## 3 Experimental Section
### Instrumentation and Data Analysis  
In Boston, CO\({}_{2}\) and H\({}_{2}\)O mixing ratios were measured at 1 Hz using a Picarro 2301 cavity ring down spectrometer (Picarro, Sunnyvale, CA, USA). CO\({}_{2}\) measurements were corrected for water vapor content [58]. To ensure data quality and correct any possible sensor drift, periodic calibrations were performed every four months using three known reference CO\({}_{2}\) standards between 350 and 460 ppm (traceable to NOAA/CMDL). Instrument coefficients were adjusted to correct for minor drift over time (less than 0.2 ppm between calibrations). A Campbell Scientific (Logan, UT, USA) CSAT3 sonic anemometer was used to measure wind velocities and wind direction. Temperature and relative humidity were measured with a Vaisala HMP45C probe (Helsinki, Finland). The system operated nearly continuously from January through December: 2011 data completeness was 93%. In Worcester, CO\({}_{2}\) mixing ratios were measured at 1 Hz with a closed path LI-6262 (LICOR, Lincoln, NE, USA) infrared gas analyzer. The system was automatically calibrated every six hours using three reference gases between 340 ppm and 460 ppm (traceable to NOAA/CMDL standards). Wind speed and direction were measured with a Met One 034B Windset 2 wind instrument. All data were recorded using a Campbell Scientific (Logan, UT, USA) CR1000 datalogger. The system operated nearly continuously from installation in late March through December: overall 2011 completeness was 70%. CO\({}_{2}\) at Harvard Forest was measured by a LI-6262 (LICOR, Lincoln, NE, USA) infrared gas analyzer. Automated calibrations of the sensor were run at least twice daily to account for instrumentation drift; at least two reference gases between 340 and 460 ppm (traceable to NOAA/CMDL) were used during these calibrations. Wind speed and direction above the canopy were measured using an Applied Technologies (Longmont, CO, USA) SATI/3K 3-D sonic anemometer. Air temperature was derived from sonic anemometer's speed of sound measurement by accounting for influence of water vapor. Relative humidity was measured by Vaisala (Vantaa, Finland) HMP45 probe in an aspirated radiation shield. Ambient atmospheric pressure was measured by a MKS Instrument (Andover, MA, USA) absolute manometer. Data at the EMS tower were digitized and recorded using a custom data acquisition and control system. The Harvard Forest EMS tower operated from January through December in 2011, but experienced several interruptions in the CO\({}_{2}\) data: 2011 data completeness was 65%. Since instrument height can significantly alter observed CO\({}_{2}\) concentrations due to plume dispersion [28, 59], gas analyzers were placed at approximately the same height above ground level at each location. While this ensured that all sites observed emissions from similarly sized source areas, each instrument was likely located at a different vertical position within its respective boundary layer. For example, observations from urban Boston and Worcester were likely affected by building topography and the corresponding changes in micrometeorology and atmospheric mixing [60, 61]. Methodological challenges (e.g., urban tall tower construction) in elevating instruments above the roughness layer combined with differences in boundary layer height at each location prevented similar instrument positioning within each plume/boundary layer. However, given their locations on roofs of similar heights, the instruments in urban Boston and Worcester were assumed to be within a similar urban canopy layer, which is supported by the similar mean wind speed observed at these two sites. On the same roof as the Worcester tower, there were also a series of large mechanical units. While we confirmed that these units were not venting fossil fuel exhaust, we also conducted wind rose analyses to verify that CO\({}_{2}\)-rich air from the interior of the building was not being vented (Supplementary Material).