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| Titel |
Measurement of NOx fluxes by eddy covariance from the BT tower, London during the ClearfLo project |
| VerfasserIn |
James Lee, Carole Helfter, Eiko Nemitz, Anja Tremper, Jenny Stocker, David Carruthers |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250088073
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| Publikation (Nr.) |
 EGU/EGU2014-2148.pdf |
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| Zusammenfassung |
| The vast majority of air pollutants are emitted directly into the atmosphere from
activities occurring at the Earth’s surface. One of the key anthropogenic pollutants is
NOx (defined as the sum of NO and NO2), which is emitted as a result of most
anthropogenic combustion processes. Whilst the chemical reactions and atmospheric
processing of NOx are reasonably well understood, and can be modelled with some skill,
large uncertainties arise in models due to uncertainty associated with the rate of
emissions. In recent years it has become clear that measured trends in certain pollutants,
for example NO2, have not followed trends predicted by inventories. Continued
exceedances of certain air pollution targets are of significant concern to governments, who
have identified reducing this uncertainty associated with emissions as key evidence
need.
As part of the UK Natural Environment Research Council (NERC) Clean Air for London
(ClearfLo) project, concentrations and fluxes of NOx were measured from the top of
the BT tower, which is a 188m high telecommunications tower, situated in central
London (51o31’17.4”N; 0o8’20.04W). The tower is surrounded by a mixture of
commercial and residential buildings with an average height of 15 m. The typical daytime
flux footprint of the tower is dominated by commercial/residential buildings and
roads (82%) but also includes urban parkland (13%) and impervious ground (5%).
High time resolution (10 Hz) chemiluminescence measurements of NO and NO2
(photolytic conversion to NO followed by chemiluminescence) were combined with fast
turbulence measurements from a sonic anemometer to calculate fluxes using the eddy
covariance technique. In brief, NOx fluxes per notional half-hourly averaging period
were obtained by maximising the covariance between instantaneous (i.e. mean for
the averaging period subtracted from each 10 Hz data point) fluctuations of NOx
mixing ratio and vertical wind velocity. 24 hour NOx flux measurements were made
on 36 days during June, July and August 2012 and 28 days in March and April
2013.
The data showed a clear diurnal cycle, with NOx flux broadly following measured traffic
flow in the surrounding streets, with a typical maximum daytime flux of 4μg m-2 s-1.
Mixing ratios of NOx can be seen to be a function of the NOx flux and the boundary layer
height evolution during the day. A clear weekday / weekend dependence is seen on the NOx
flux measurements, again following the traffic flow data. The measured fluxes were averaged
over 24 hours and scaled up to give a ‘top down’ estimate of the annual emission rate of 79.6
T km-2 yr-1. This compares well to estimates from the UK National Atmospheric Emissions
Inventory, however some differences are seen when the data is separated into different
wind sectors. Conversely, a ‘bottom up’ dispersion model (ADMS-Urban) was
run using measured meteorological data and the detailed London Atmospheric
Emissions Inventory as input. ADMS-Urban has previously been validated using
near-surface measurements, but the current work demonstrates that the model gives good
agreement with the flux measurements, both in terms of magnitude and diurnal cycle. |
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