Observing System Choice Can Minimize Interference of the Biosphere in Studies of Urban CO2 Emissions

Abstract

Cities around the world have introduced initiatives to reduce CO2 emissions. Atmospheric observations can provide evaluation and assessment of these initiatives by quantifying emissions, considering local sources and sinks. The relative importance of the urban biosphere, which can act as both a source (respiration) and sink (photosynthesis) of CO2, has previously been suggested to strongly impact urban CO2 measurements, confounding the ability to use observations to study fossil emissions. However, if using an observing framework that measures a local urban background and the direct urban core outflow, for example, along a downwind airborne transect, the biosphere’s role may be minimized. Here, we combine real, airborne observations of CO2 downwind of select cities in the Northeast US with high-resolution, back-trajectory modeling and spatially and temporally resolved surface biosphere and fossil fuel fluxes to characterize the relative biosphere importance to urban CO2 profiles. We show the biosphere influence using this urban observing system to be small, averaging only 15% of the local CO2 enhancement annually, <10% outside of summer, and with a maximum influence of 29% in summer when the biosphere drawdown is most pronounced. Furthermore, when considering two biosphere models that differ by >80%, the impact on observed urban CO2 signals is reduced to only 12% on average. Urban observing frameworks that utilize this local background approach—including those via aircraft or satellite observations—can minimize the biosphere’s influence and thus help facilitate robust assessments of urban fossil fuel CO2 emissions.Plain Language SummaryCities around the world have announced plans to reduce CO2 emissions. Atmospheric CO2 observations provide a potential pathway toward independent assessment of implemented policies. However, these measurements can be strongly influenced by the urban biosphere, which can act as both a source (respiration) and sink (photosynthesis) of CO2. If using an observing approach that introduces a local, urban background—for example, observations via a downwind airborne transect that captures an entire urban outflow—the relative role of the biosphere may be minimized. Here, we combine back trajectory modeling with high-resolution surface fossil fuel and biosphere CO2 fluxes across six cities and one powerplant in the NE US to demonstrate that observing strategies using this approach can greatly reduce biosphere interferences in studies of urban CO2 (<10% biosphere interference outside of summer months, on average) and pave the way to conduct robust studies of urban fossil fuel CO2 emissions.Key PointsUrban fossil CO2 emissions can be isolated from biosphere influences using observation approaches that define a local backgroundHigh variability of biosphere representation has minimal influence on bio contribution to urban CO2 using local background framework