Local to Regional Heterogeneity in Greenhouse Gas - Air Pollutant Emission Control Co-Benefits: New Insights from Emission Inventory and Ambient Measurement Syntheses in Boston and Salt Lake City

Emerging urban commitments to reducing greenhouse gas emissions are expected to have co-benefits for local air quality due to the co-emission of important air pollutants including nitrogen oxides (NO_x). To what extent will specific carbon-focused policies improve urban air quality? The expected benefits from a particular policy may vary from city-to-city, and even spatially within a single city due to the heterogeneity of dominant emission sources and the role of meteorology, among other factors. We use a state-of-the-art CO₂ bottom-up emission inventory at 1 x 1 km scale in combination with state-specific NO_x:CO₂ emission ratios by sector to develop a new spatially and temporally resolved NO_x emission inventory. With these inventories, we quantify the potential impact of specific sectors on both greenhouse gas and air pollutant emissions for Boston and Salt Lake City and examine the spatial heterogeneity and sensitivity of co-benefits to prevailing meteorology. We choose these cities due to their contrasting urban form and emission composition, and the unique availability of urban CO₂ concentration observations co-located with air quality measurements.

As expected, on-road emissions are consistently large sources of both greenhouse gas and NO_x emissions in both cities across all seasons. This sector also exhibits the most pronounced diurnal and day of week behavior in the inventories, so that air quality benefits may depend on what time of day these impacts are most relevant to human health. However, we find that dominant emissions sectors are not the same between cities, and have strong seasonal variation. For example, the electrical generation and residential sectors dominate CO₂ emissions (~50% of total) in Boston during the winter, but only contribute 5% and 16% respectively of urban NO_x emissions for the same season. In contrast, in Salt Lake City wintertime CO₂ emissions are dominated by on-road gasoline and diesel vehicle emissions (~50% of total), which also contribute 60% of urban NO_x emissions. We connect this inventory analysis to ambient measurements of CO₂ and NO_x in our study areas to confirm the importance of various emission sectors on ambient air quality and greenhouse gas concentrations over space and time. For example, the weekday-weekend difference in Boston NO_x emissions (~20%), dominated by changes in diesel emissions (30%), is consistent with ambient measurements of NO_x enhancements. Likewise, ambient measurements of NO_x and CO₂ in Boston and Salt Lake City sorted by air mass history show evidence of spatial heterogeneity in the emissions sources consistent with our prior inventories. For example, northwest of Boston the integrated NO_x:CO₂ emission ratio is substantially higher (1.35 g/kg) than southwest of the city (1.05 g/kg), due to the spatial distribution of electricity generating units within the city. We leverage these new observational constraints in combination with our bottom-up inventory analysis to infer how air quality benefits from specific greenhouse-gas focused interventions may depend on the intersection of neighborhood-level emissions, population spatial distribution, and prevailing meteorology.