## 2 Results and Discussion
### Enhanced Vegetation Index (EVI) Time Series and Phenology Timing  
While emissions variations are clearly associated with urban areas, less direct effects of urbanization can also influence CO\({}_{2}\) fluxes and observed CO\({}_{2}\) mixing ratios. The urban heat island effect, in particular, may alter the balance of respiration and photosynthesis in urban areas relative to nearby rural counterparts [11]. Urban heat islands may also alter seasonal anthropogenic emissions due to changes in heating and cooling degree days. Temperature gradients are frequently observed between  
Figure 5: **(a)** A comparison between diurnal Vulcan mobile source emission estimates for the focal areas surrounding the Boston and Worcester (combined east and west sectors) tower sites for summer weekends and weekdays. **(b)** A weekend and weekday CO\({}_{2}\) mixing ratio comparison at the same sites. Confidence intervals (C.I.) were boostrapped and reflect 90% confidence. urban and rural areas [9, 43]. We observed mean summer temperatures of 21.7, 21.1, and 20.6 \({}^{\circ}\)C in Boston, Worcester, and Harvard Forest, respectively. Increases in temperature across the gradient were likely related to elevation differences, increased incoming longwave radiation (due to a reduced sky view factor and by the presence of atmospheric pollution), decreased latent heat fluxes, increased building and road storage heat flux, and anthropogenic heat emissions. [9, 10, 44]. Higher temperatures associated with urbanization can result in altered vegetation phenology [11, 45]. For example, urban green-up (defined here as 25% leaf emergence) and brown-down (defined here as 90% leaf senescence) tend to occur earlier and later, respectively, than in nearby rural and suburban areas [12]. To examine trends in phenology across the gradient, we used the Enhanced Vegetation Index (EVI), which measures surface greenness. Based on the absolute EVI time series, green-up in 2011 occurred on approximate day of year (DOY) 126, 136, and 141 in Boston, Worcester and Harvard Forest, respectively (Figure 6). Brown-down in 2011 occurred on approximate DOY 304, 284, and 288, respectively. The total growing season length difference between Boston and Harvard Forest in 2011 was 31 days, a potential 20% lengthening in the period for biogenic carbon uptake. While the differential impacts of earlier green-up and later brown-down are still being quantified, for each one-day increase in growing season length, net ecosystem carbon uptake has been found to increase by 4.3 g C m\({}^{-2}\) day\({}^{-1}\) across a range of temperate deciduous forests [46]. Assuming similar productivity per unit canopy cover and a 31 day phenologic change, the extended urban growing season could potentially increase net biogenic carbon sequestration in Boston (27% canopy cover) by as much as 0.36 mg C ha\({}^{-1}\) yr\({}^{-1}\), a 50% increase in net biogenic exchange. However, abiotic growing conditions that affect ecosystem productivity, such as soil moisture, nitrogen availability, and atmospheric ozone, differ significantly between urban and rural areas [13]. As a result, scaling ecosystem productivity by canopy cover should only be considered a first order estimate of the effect of longer growing seasons on carbon uptake. Further, while it is clear that the urban heat island effect significantly alters air and soil temperatures and growing season length in the region, it is difficult to determine what fraction of the lengthened growing season in Boston, relative to Harvard Forest, is due to heat island effects _versus_ local climate and topographic differences between the sites. The timing of green-up and brown-down occurred at different points along the seasonal CO\({}_{2}\) trend (Figure 3). In Boston, CO\({}_{2}\) mixing ratios began to decline in early February, well before the onset of photosynthesis in early May.