Factors associated with NO₂ and NO_X concentration gradients near a highway

The objective of this research is to learn how the near-road gradient, in which NO₂ and NO_X (NO + NO₂) concentrations are elevated, varies with changes in meteorological and traffic variables. Measurements of NO₂ and NO_X were obtained east of I-15 in Las Vegas and fit to functions whose slopes (dC_NO2 /dx and dC_NOX /dx, respectively) characterize the size of the near-road zone where NO₂ and NO_X concentrations from mobile sources on the highway are elevated. These metrics were used to learn about the near-road gradient by modeling dC_NO2 /dx and dC_NOX /dx as functions of meteorological variables (e.g., wind direction, wind speed), traffic (vehicle count), NO_X concentration upwind of the road, and O₃ concentration at two fixed-site ambient monitors. Generalized additive models (GAM) were used to model dC_NO2 /dx and dC_NOX /dx versus the independent variables because they allowed for nonlinearity of the variables being compared. When data from all wind directions were included in the analysis, variability in O₃ concentration comprised the largest proportion of variability in dC_NO2 /dx, followed by variability in wind direction. In a second analysis constrained to winds from the west, variability in O₃ concentration remained the largest contributor to variability in dC_NO2 /dx, but the relative contribution of variability in wind speed to variability in dC_NO2 /dx increased relative to its contribution for the all-wind analysis. When data from all wind directions were analyzed, variability in wind direction was by far the largest contributor to variability in dC_NOX /dx, with smaller contributions from hour of day and upwind NO_X concentration. When only winds from the west were analyzed, variability in upwind NO_X concentration, wind speed, hour of day, and traffic count all were associated with variability in dC_NOX /dx. Increases in O₃ concentration were associated with increased magnitude near-road dC_NO2 /dx, possibly shrinking the zone of elevated concentrations occurring near roads. Wind direction parallel to the highway was also related to an increased magnitude of both dC_NO2 /dx and dC_NOX /dx, again likely shrinking the zone of elevated concentrations occurring near roads. Wind direction perpendicular to the road decreased the magnitude of dC_NO2 /dx and dC_NOX /dx and likely contributed to growth of the zone of elevated concentrations occurring near roads. Thus, variability in near-road concentrations is influenced by local meteorology and ambient O₃ concentration.