## 3 Experimental Section  
### Site Description  
This research was conducted at three atmospheric measurement sites that extended across an urban-to-rural gradient from downtown Boston, to the medium-sized city of Worcester, MA, and to the Harvard Forest Long Term Ecological Research (LTER) site in Central Massachusetts (Figure 8). Our eastern and most urban measurement location was a 2.0 m tall instrument tower located on the 29.0 m tall roof of the College of Arts and Sciences (CAS) building on the campus of Boston University(42.35 N, 71.10 W), in Boston, MA, USA. The base of the building is approximately 3.8 m above sea level (m a.s.l.). The tower is located within a heavily developed urban area \(\sim\)0.16 km from Interstate 90 and \(\sim\)4 km from the downtown high-rise buildings. The BU Law building (\(\sim\)60 m a.s.l.) and the Warren Towers (\(\sim\)45 m a.s.l.) are approximately 70 and 180 m from the test site, respectively. Boston, with a population of approximately 600,000 people, is characterized by high-density urban development with parks interspersed. There are three power plants (all utilizing natural gas or oil) and one large regional airport within 16 km of the tower site [52].  
Our central MA location was 65 km west of downtown Boston on a 21.7 m tall building roof on the Worcester State University campus (42.27 N, 71.84 W), which is on the western side of Worcester, MA. The base of the building is approximately 173.5 m a.s.l. Worcester is a secondary urban center with a population of 180,000 people. This site is characterized by large tracts of forest to the west and urbanized areas to the east. The Worcester Regional Airport and Interstate 290 are located 1.6 km to the west and 4 km to the east of the tower site, respectively. There are also several large industrial point source emissions within 16 km of the Worcester tower site including the Saint-Gobain Abrasives and Wheelabrator Millbury, Inc power plants [52].  
Our western MA and most rural site is located 41 km northwest of Worcester and 94 km northwest of downtown Boston at the Harvard Forest LTER site in Petersham, MA. Atmospheric observations were made on the Prospect Hill tract (42.54 N, 72.17 W, elevation 340 m) at the Environmental Monitoring Site (HF EMS), which is a 30-m tall tower that extends above the forest canopy [54, 55].  
Figure 8: Land cover across our eastern Massachusetts study area, derived from Mass GIS data layers [53]. Impervious surface fraction is between 0 (no constructed impervious surfaces) and 1 (completely covered by constructed impervious surface).  
The base of the tower is approximately 349.4 masl. This area is characterized by low human population and by a mixed broadleaf forest dominated by red oak (_Quercus rubra_) and red maple (_Acer rubrum_) with moderately high biomass (115 mg C ha\({}^{-1}\)) [56]. The most proximate source of anthropogenic CO\({}_{2}\) emissions is Route 2, located \(\sim\)5 km north of the tower. There are no large point sources of CO\({}_{2}\) within the Harvard Forest study area [52].  
For this study, we examined meteorological observations and CO\({}_{2}\) mixing ratios at all three tower locations for 2011. We considered the biogenically dominated CO\({}_{2}\) mixing ratios from Harvard Forest to be the background signature for Eastern Massachusetts. Worcester and Boston represented points of increasing anthropogenic influence along Boston's urbanization gradient. The Boston, Worcester, and Harvard Forest areas had population densities of approximately 4,900, 2,800, and 9 people/km\({}^{2}\), respectively [57]. Viewed together, these three sites constituted both an urban to rural gradient and an anthropogenic to biogenic carbon flux gradient.