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| Titel |
Sources of greenhouse gases and carbon monoxide in central London (UK) |
| VerfasserIn |
Carole Helfter, Anja Tremper, Giulia Zazzeri, Janet F. Barlow, Eiko Nemitz |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250106225
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| Publikation (Nr.) |
 EGU/EGU2015-5885.pdf |
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| Zusammenfassung |
| Biosphere-atmosphere exchange of carbon dioxide (CO2) has been on the scientific agenda
for several decades and new technology now also allows for high-precision, continuous
monitoring of fluxes of methane (CH4) and nitrous oxide (N2O). Compared to the natural
environment, flux measurements in the urban environment, which is home to over 50%
of the population globally, are still rare despite high densities of anthropogenic
sources of pollutants. We report on over three years of measurements atop a 192
m tower in central London (UK), Europe’s largest city, which started in October
2011.
Fluxes of methane, carbon monoxide (CO) and carbon dioxide are measured by
eddy-covariance (EC) at the British Telecom tower (51°31’ 17.4” N 0Ë 8’ 20.04” W). In
addition to the long-term measurements, EC fluxes of nitrous oxide (N2O) were measured in
February 2014.
All four trace gases exhibit diurnal trends consistent with anthropogenic activities with
minimum emissions at night and early afternoon maxima. Segregating emissions by wind
direction reveals heterogeneous source distributions with temporal patterns and source
strengths that differ between compounds. The lowest emissions for CO, CO2 and CH4 were
recorded for NW winds. The highest emissions of methane were in the SE sector, in the NE
for CO2 and in the W for CO. Fluxes of all 3 gases exhibited marked seasonal trends
characterised by a decrease in emissions in summer (63% reduction for CO, 36% for CO2
and 22% for CH4). Monthly fluxes of CO and CO2 were linearly correlated to air temperature
(R2 = 0.7 and 0.59 respectively); a weaker dependence upon temperature was also observed
for CH4 (R2 = 0.31).
Diurnal and seasonal emissions of CO and CO2 are mainly controlled by local fossil fuel
combustion and vehicle cold starts are thought to account for 20-30% of additional
emissions of CO during the winter. Fugitive emissions of CH4 from the natural gas
distribution network are thought to be substantial, which is consistent with the weaker
seasonality of CH4 fluxes compared with CO and CO2. Annual estimates of CO2
emissions (41 kt km-2) obtained by EC were consistent with data upscaled from the
London Atmospheric Emissions Inventory (LAEI; 46 kt km-2). Good agreement
between measurements and inventory data was also found for CO (measured 156 t
km-2; LAEI 145 t km-2) and for N2O (measured 0.36 t km-2; LAEI 0.42 t km-2),
although based on a much shorter measurement period. By contrast, a two-fold
difference was found between inventory and measured CH4 fluxes (measured 75 t
km-2; LAEI 34 t km-2), which could indicate an underestimation by the inventory
of CH4 emissions from anthropogenic sources or the existence of unaccounted
biogenic sources. Measurements of isotopic CH4 taken 2 km SE of the tower near
the banks of the river Thames reveal multiple episodes of 13C-depleted morning
peaks consistent with biogenic sources. We speculate that the Thames can act as an
additional significant source of biogenic methane especially at low tide and after
heavy rainfall, which could explain the large emissions observed in the S-SE sector. |
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