Atmospheric measurements of ratios between CO<sub>2</sub> and co-emitted species from traffic: a tunnel study in the Paris megacity
Measurements of CO<sub>2</sub>, CO, NO<sub>x</sub> and selected Volatile Organic Compounds (VOCs) mole fractions were performed continuously during a 10-day period in the Guy Môquet tunnel in Thiais, a peri-urban area about 15 km south of the centre of Paris, between 28 Septe...
| Main Authors: | , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2014-12-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/14/12871/2014/acp-14-12871-2014.pdf |
| Summary: | Measurements of CO<sub>2</sub>, CO, NO<sub>x</sub> and selected Volatile Organic Compounds
(VOCs) mole fractions were performed continuously during a 10-day period in
the Guy Môquet tunnel in Thiais, a peri-urban area about 15 km south
of the centre of Paris, between 28 September and 8 October 2012. This
data set is used here to identify the characteristics of traffic-emitted
CO<sub>2</sub> by evaluating its ratios to co-emitted species for the first time
in the Paris region. High coefficients of determination (<i>r</i><sup>2</sup> > 0.7) are observed between CO<sub>2</sub> and certain compounds that are
characteristic of the traffic source (CO, NO<sub>x</sub>, benzene, xylenes and
acetylene). Weak correlations (<i>r</i><sup>2</sup> < 0.2) are found with species
such as propane, n-butane and <i>i</i>-butane that are associated with fuel
evaporation, an insignificant source for CO<sub>2</sub>. To better characterise
the traffic signal we focus only on species that are well-correlated with
CO<sub>2</sub> and on rush-hour periods characterised by the highest
traffic-related mole fractions. From those mole fractions we remove the
nighttime-average weekday mole fraction obtained for each species that we
infer to be the most appropriate background signal for our study. Then we
calculate observed Δspecies / ΔCO<sub>2</sub> ratios, which we compare
with the ones provided by the 2010 bottom–up high-resolved regional emission
inventory from Airparif (the association in charge of monitoring the air
quality in Île-de-France), focusing on local emission data for the
specific road of the tunnel. We find an excellent agreement (2%) between
the local inventory emission CO / CO<sub>2</sub> ratio and our observed ΔCO / ΔCO<sub>2</sub> ratio. Former tunnel experiments carried out elsewhere
in the world provided observed ΔCO / ΔCO<sub>2</sub> ratios that
differ from 49 to 592% to ours. This variability can be related to
technological improvement of vehicles, differences in driving conditions, and
fleet composition. We also find a satisfactory agreement with the Airparif
inventory for n-propylbenzene, n-pentane and xylenes to CO<sub>2</sub> ratios. For
most of the other species, the ratios obtained from the local emission
inventory overestimate the observed ratios to CO<sub>2</sub> by 34 to more
than 300%. However, the emission ratios of NO<sub>x</sub>, <i>o</i>-xylene and <i>i</i>-pentane
are underestimated by 30 to 79%. One main cause of such high
differences between the inventory and our observations is likely the
obsolete feature of the VOCs speciation matrix of the inventory that has not
been updated since 1998, although law regulations on some VOCs have occurred since
that time. Our study bears important consequences, discussed in the conclusion, for the characterisation of the urban CO<sub>2</sub> plume and for atmospheric inverse modelling of urban CO<sub>2</sub> emissions. |
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| ISSN: | 1680-7316 1680-7324 |