## 1 Introduction  
The world's population has been rapidly shifting from rural and agrarian to urban areas, with the percent of global population living in cities increasing from 29.4% to 51.6% between 1950 and 2010 [1]. This urbanization trend is forecast to continue with models suggesting that by 2050 nearly 70% of the global population will live in urban areas [1]. While urban areas currently comprise less than 2% of global land area [2], their impact extends far beyond the city limits through environmental teleconnections [3] and demand for goods and services [4]. Urban areas are estimated to consume 67% of global energy and emit 71% of energy-related CO\({}_{2}\) emissions [5]. Despite the significant role these areas play in anthropogenic emissions, most research relating to atmospheric CO\({}_{2}\) dynamics has avoided areas close to or heavily influenced by cities [6,7]. Efforts to quantify terrestrial carbon exchange have instead focused on areas dominated by biogenic fluxes and homogenous land use patterns such as forest and agriculture [8]. By contrast, urban areas are often comprised of heterogeneous land cover and complex topography, which complicate measurements and source attribution of both CO\({}_{2}\) fluxes and mixing ratios. A range of environmental gradients has been observed between urban and adjacent rural locations. For example, urban heat islands, where temperatures can be several degrees higher than adjacent rural areas, develop due to reductions in latent heat fluxes and surface albedo changes associated with paving, among other reasons [9,10]. Urban canyons created by tightly spaced buildings and roadways can change airflow patterns and increase downwelling longwave radiation by reducing the sky view factor. This in turn raises temperatures. Importantly, these increases in temperature have also been shown to extend the growing season [11,12] and likely also affect biogenic carbon exchange in cities. Differences in hydrology, floral and faunal species diversity, soil nitrogen and carbon stocks, and concentrations of atmospheric pollutants have also been observed along urbanization gradients [13], although not always according to expectations. While some of the environmental gradients associated with urbanization have been better defined [14], the influence of these variables on atmospheric CO\({}_{2}\) mixing ratios has just recently begun to be assessed. For example, CO\({}_{2}\) mixing ratios have been found to be higher in urban centers compared to adjacent rural locations in Phoenix [15], Salt Lake City [16], and Baltimore [17]--a phenomenon known as an \"urban CO\({}_{2}\) dome\". These higher mixing ratios are due in part to local traffic emissions, as seen in Helsinki [18], Mexico City [19], and Basel [20], but may also be effected by residential, commercial, and industrial emissions. Unique patterns of CO\({}_{2}\) across urbanization gradients have also been demonstrated in Melbourne [21], Phoenix [22], and Rome [23], suggesting an association between urban land uses, urban density, and observed CO\({}_{2}\)[24]. There have also been attempts to map emissions at finer spatial scales in urban areas using mass flux measurements of carbon dioxide, among other data sources [25, 26, 27], but these results can be very difficult to interpret due to complex urban micrometeorology [28]. Urban areas' influence on atmospheric CO\({}_{2}\) is often framed in terms of anthropogenic emissions; however, biomass and biogenic CO\({}_{2}\) flux in urban areas can approach that of nearby forest-dominated areas [29, 30, 31]. In remote sensing products such as MODIS NPP, urban areas are masked out and assumed to have little productivity, but the biomass present in these areas suggests that biogenic fluxes are also important. Ecological processes in human-dominated ecosystems such as urban areas are expected to differ from adjacent, predominantly rural locations [32, 33, 34], but these differences in ecosystem function are poorly understood.