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 NOX/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 NOX 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 NOX and 0.74 for NO2. The leave-one-out cross-validated R2 values for NOX 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 NOX 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 NOX emission factors were 8.9 (SE = 0.7) and 0.16 (SE = 0.12) grams NOX/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.