city located on the international border between the United States and Mexico. Integrated measurements of NO2 were collected over 7 days at 20 elementary schools and 4 air quality monitoring stations located throughout the city during typical winter atmospheric conditions. Replicate passive monitors were co-located with chemiluminescence analyzers at the monitoring stations for two consecutive 7-day periods. The passive measurements correlated with the analyzer measurements (R2=0.74) with precision of 2.5±2.2 ppb. Nitrogen dioxide concentrations ranged from 11.0 to 37.5 ppb (mean 20.6±7.1 ppb). In a multivariate regression model, the site elevation and distances to a main highway and to an international port of entry from Mexico explained 81% of the variance in the passive measurements. The results of this pilot study indicate that proximity to vehicle-related sources of NO2 and site elevation are key predictors for future, more detailed assessments of vehicle-related air pollution exposure in the El Paso region