## 2 Results and Discussion
### Trends in Observed CO\({}_{2}\)
The daily minimum in CO\({}_{2}\) occurred in the early afternoon hours as atmospheric mixing and photosynthesis were maximized. At Harvard Forest, this same diurnal pattern occurred during the summer when photosynthesis and respiration were both large, but was absent during the winter months when CO\({}_{2}\) hovered around 400 ppm, reflecting the low local anthropogenic emissions (Figure 2), minimal photosynthesis, and reduced ecosystem respiration due to low temperatures [42]. We observed the largest diurnal variability in CO\({}_{2}\) during the summer months with maximum diurnal amplitudes of 29.2, 31.6, 31.1, and 29.0 ppm at Boston, East Worcester, West Worcester, and Harvard Forest, respectively (Figure 4). These results vary slightly from observations across Portland, OR's urbanization gradient during summer and fall where amplitudes were higher in rural (33 ppm) and suburban (29.5 ppm) areas compared to the downtown core (25 ppm) [41]. Differences in both the absolute magnitude in CO\({}_{2}\) mixing ratios and their relationship with urban development between Boston and Portland's urbanization gradients reflect local meteorology, emissions, and the influence of deciduous _versus_ evergreen vegetation exchange dynamics. For example, the Portland area has a greater number of conifers and a more temperate climate than Boston, which could result in biogenic fluxes that are greater in magnitude and driven more by moisture availability. Diurnal patterns in mobile and total emissions in Boston and Worcester (east and west sectors combined) reflected human activity with overall emissions increasing around 7 am and remaining high through 8 pm (Figure 5). Mid-day weekday CO\({}_{2}\) mixing ratios in Boston and Worcester were 5.1 and 2.3 ppm greater than on weekends, respectively. There was no statistically significant weekend effect observed at Harvard Forest. When integrated across the day, Vulcan mobile source emission estimates were 42.7% and 58.7% higher during the weekday compared to weekends in Boston and Worcester,  
Figure 4: Seasonal deviation from the 24 hour median CO\({}_{2}\) mixing ratio in (**a**) Harvard Forest, (**b**) West Worcester, (**c**) East Worcester, and (**d**) Boston. The Worcester system was established in April of 2011. Therefore, a full seasonal analysis was not possible. respectively, which is consistent with elevated CO\({}_{2}\) mixing ratios observed during weekdays at each site. Observational studies in Portland and Phoenix showed weekday/weekend differences as high as 4.0 and 14.4 ppm, respectively [15; 41], while a study from suburban Baltimore showed no significant weekend difference [17]. Weekend effects reflect the importance of local commuting patterns on observations of CO\({}_{2}\) mixing ratios. Despite being a relatively well-mixed gas, the imprint of human and biogenic activity can be seen in both the short-term and long-term signals of CO\({}_{2}\) across Boston's urbanization gradient. Moreover, many of the changes in CO\({}_{2}\) mixing ratios across the gradient were caused by alteration of land cover and the concomitant changes in vegetated fraction and anthropogenic emissions, as seen in Figure 2. These data suggest significant, direct alteration of CO\({}_{2}\) mixing ratios due to urban land cover change and associated anthropogenic activities.