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)/NO(2) in Los Angeles, CA and Seattle, WA. The Caline3 model prediction assumed a unit emission factor for all roadway segments (1.0g/vehicle-mile). The NO(X) and/or NO(2) 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 2weeks 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 R(2) values of 0.53 for NO(X) and 0.74 for NO(2). The leave-one-out cross-validated R(2) values for NO(X) and NO(2) were 0.45 and 0.71, respectively. The equivalent "standard" NO(2) model for Seattle had an R(2) of 0.72 and a leave-one-out cross-validated R(2) of 0.63. When the Caline3 variable was included in the LA hybrid model, the R(2) values were 0.71 and 0.79 for NO(X) and NO(2), respectively. The corresponding cross-validated R(2) values were 0.66 and 0.77, for NOX and NO2, respectively. In Seattle, the inclusion of the Caline3 variable resulted in a NO(2) model with an R(2) of 0.81 and a corresponding cross-validated R(2) 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.