Abstract
Abstract. Providing timely information on urban greenhouse gas (GHG) emissions and their trends to stakeholders relies on reliable measurements of atmospheric concentrations and the understanding of how local emissions and atmospheric transport influence these observations. Portable Fourier transform infrared (FTIR) spectrometers were deployed at five stations in the Paris metropolitan area to provide column-averaged concentrations of CO2 (XCO2) during a field campaign in spring of 2015, as part of the Collaborative Carbon Column Observing Network (COCCON). Here, we describe and analyze the variations of XCO2 observed at different sites and how they changed over time. We find that observations upwind and downwind of the city centre differ significantly in their XCO2 concentrations, while the overall variability of the daily cycle is similar, i.e. increasing during night-time with a strong decrease (typically 2–3 ppm) during the afternoon. An atmospheric transport model framework (CHIMERE-CAMS) was used to simulate XCO2 and predict the same behaviour seen in the observations, which supports key findings, e.g. that even in a densely populated region like Paris (over 12 million people), biospheric uptake of CO2 can be of major influence on daily XCO2 variations. Despite a general offset between modelled and observed XCO2, the model correctly predicts the impact of the meteorological parameters (e.g. wind direction and speed) on the concentration gradients between different stations. When analyzing local gradients of XCO2 for upwind and downwind station pairs, those local gradients are found to be less sensitive to changes in XCO2 boundary conditions and biogenic fluxes within the domain and we find the model–data agreement further improves. Our modelling framework indicates that the local XCO2 gradient between the stations is dominated by the fossil fuel CO2 signal of the Paris metropolitan area. This further highlights the potential usefulness of XCO2 observations to help optimize future urban GHG emission estimates.
Highlights
Atmospheric background concentrations of CO2 measured since 1958 in Mauna Loa, USA, have passed the symbolic milestone of 400 ppm as of 2013 (Jones, 2013)
We focus our attention on cities, which are a critical part of this endeavour as emissions from urban areas are currently estimated to represent from 53 % to 87 % of global fossil fuel CO2 emissions (FFCO2), depending on the accounting method considered, and are predicted to increase further (IPCC, 2013; IEA, 2008; Dhakal, 2009)
We find that the wind speeds during daytime nearly always surpass 3 m s−1, which has been identified by Bréon et al (2015) and Staufer et al (2016) as the cut-off wind speed above which the atmospheric transport model CHIMERE performs best in modelling CO2 concentration gradients in the mixed layer
Summary
Atmospheric background concentrations of CO2 measured since 1958 in Mauna Loa, USA, have passed the symbolic milestone of 400 ppm (monthly mean) as of 2013 (Jones, 2013). Quantifying fossil fuel CO2 emissions (FFCO2) can contribute to defining effective climate mitigation strategies. We focus our attention on cities, which are a critical part of this endeavour as emissions from urban areas are currently estimated to represent from 53 % to 87 % of global FFCO2, depending on the accounting method considered, and are predicted to increase further (IPCC, 2013; IEA, 2008; Dhakal, 2009). As stated in the IPCC Fifth assessment report, “current and future urbanization trends are significantly different from the past” and “no single factor explains variations in per capita emissions across cities and there are significant differences in per capita greenhouse gas (GHG) emissions between cities within a single country” (IPCC, 2014). The large uncertainty of the global contribution of urban areas to CO2 emissions today and in the future is why a new generation of city-scale observing and modelling systems is needed
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