## 2 Results and Discussion
### Trends in Observed CO\({}_{2}\)  
The spatial and temporal variability in carbon fluxes hinders simple characterization of the primary determinants of local CO\({}_{2}\) observations, especially in urban areas where the land cover is heterogeneous and topography is complex. Despite these challenges, we observed variations in CO\({}_{2}\) mixing ratios across Boston's urbanization gradient that were consistent with vegetation and urbanization patterns at each site (Figure 2). For the Worcester site, which is the midpoint in our urbanization gradient, we separately characterized the results for air originating from the urbanized area to the east (0 to 180 degrees) and the rural area to the west (180 to 360 degrees) of the site. Henceforth, we will refer to these urban and rural sectors as East Worcester and West Worcester, respectively. Figure 2: Estimates of carbon flux and canopy cover across Boston’s urbanization gradient. Anthropogenic emissions estimates (\(\uparrow\)) are based on the Vulcan [39] dataset and estimates of human respiration for the 33 km \(\times\) 33 km focal areas shown in each panel. Canopy percentage and biogenic fluxes (both \(\uparrow\) and \(\downarrow\)) were estimated within the 1 km radius around each tower (red circles). In Worcester, statistics were split into easterly and westerly sectors that represent half the areal coverage and are delineated by the dashed line. **GPP** = Gross primary production, **E.R. ** = Ecosystem respiration, **Human** = Human CO\({}_{2}\) respiration, **Mob. ** = Mobile source emissions, **Res. ** = Residential emissions, and **Other** = All other fossil fuel emissions. All fluxes are in mg C ha\({}^{-1}\) yr\({}^{-1}\). All atmospheric CO\({}_{2}\) measurements are time weighted annual means with bootstrapped 95% confidence intervals. In 2011, mean CO2 mixing ratios in Boston were 8.8 ppm higher than air originating from East Worcester, 15.5 ppm higher than air originating from West Worcester, and 20.6 ppm higher than observations at Harvard Forest. These observations were consistent with the patterns in local anthropogenic and biogenic fluxes. Across all sites these differences amounted to a roughly 5% difference in mean annual CO2 mixing ratios, despite the combination of a large biotic imprint on atmospheric CO2 in the rural areas and large anthropogenic emissions in the urban areas. The 2011 annual mean observed CO2 mixing ratios in Boston, East Worcester, West Worcester, and Harvard Forest were 393.4 \(\pm\)0.15, 398.5 \(\pm\)0.23, 405.2 \(\pm\)0.45, and 414.0 \(\pm\)0.21 ppm, respectively (Figure 2). The trends in CO2 mixing ratios at all sites showed seasonal shifts with winter enhancement--associated with heating related emissions and ecosystem respiration--and summer draw-down, coinciding with enhanced ecosystem productivity and reduced anthropogenic emissions. Anthropogenic emissions for all sectors decreased significantly from Boston to Harvard Forest (Figure 2). Total annual estimated fossil fuel emissions for Boston (excluding the area covered by water), East and West Worcester, and Harvard Forest were 34.7, 5.9, 1.97, and 1.53 Mg C ha-1 yr-1, respectively. The composition of anthropogenic emissions also changed across Boston's urbanization gradient: emissions from other sources (such as industrial and commercial) decreased as a percentage of total emissions as urbanization decreased. There were also large seasonal differences in anthropogenic emissions: the \(\sim\)47% increase in total emissions between summer and winter at all sites (Figure 1) was driven by the \(\sim\)480% increase in residential emissions between these seasons. Patterns in the estimated biogenic fluxes showed the opposite trend; gross primary productivity (photosynthesis) and ecosystem respiration increased from urban Boston to rural Harvard Forest (Figure 2). This increase in biogenic fluxes was associated with the increase in forest canopy from east to west across the region.