## 3 Experimental Section
### Anthropogenic and Biogenic Carbon Fluxes  
With the exception of human respiration, anthropogenic emission estimates for this analysis were based on the Vulcan CO\({}_{2}\) fossil fuel emission estimates [39, 65]. The Vulcan product includes hourly anthropogenic CO\({}_{2}\) emissions estimates at a 0.1 \(\times\) 0.1 degree (\(\sim\)121 km\({}^{2}\) in the Boston area) spatial resolution for the entire continental US for the year 2002. Vulcan emissions are partitioned into a variety of sectors, including total, residential, commercial, and on-road emissions. For this analysis, total emissions from all sectors were extracted for a 9 pixel area surrounding each tower site. A 9 pixel area was chosen to ensure that the centroids of the three study areas were within 3 km of the tower locations. Annual, seasonal, daily, and diurnal anthropogenic emissions estimates were calculated over the 9 pixel area (1,089 km\({}^{2}\) areas). At all three sites, the emissions and mixing ratios were analyzed by 45 \({}^{\circ}\) and 180 \({}^{\circ}\) sections aligned to the cardinal directions. Only the results from Worcester were significantly variable by sector (and only for the east and west 180 \({}^{\circ}\) sectors) due to the distinct boundaries between vegetation and more intensively developed areas. These emissions represent estimated local emissions, rather than demand-induced emissions elsewhere. For example, the effect of emissions from air conditioning at these tree sites is difficult to assess since most of the electricity was generated outside of our study areas. In contrast, most space heating emissions were captured in our analysis since 82%-90% of all residential homes in the Worcester and Boston areas used natural gas and fuel oil for heating [66].  
To estimate CO\({}_{2}\) flux from human respiration, we created a spatially explicit data layer based on a Massachusetts population density map [29] and an estimate of mean per-capita CO\({}_{2}\) flux from respiration (257 g C day\({}^{-1}\) per person [67]). The population density map was based on block-level data from the 2010 US Census [57] and allocated the population within a given census block to the residential land area [53] of that census block.  
At each tower site, we digitized and calculated the canopy cover within a 1 km radius using high resolution QuickBird imagery downloaded from Google Maps [68] and the ImageJ image analysis software [69]. The QuickBird images used for this analysis represent land cover from recent, unspecified dates. The vegetative fraction at each site includes only large woody vegetation, excluding lawns and shrubs. Average annual gross primary production and ecosystem respiration at Harvard Forest was 14.0 and 11.5 mg C ha\({}^{-1}\) yr\({}^{-1}\), respectively, between 1992 and 2004 [42]. Since long-term flux tower based estimates of gross primary production are not available for urban areas in our region, we scaled gross primary production and ecosystem respiration linearly based on percent canopy cover, using Harvard Forest as our baseline (97% tree canopy cover). In Boston, a robust relationship between canopy cover and biomass was observed [29], supporting the use of canopy cover as a proxy for biomass and leaf area. These first order biogenic flux estimates were calculated for the 1 km\({}^{2}\) area around each site.