Analysis of the potential of near-ground measurements of CO<sub>2</sub> and CH<sub>4</sub> in London, UK, for the monitoring of city-scale emissions using an atmospheric transport model

Abstract

Abstract. Carbon dioxide (CO 2 ) and methane (CH 4 ) mole fractions were measured at four near-ground sites located in and around London during the summer of 2012 with a view to investigating the potential of assimilating such measurements in an atmospheric inversion system for the monitoring of the CO 2 and CH 4 emissions in the London area. These data were analysed and compared with simulations using a modelling framework suited to building an inversion system: a 2 km horizontal resolution south of England configuration of the transport model CHIMERE driven by European Centre for Medium-Range Weather Forecasts (ECMWF) meteorological forcing, coupled to a 1 km horizontal resolution emission inventory (the UK National Atmospheric Emission Inventory). First comparisons reveal that local sources, which cannot be represented in the model at a 2 km resolution, have a large impact on measurements. We evaluate methods to filter out the impact of some of the other critical sources of discrepancies between the measurements and the model simulation except that of the errors in the emission inventory, which we attempt to isolate. Such a separation of the impact of errors in the emission inventory should make it easier to identify the corrections that should be applied to the inventory. Analysis is supported by observations from meteorological sites around the city and a 3-week period of atmospheric mixing layer height estimations from lidar measurements. The difficulties of modelling the mixing layer depth and thus CO 2 and CH 4 concentrations during the night, morning and late afternoon lead to focusing on the afternoon period for all further analyses. The discrepancies between observations and model simulations are high for both CO 2 and CH 4 (i.e. their root mean square (RMS) is between 8 and 12 parts per million (ppm) for CO 2 and between 30 and 55 parts per billion (ppb) for CH 4 at a given site). By analysing the gradients between the urban sites and a suburban or rural reference site, we are able to decrease the impact of uncertainties in the fluxes and transport outside the London area and in the model domain boundary conditions. We are thus able to better focus attention on the signature of London urban CO 2 and CH 4 emissions in the atmospheric CO 2 and CH 4 concentrations. This considerably improves the statistical agreement between the model and observations for CO 2 (with modeldata RMS discrepancies that are between 3 and 7 ppm) and to a lesser degree for CH 4 (with model-data RMS discrepancies that are between 29 and 38 ppb). Between one of the urban sites and either the rural or suburban reference site, selecting the gradients during periods wherein the reference site is upwind of the urban site further decreases the statistics of the discrepancies in general, though not systematically. In a further attempt to focus on the signature of the city anthropogenic emission in the mole fraction measurements, we use a theoretical ratio of gradients of carbon monoxide (CO) to gradients of CO 2 from fossil fuel emiss...

The first 1-year-long estimate of the Paris region fossil fuel CO<sub>2</sub> emissions based on atmospheric inversion

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A multi-model approach to monitor emissions of CO<sub>2</sub> and CO in an urban-industrial complex

“…The resolution appears to be too low to fully represent the transport of the urban signals, especially at the urban site, 440 similar to previous findings related to power plant plumes (Lindenmaier et al, 2014) and megacities (Boon et al, 2016). WRF-Chem instantaneously mixes all emissions throughout a grid cell, which results in underestimated downwind concentrations and a large negative bias for both species.…”

Diurnal, synoptic and seasonal variability of atmospheric CO<sub>2</sub> in the Paris megacity area

“…We plan to improve our instrumental setup on the Eiffel Tower with two additional sampling heights to gather vertical CO 2 profiles and associated meteorological data: this will be of great help to understand the coupling between CO 2 sources and atmospheric dynamics over the Paris megacity in the future. This recalls as well that the altitude relative to ground level and the distance to the emissions of a station are very important factors to take into account in the network capacity to properly detect a CO 2 urban plume (see also the discussion about this topic in Boon et al, 2016).…”

“…), located in a remote peri-urban area ∼ 20 km SW of Paris. Furthermore, the Mace Head station (MHD -west coast of Ireland) is generally used as the reference site for European CO 2 background measurement (Bousquet et al, 1996), as it has been the case in the Heidelberg (Germany) study of Vogel et al (2010) or in the Paris study of Lopez et al (2013). The relevance of this remote coastal site as a regional background site, especially for studying the regional impact of the Paris megacity on atmospheric CO 2 remains to be assessed at the diurnal to the seasonal scales, as no regional in situ network measurements were available to tackle this question.…”