Observational Inferences of NO_<i>x</i> and CO Emission Factors for Vehicles and Homes in the San Francisco Bay Area

We present the seasonal variations of enhancement ratios (ERs, i.e., ΔNOx/ΔCO2 and ΔCO/ΔCO2) as a function of distance from highways in the San Francisco Bay Area, using observations from the Berkeley Environmental Air Quality and CO2 Network (BEACO2N) at 40 locations. The spatial patterns exhibit exponential distance-decay relationships, with higher NOx and CO ERs near highways and more uniform ERs at distances beyond 3 km. These patterns are used to infer emission factors (EFs) for transportation and residential buildings. BEACO2N-derived EFs for CO (7.8 ± 0.6 ppbv/ppmv) and NOx (1.0 ± 0.02 ppbv/ppmv) from transportation agree with inventory estimates. In contrast, the residential NOx EF (0.15 ± 0.01 ppbv/ppmv) is four times lower than inventory estimates, and the residential CO EF (4.3 ± 0.3 ppbv/ppmv) is 33% lower than the California state inventory estimate.