Abstract
Abstract. The emission of CO2 from the burning of fossil fuel is a prime determinant of variations in atmospheric CO2. Here, we simulate this fossil-fuel signal together with the natural and background components with a regional high-resolution atmospheric transport model for central and southern Europe considering separately the emissions from different sectors and countries on the basis of emission inventories and hourly emission time functions. The simulated variations in atmospheric CO2 agree very well with observation-based estimates, although the observed variance is slightly underestimated, particularly for the fossil-fuel component. Despite relatively rapid atmospheric mixing, the simulated fossil-fuel signal reveals distinct annual mean structures deep into the troposphere, reflecting the spatially dense aggregation of most emissions. The fossil-fuel signal accounts for more than half of the total (fossil fuel + biospheric + background) temporal variations in atmospheric CO2 in most areas of northern and western central Europe, with the largest variations occurring on diurnal timescales owing to the combination of diurnal variations in emissions and atmospheric mixing and transport out of the surface layer. The covariance of the fossil-fuel emissions and atmospheric transport on diurnal timescales leads to a diurnal fossil-fuel rectifier effect of up to 9 ppm compared to a case with time-constant emissions. The spatial pattern of CO2 from the different sectors largely reflects the distribution and relative magnitude of the corresponding emissions, with power plant emissions leaving the most distinguished mark. An exception is southern and western Europe, where the emissions from the transportation sector dominate the fossil-fuel signal. Most of the fossil-fuel CO2 remains within the country responsible for the emission, although in smaller countries up to 80 % of the fossil-fuel signal can come from abroad. A fossil-fuel emission reduction of 30 % is clearly detectable for a surface-based observing system for atmospheric CO2, while it is beyond the edge of detectability for the current generation of satellites with the exception of a few hotspot sites. Changes in variability in atmospheric CO2 might open an additional door for the monitoring and verification of changes in fossil-fuel emissions, primarily for surface-based systems.
Highlights
With annual CO2 emissions from fossil-fuel burning and cement production having soared in recent decades and approaching 10 Pg C yr−1 (Raupach et al, 2007; Friedlingstein et al, 2014; Le Quéré et al, 2016), these fluxes have reached the same order of magnitude as the natural exchange fluxes between the atmosphere and land surface and between the atmosphere and the ocean, respectively (Sarmiento and Gruber, 2002; Le Quéré et al, 2016)
The modeled daily mean atmospheric CO2 at these four sites agrees generally well with the corresponding observations, the agreement deteriorates with proximity to the ground
Influence from air pollution is only observed during episodes of transport from the UK and continental Europe, which are very well captured by the model
Summary
With annual CO2 emissions from fossil-fuel burning and cement production having soared in recent decades and approaching 10 Pg C yr−1 (Raupach et al, 2007; Friedlingstein et al, 2014; Le Quéré et al, 2016), these fluxes have reached the same order of magnitude as the natural exchange fluxes between the atmosphere and land surface and between the atmosphere and the ocean, respectively (Sarmiento and Gruber, 2002; Le Quéré et al, 2016). The fossil-fuel emissions have become a key driver for the spatiotemporal dynamics of atmospheric CO2, close to major sites of emissions and far downstream (Peylin et al, 2011; Keppel-Aleks et al, 2013; Nassar et al, 2013) This represents simultaneously a challenge and an opportunity. The large fossil-fuel CO2 signal complicates the use of atmospheric CO2 observations to determine sources and sinks of CO2 driven by the land biosphere through atmospheric inverse modeling methods This requires the separation of the biospheric signal in atmospheric CO2 from the total signal, which is usually accomplished by subtracting an estimate of the fossil-fuel component from the measured atmospheric CO2 concentration. In order to benefit from the monitoring and verification opportunity as well as to minimize the magnitude of the challenge associated with atmospheric inversions, it is paramount to characterize the fossil-fuel component in atmospheric CO2 well in time and space
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