Improving spatial concentration estimates for nitrogen oxides using a hybrid meteorological dispersion/land use regression model in Los Angeles, CA and Seattle, WA

Predictions from a simple line source dispersion model, Caline3, were included as a covariate in a land use regression (LUR) model for NO_X/NO₂ in Los Angeles, CA and Seattle, WA. The Caline3 model prediction assumed a unit emission factor for all roadway segments (1.0 g/vehicle-mile). The NO_X and/or NO₂ measurements for LA and Seattle were obtained from a comprehensive measurement campaign that is part of the Multi-Ethnic Study of Atherosclerosis Air Pollution Study (MESA Air). The measurement campaigns in both cities were approximately 2 weeks in duration employing approximately 145 measurement sites in Greater LA and 26 sites in Seattle. The best “standard” LUR model (obtained without the inclusion of the Caline3 predictions) in LA had R² values of 0.53 for NO_X and 0.74 for NO₂. The leave-one-out cross-validated R² values for NO_X and NO₂ were 0.45 and 0.71, respectively. The equivalent “standard” NO₂ model for Seattle had an R² of 0.72 and a leave-one-out cross-validated R² of 0.63. When the Caline3 variable was included in the LA hybrid model, the R² values were 0.71 and 0.79 for NO_X and NO₂, respectively. The corresponding cross-validated R² values were 0.66 and 0.77, for NOX and NO2, respectively. In Seattle, the inclusion of the Caline3 variable resulted in a NO₂ model with an R² of 0.81 and a corresponding cross-validated R² of 0.67. In LA, hybrid model performance was not affected by excluding roadways with annual average daily traffic volumes (AADT) < 100,000. When the Caline3 predictions for heavy-duty trucks and lighter-duty vehicles were modelled as separate terms, the estimated fleet average NO_X emission factors were 8.9 (SE = 0.7) and 0.16 (SE = 0.12) grams NO_X/vehicle mile for heavy-duty and lighter-duty vehicles, respectively. These values are consistent with fleet average emission factors computed for LA with EMFAC 2007.