Cities as Net Sources of CO2: Review of Atmospheric CO2 Exchange in Urban Environments Measured by Eddy Covariance Technique

Cities are main contributors to the CO2 rise in the atmosphere. It is clear that accurate estimates of the magnitude of anthropogenic and natural urban emissions are needed to assess their influence on the carbon balance. Increasingly the eddy covariance (EC) method is applied in urban environments to quantify CO2 fluxes. The technique has many advantages over other methods. It is a direct measure of the flux that includes all major and minor natural and anthropogenic sources and sinks, is in situ, non-intrusive, quasi-continuous and with proper selection of the footprint can represent a large upwind extent similar to the size of a complete urban neighborhood. This article reviews the basic principles and requirements of the EC technique, discusses its application in the urban context and summarizes observations from over 30 EC systems, primarily deployed in midlatitude cities. The results show that the urban surface is a net source of CO2, with vehicle exhaust and domestic heating as the major contributors while urban vegetation is not capable to completely offset the anthropogenic emissions. The largest CO2 fluxes have been observed in densely built up locations in city centers, followed by sites within urban core, while the lowest fluxes are found in suburban neighborhoods. The daily net CO2 exchange depicts a strong relationship with vegetation fraction, but not with population density.