Intra-Urban Heterogeneity in Greenhouse Gas - Air Pollutant Emissions: A Multi-Year Emission Inventory, Ambient Measurement, STILT Model Synthesis in Boston

Current quantifications of urban anthropogenic GHG and air quality (AQ) emissions exist at coarse spatial and temporal resolutions unable to resolve heterogeneity within urban areas. To better quantify sector contributions driving emission patterns, finer scale sub-city level analysis is needed. Here we combine new high resolution GHG and AQ inventories, co-located in-situ monitor observations of CO₂ and NOx, and STILT model footprints to understand where and when intra-urban CO₂ and co-emitted nitrogen oxides (NOx) are being emitted and how these patterns are changing over time. This synthesis allows us to know what specific sectors are major contributors to CO₂ (and NOx) in an urban area and can inform the expected co-benefits of specific urban CO₂ reduction policies.

We combined the hourly-resolved 1km x 1km ACES CO₂ inventory with state-level sector specific NOx:CO₂ emission ratios to create a new sector-resolved NOx emission inventory at the same resolution. We used STILT-modeled footprints at our NOx and CO₂ measurement location to examine the expectations of local sector contributions to ambient CO₂, NOx concentrations at a downtown Boston location. The correlation between the modeled (footprint + inventory) and observed CO₂ and NOx enhancements is 0.61. The strength of this relationship varies depending on the predominant emission sector contributing to the atmospheric enhancement. This may be evidence that the inventory captures specific sectors better than others. For example, when measurement footprints are primarily composed of on-road gasoline vehicle emissions the correlation increases to 0.67, and when they are dominated by residential emissions it increases again to 0.78. In contrast, the correlation is worse when commercial sectors dominate (0.54).

We also contrast the expectations at our measurement location with city- and state-wide total emissions. Our work corroborates expectations that on-road gasoline vehicle emissions are large sources of greenhouse gas and NOx emissions. At our measurement location on-road gasoline emissions are the largest source (21.8%) of CO₂, though this is lower than their statewide contribution of 36.7%. We find citywide CO₂ emissions are actually dominated by airport emissions (36.4%), although they only account for 1.3% of CO₂ and 0.6% of NOx emissions sampled at our measurement location. This emphasizes the importance of measurement location and atmospheric transport in determining the importance of specific sectors to air quality. The other largest sector of CO₂ emissions in Boston is electricity generating facilities (19.6%), but these do not contribute significantly to NOx emissions (5%). Conversely, on-road diesel vehicles are a large source of NOx emissions (12.8%) with minimal relative CO₂ emissions (4.3%). Our work points towards important challenges urban areas will face in creating policy that captures both greenhouse gas and air quality co-benefits.