Diurnal, synoptic and seasonal variability of atmospheric CO2 in the Paris megacity area
Authors: Xueref-Remy Irène , Dieudonné Elsa , Vuillemin Cyrille , Lopez Morgan , Lac Christine , Schmidt Martina , Delmotte Marc , Chevallier Frédéric , Ravetta François , Perrussel Olivier , Ciais Philippe , Bréon François-Marie , Broquet Grégoire , Ramonet Michel , Spain T. Gerard , Ampe Christophe 
Published: March 2018

Abstract. Most of the global fossil fuel CO2 emissions arise
from urbanized and industrialized areas. Bottom-up inventories quantify
them but with large uncertainties. In 2010–2011, the first atmospheric
in situ CO2 measurement network for Paris, the capital of France, began
operating with the aim of monitoring the regional atmospheric impact of
the emissions coming from this megacity. Five stations sampled air along a
northeast–southwest axis that corresponds to the direction of the dominant
winds. Two stations are classified as rural (Traînou – TRN; Montgé-en-Goële – MON), two are
peri-urban (Gonesse – GON; Gif-sur-Yvette – GIF) and one is urban (EIF, located on top of the Eiffel
Tower). In this study, we analyze the diurnal, synoptic and seasonal
variability of the in situ CO2 measurements over nearly 1 year
(8 August 2010–13 July 2011). We compare these datasets with remote CO2
measurements made at Mace Head (MHD) on the Atlantic coast of Ireland and
support our analysis with atmospheric boundary layer height (ABLH)
observations made in the center of Paris and with both modeled and observed
meteorological fields. The average hourly CO2 diurnal cycles observed
at the regional stations are mostly driven by the CO2 biospheric cycle,
the ABLH cycle and the proximity to urban CO2 emissions. Differences
of several µmol mol−1 (ppm) can be observed from one
regional site to the other. The more the site is surrounded by urban sources
(mostly residential and commercial heating, and traffic), the more the
CO2 concentration is elevated, as is the associated variability which
reflects the variability of the urban sources. Furthermore, two sites with
inlets high above ground level (EIF and TRN) show a phase shift of the
CO2 diurnal cycle of a few hours compared to lower sites due to a
strong coupling with the boundary layer diurnal cycle. As a consequence, the
existence of a CO2 vertical gradient above Paris can be inferred, whose
amplitude depends on the time of the day and on the season, ranging from a
few tenths of ppm during daytime to several ppm during nighttime. The
CO2 seasonal cycle inferred from monthly means at our regional sites
is driven by the biospheric and anthropogenic CO2 flux seasonal
cycles, the ABLH seasonal cycle and also synoptic variations.
Enhancements of several ppm are observed at peri-urban stations compared to
rural ones, mostly from the influence of urban emissions that are in the
footprint of the peri-urban station. The seasonal cycle observed at the
urban station (EIF) is specific and very sensitive to the ABLH cycle. At
both the diurnal and the seasonal scales, noticeable differences of several
ppm are observed between the measurements made at regional rural stations
and the remote measurements made at MHD, that are shown not to define
background concentrations appropriately for quantifying the regional
(∼ 100 km) atmospheric impact of urban CO2 emissions. For
wind speeds less than 3 m s−1, the accumulation of local CO2
emissions in the urban atmosphere forms a dome of several tens of ppm at the
peri-urban stations, mostly under the influence of relatively local
emissions including those from the Charles de Gaulle (CDG) Airport facility
and from aircraft in flight. When wind speed increases, ventilation
transforms the CO2 dome into a plume. Higher CO2 background
concentrations of several ppm are advected from the remote Benelux–Ruhr and
London regions, impacting concentrations at the five stations of the network
even at wind speeds higher than 9 m s−1. For wind speeds ranging
between 3 and 8 m s−1, the impact of Paris emissions can be detected in
the peri-urban stations when they are downwind of the city, while the rural
stations often seem disconnected from the city emission plume. As a
conclusion, our study highlights a high sensitivity of the stations to wind
speed and direction, to their distance from the city, but also to the ABLH
cycle depending on their elevation. We learn some lessons regarding the
design of an urban CO2 network: (1) careful attention should be paid to
properly setting regional (∼ 100 km) background sites that
will be representative of the different wind sectors; (2) the downwind
stations should be positioned as symmetrically as possible in relation
to the city center, at the peri-urban/rural border; (3) the stations should
be installed at ventilated sites (away from strong local sources) and the
air inlet set up above the building or biospheric canopy layer, whichever is
the highest; and (4) high-resolution wind information should be available
with the CO2 measurements.