Understanding the influence of biogenic carbon in world cities: Implications for emissions attribution

Collectively, cities are responsible for more than 70% of anthropogenic carbon dioxide (CO2) emissions, and this percentage is expected to rise in coming decades as urban populations increase. For this reason, cities worldwide are increasingly working to monitor and reduce their emissions. However, attributing emissions to anthropogenic sources is challenging because variations in CO2 concentrations can be caused by weather systems, biospheric fluxes, or non-local sources. Combining CO2 measurements with measurements of gases co-emitted by combustion sources, such as carbon monoxide (CO) or nitrogen oxides, is a promising approach that can provide additional information in support of attribution, but its application requires careful examination of the role of biogenic hydrocarbons in the evolution of these gases. Here, we present results of a modeling study using NASA's Goddard Earth Observing System Model (GEOS), which simulated the emission and transport of CO2 and CO at 25-km globally. We use the ability of GEOS to separately simulate anthropogenic and biospheric CO2 as well as separating CO emitted from anthropogenic sources and from oxidation of biogenic hydrocarbons. Using this configuration we performed two parallel experiments. In the first, we use year-specific biospheric CO2 fluxes and CO emissions that are influenced by synoptic scale variations in temperature and in the second, we apply climatological biospheric emissions that lack this source of variability. The results are used to quantify how much of the variability in atmospheric CO2 is due to biospheric flux and anthropogenic emissions in urban areas across the world. We also quantify the role of biogenic CO sources in controlling variations in CO:CO2 ratios in different cities. The results indicate that in a number of cities, particularly mid-size cities located near forests, the role of biogenic CO can be a substantial and complicating factor for attribution studies. In order to mitigate the risk of improper attribution in such locations, it is necessary to realistically simulated the temperature-dependent variations in biogenic hydrocarbons and in respiration fluxes.