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| Titel |
Integral emission factors for methane determined using urban flux measurements and local-scale inverse models |
| VerfasserIn |
Andreas Christen, Mark Johnson, Marina Molodovskaya, Rick Ketler, Zoran Nesic, Ben Crawford, Marco Giometto, Mike van der Laan |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250077391
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| Zusammenfassung |
| The most important long-lived greenhouse gas (LLGHG) emitted during combustion of fuels
is carbon dioxide (CO2), however also traces of the LLGHGs methane (CH4) and nitrous
oxide (N2O) are released, the quantities of which depend largely on the conditions of the
combustion process. Emission factors determine the mass of LLGHGs emitted per energy
used (or kilometre driven for cars) and are key inputs for bottom-up emission modelling.
Emission factors for CH4 are typically determined in the laboratory or on a test stand for a
given combustion system using a small number of samples (vehicles, furnaces), yet
associated with larger uncertainties when scaled to entire fleets. We propose an alternative,
different approach - Can integrated emission factors be independently determined using
direct micrometeorological flux measurements over an urban surface? If so, do emission
factors determined from flux measurements (top-down) agree with up-scaled emission factors
of relevant combustion systems (heating, vehicles) in the source area of the flux
measurement?
Direct flux measurements of CH4 were carried out between February and May, 2012
over a relatively densely populated, urban surface in Vancouver, Canada by means of eddy
covariance (EC). The EC-system consisted of an ultrasonic anemometer (CSAT-3,
Campbell Scientific Inc.) and two open-path infrared gas analyzers (Li7500 and
Li7700, Licor Inc.) on a tower at 30m above the surface. The source area of the EC
system is characterised by a relative homogeneous morphometry (5.3m average
building height), but spatially and temporally varying emission sources, including two
major intersecting arterial roads (70.000 cars drive through the 50% source area per
day) and seasonal heating in predominantly single-family houses (natural gas). An
inverse dispersion model (turbulent source area model), validated against large eddy
simulations (LES) of the urban roughness sublayer, allows the determination of the
spatial area that contributes to each measurement interval (30 min), which varies
with wind direction and stability. A detailed geographic information system of the
urban surface combined with traffic counts and building energy models makes it
possible to statistically relate fluxes to vehicle density (km driven) and buildings
(gas heated volume) - and ultimately quantify the contribution of space heating,
transport sector and fugitive emissions to the total emitted CH4 from an urban
environment.
The measured fluxes of CH4 over the selected urban environment averaged to 22.8 mg
CH4 m-2 day-1 during the study period. Compared with the simultaneously measured
CO2 emissions, the contribution of CH4, however, accounts for only about 3% of the
total LLGHG emissions from this particular urban surface. Traffic contributed 8.8
mg CH4 m-2 day-1, equivalent to 39% of the total CH4 flux. The determined
emission factor for the typical fleet composition is 0.062 g CH4 per km driven
which is higher than upscaled fleet emission factors (EPA) by a factor of two. This
discrepancy can be partially explained through the slower city traffic with frequent
idling (traffic congestion), fleet composition and cold starts. Emissions of CH4 by
domestic space heating (55% of the total CH4 flux or 12.7 mg CH4 m-2 day-1) are
also higher than estimated from upscaled emission factors. There is no evidence
of substantial unknown sources such as soil processes, combustion of wood, and
leakages from gas distribution pipes (residual: 6% or 1.3 mg CH4 m-2 day-1). The
presented study is among the first direct measurements of CH4 emissions over an
urban surface and demonstrates that flux measurements of greenhouse gases can
be used to determine sources and emission factors in complex urban situations. |
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