Carbon cycling across the Boston urban to rural gradient: Integrating emissions estimates and atmospheric observations

Carbon dioxide is a well-mixed greenhouse gas, but how, where, and when it is exchanged with Earth's surface is a complex spatio-temporal, coupled natural-human problem. Nowhere is this challenge more pronounced than in the urban environment. Fixed objects like buildings and trees, and mobile elements like cars and people, exchange carbon (C) across a wide range of spatial and temporal scales with a dynamic set of driving variables. As cities, states, and nations undertake efforts to reduce and regulate greenhouse gas emissions, we must understand these space-time variations and the underlying drivers of biogenic and anthropogenic exchange in order to develop robust monitoring, reporting, and verification systems. Most research to date has focused on urban carbon emissions, or separately on carbon exchange in rural, forested environments. Thus, we are currently lacking empirical data on carbon cycling across urban to rural gradients that are critical for effectively reducing emissions and advancing urban sustainability efforts. Using the Boston, MA region as a case study, we explore the spatial and temporal patterns in anthropogenic emissions and atmospheric concentrations across an urban to rural gradient. Micrometeorological, biometric, econometric, and remote sensing methods are combined to characterize these relationships and determinants. We find that concentrations of CO2 within Boston's urban core are 12.7 ± 1.4 ppm greater that concurrent measurements at the rural Harvard Forest study area (~92 km away). Weekday concentrations are 2.1 ppm greater than weekend concentrations, with an 11.1 ppm and 5.1 ppm amplitude in the diurnal cycle on weekdays and weekends, respectively. Careful fusion of emissions estimates with direct atmospheric observations are a critical component for developing robust carbon validation methods. Within urban areas, the concept of urban metabolism provides a framework for monitoring, reporting, and verification that allows us to account for imports, exports, and transformations of carbon within urban areas.