## 4 Conclusions  
In this study we examined the spatial and temporal variations in atmospheric CO\({}_{2}\) mixing ratios and carbon fluxes across Boston's urbanization gradient. There were large differences in estimated biogenic and anthropogenic carbon fluxes across this gradient with total anthropogenic emissions ranging from 37.3 mg C ha\({}^{-1}\) yr\({}^{-1}\) in urban Boston to 1.5 mg C ha\({}^{-1}\) yr\({}^{-1}\) at the rural Harvard Forest. Despite the \(\sim\)25-fold difference in local emissions, the mean annual difference in atmospheric CO\({}_{2}\) mixing ratios was only 20.6 \(\pm\) 0.4 ppm from the most rural to the most urban ends of our Boston gradient. The atmospheric signal from vegetation was clear in the observed seasonality at all sites, regardless of the amount of local vegetation (Figure 1). We observed significant differences in growing season length across the gradient, with Boston's growing season exceeding Harvard Forest's by 31 days in 2011. This extended growing season in Boston could potentially increase Boston's annual net carbon sequestration by as much as 50%. In densely populated urban Boston, we estimated that human respiration contributed nearly as much CO\({}_{2}\) as ecosystem respiration with each contributing 2.8 and 3.1 mg C ha\({}^{-1}\) yr\({}^{-1}\), respectively, similar to results found in Vancouver, CA, USA [75]. Heterogeneity in land cover across the urban-rural gradient, combined with variable meteorology and concomitant shifts in source areas, created a profound challenge in disaggregating the contributions to the CO\({}_{2}\) signal. The estimated carbon fluxes in this study highlight potential carbon mitigation strategies in urban areas, which are responsible--directly or indirectly--for the majority of anthropogenic emissions. For example, it has already been suggested that net carbon uptake from vegetation and corresponding soils can be significant, even in urban areas [35]. Woody vegetation in Boston is not actively managed to reduce carbon emissions, but our first order estimate of urban vegetation sequestration is nearly 0.7 mg C ha\({}^{-1}\) yr\({}^{-1}\). Grow Boston Greener, which seeks to increase the city's tree canopy cover to 35%, might with time increase the sequestration by an additional 0.2 mg C ha\({}^{-1}\) yr\({}^{-1}\) (assuming productivity scales linearly with canopy cover), but the establishment and maintenance of those new trees can also have significant associated emissions [36]. Our results suggest other strategies may have a larger effect on the area's carbon budget. For example, urban growth strategies which focus on densifying suburban areas rather than clearing new exurban areas could reduce both transportation related emissions (currently estimated at 10.6 mg C ha\({}^{-1}\) yr\({}^{-1}\) in Boston) [76] and emissions associated with forest cover loss [30]. Further, residential emissions are the second largest CO\({}_{2}\) source in Boston (8 mg C ha\({}^{-1}\) yr\({}^{-1}\); Figure 2). Given that over 60% of Boston's housing stock was built before 1939 [77], there are ample opportunities for efficiency improvements from this sector [66]. As cities, regions, and nations move forward with climate action plans and treaties, we need to improve our capacity to measure emissions and carbon exchange within urban areas. With our current measurement networks, modeling techniques, and fundamental understanding of the urban carbon cycle, we are not yet able to adequately characterize urban carbon sources and sinks or to define the influence of urban ecosystems on regional atmospheric composition. The significant overlap of biogenic and anthropogenic processes in these areas makes partitioning of atmospheric mixing ratios into component fluxes difficult. Despite scientific uncertainties, current literature clearly suggests pathways for policymakers and highlights the imperative for CO\({}_{2}\) emission reductions.