## 2 Results and Discussion
### Trends in Observed CO\({}_{2}\)
Biogenic fluxes dominated carbon exchange processes at Harvard Forest and West Worcester and reflect the largely undeveloped, forested character of these areas. Tree canopy cover was 27% within a 1 km radius of the Boston tower: this is consistent with the 29% average overall canopy for the City of Boston [40]. Gross primary production and ecosystem respiration each constitute a substantial portion of total CO2 fluxes in Boston, suggesting considerable biotic influence even within dense urban areas. Differences between the human and vegetation-dominated environments across Boston's urbanization gradient were reflected in the annual standard deviations of CO2 mixing ratios in Boston (17.8 ppm), East Worcester (21.5 ppm), West Worcester (15.9 ppm) and Harvard Forest (14.0 ppm). Higher overall and diurnal variability in Boston and East Worcester was likely due to proximate anthropogenic emissions, such as local traffic, combined with higher air entrainment from surrounding buildings. This variability was also exhibited in the hourly average CO2 mixing ratios and the corresponding seasonal trends (Figure 3). In Harvard Forest, total carbon emissions were relatively low in winter due to low biogenic and anthropogenic fluxes, resulting in CO2 mixing ratios that remained relatively constant over time. Moving towards Boston, CO2 mixing ratios quickly became more sinusoidal and reflected greater levels of anthropogenic emissions, which were quite high during winter months. The variability in the trend of seasonal CO\({}_{2}\) is supported by the changes in the heteroscedasticity and skewness of the frequency distributions of CO\({}_{2}\) observations at these three sites (Figure 3). For example, the data distributions from Boston and East Worcester exhibited a strong positive skewness of +2.8 and +1.1, respectively. On the other hand, hourly CO\({}_{2}\) mixing ratios in West Worcester and Harvard Forest had a slight positive (+0.6) and negative skewness (-0.8), respectively, and exhibited a much lower variance. Without large proximate anthropogenic emissions at these two sites, mixing ratios rarely exceeded 450 ppm. The negative skew at Harvard Forest likely resulted from strong photosynthetic activity. While other studies have observed urban CO\({}_{2}\) mixing ratios well above background levels, the magnitudes of these CO\({}_{2}\) domes varied greatly by location [15,21,41]. For example, mean peak city-center mixing ratios in Phoenix, AZ were 28%-76% higher than local background values, although this finding was likely influenced by highly stable atmospheric conditions resulting from local wintertime atmospheric inversion. In Portland, OR and Melbourne, Australia mean CO\({}_{2}\) mixing ratios at more developed sites were as much as 6 and 12 ppm greater, respectively, than those in corresponding lesser-developed locations. The strength of urban CO\({}_{2}\) domes, including Boston's, is sensitive to local meteorological conditions, emissions, biogenic processes, and the height of the gas analyzer above the surface. These local influences complicate simple generalization or extrapolation of urban carbon domes. Observed diurnal patterns in CO\({}_{2}\) mixing ratios across Boston's urbanization gradient exhibited predictable behavior associated with stratification of the atmosphere, but also showed marked differences as urbanization increased (Figure 4). The diurnal patterns at Boston and Worcester broadly  
Figure 3: Hourly average CO\({}_{2}\) mixing ratios in (**a**) Harvard Forest, (**b**) West Worcester, (**c**) East Worcester, and (**d**) Boston with a LOESS regression trend line. To the right of each panel, a box and whisker plot summarizes the annual data. Open circles represent observations that are more than 1.5 times greater than the inter-quartile range. showed a daily maximum occurring between 4:00 am and 7:00 am, followed by a rapid decrease occurring with sunrise and the associated break-up of the nocturnal boundary layer [20,37].