## 2 Results and Discussion
### Land Cover and Relationship to CO\({}_{2}\)  
The differences we observed between our three study sites suggest that the land cover around a measurement location can influence observed mixing ratios of CO\({}_{2}\). For example, we found that observations near a densely populated area with high traffic emissions (Boston) exhibited higher atmospheric mixing ratios than a site surrounded by forest (Harvard Forest). However, measurements in heterogeneous urban or suburban areas reflect a mosaic of land covers and depend on local meteorological conditions. To better quantify the influence of land cover on atmospheric CO\({}_{2}\), we conducted a more detailed analysis of the Worcester site, which has both large tracts of forests and urban development nearby (Figure 7). To the west of the Worcester site, forests dominate the land cover. To the east, residential, commercial/industrial, and other developed land uses dominate. Downtown Worcester--including Interstates 90, 190, and 290--is located between 40 and 170 degrees relative to the Worcester tower. Figure 6: A Landsat Enhanced Vegetation Index (EVI) time series for Boston, Worcester and Harvard Forest. Curves were fit using LOESS. Up arrows (\(\uparrow\)) and down arrows (\(\downarrow\)) represent green-up and brown-down timing, respectively, at each site. Total impervious surface area (ISA), which includes buildings, roads, and compacted man made soils, and mean EVI reflect these land covers (Figure 7). CO\({}_{2}\) mixing ratios measured from the primarily forested sector to the west (between 180 and 360 degrees) were on average 6.7 \(\pm\) 1.8 ppm lower than CO\({}_{2}\) mixing ratios observed from the urban sector to the east (between 0 and 180 degrees) (Figure 2). This provides further evidence that biogenic and anthropogenic fluxes associated with different land cover types likely influence observed atmospheric CO\({}_{2}\) mixing ratios. For example, West Worcester exhibited much higher tree canopy cover (65%) and corresponding estimated biogenic fluxes (-9.1 and +7.48 mg C ha\({}^{-1}\) yr\({}^{-1}\) for GPP and E.R., respectively) than East Worcester (46% canopy, -6.44 and +5.29 mg C ha\({}^{-1}\) yr\({}^{-1}\) for GPP and E.R., respectively). Anthropogenic emissions for East Worcester (11.16 mg C ha\({}^{-1}\) yr\({}^{-1}\)) were relatively high compared to West Worcester and Harvard Forest, suggesting that lower levels of canopy cover can serve as a proxy for human activity and associated anthropogenic emissions [51]. There were also seasonal differences exhibited between the sectors. The mean summer CO\({}_{2}\) mixing ratio for West Worcester was 389.4 ppm, compared to 395.2 ppm for East Worcester. These lower values in the west sector suggest increased photosynthesis, which is reflected in the higher EVI (0.56) and lower ISA (13.6%) exhibited by the west sector. Despite higher ISA (44.0%) and lower EVI (0.41) in East Worcester, the estimated ecosystem respiration comprised the largest single source of CO\({}_{2}\) to  
Figure 7: A basemap with EVI of the two sectors at Worcester tower site, derived from [PERSON] and [PERSON], 2012 [50]. Worcester was chosen for this analysis due to its proximity to large tracts of forest to the west and to Worcester’s urban core, visible to the southeast. The 1 km and 5 km radii test areas were used to estimate biogenic emissions and correlate surrounding land cover to CO\({}_{2}\) observations, respectively. the atmosphere (3.1 mg C ha\({}^{-1}\) yr\({}^{-1}\)), underscoring the importance of vegetation cover on local carbon fluxes, even within developed areas. In fall, the mean CO\({}_{2}\) mixing ratios for West and East Worcester were 404.0 and 416.1 ppm, and in winter, they were 407.0 and 419.6 ppm, respectively. The larger difference in mixing ratios between the two sectors in the fall and winter could be a result of proportionally greater increases in residential anthropogenic emissions in East Worcester relative to less-developed West Worcester during the cooler months.