Interpreting continuous in-situ observations of carbon dioxide and carbon monoxide in the urban port area of Rotterdam

Large networks of expensive instruments are often used to independently quantify and monitor urban CO2 emissions with sufficient level of detail. However, many developing regions cannot afford such a monitoring effort. We explore the use of a simple, less costly method to constrain urban emissions using only two measurement sites, one upwind and one downwind of the city of Rotterdam in the Netherlands. This provides an interesting dataset of concentration gradients of multiple combustion tracers over an urban-industrial complex. We find clear emission signals from three source sectors, mainly related to industrial activities in the port and from residential areas. We estimate the anthropogenic CO2 emissions for three footprints from our observations and find them in reasonable agreement with the Dutch National Emission Registration (NER) database after accounting for biogenic fluxes. The large confidence interval for one of the footprints illustrates that the presence of point sources complicates the flux estimates. Additionally, we were able to pinpoint a limitation in the emission database using observed fossil fuel CO:CO2 ratios, although the applicability of this method is limited for the footprint with a large influence from point source emissions. There is also a large variability in the observed ratios per footprint, which indicates that the dominant source type varies over time. Finally, we show that the fossil fuel CO concentration can be used to calculate fossil fuel CO2 if their emission ratio is well-known.