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| Titel |
Determination of the NO2 concentration and its distribution with different DOAS techniques - a study in the metropolitan area of Hong Kong |
| VerfasserIn |
M. Horbanski, K. L. Chan, D. Pöhler, M. O. Wenig, U. Platt |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250064170
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| Zusammenfassung |
| In Hong Kong, like in other urban areas, nitrogen dioxide (NO2) emissions still remain one of
the most problematic pollutions. Even though a lot of effort has already been made towards
their reduction, concentrations are often much above the recommended limits. Therefore, the
Hong Kong Environmental Protection Department continuously monitors the NO2
concentration levels. These stations only give in-situ information at a few fixed locations,
which are unable to capture the strong spatial gradients that are present in urban
areas. These gradients are mainly caused by differences in automobile traffic, the
population densities and industries that are found in the individual city areas. The
gradients are further enhanced by street canyon effects. Hence, it can be assumed that
there are areas with much higher pollution compared to the values at the official
measuring sites. At present the uncertainties about the spatial NO2 distribution place a
limitation on the assessment of health risks and on the quality of chemical model
calculations.
We applied a new Cavity Enhanced (CE-) DOAS instrument to car based measurements
in the city of Hong Kong for eight days in December 2010. Measurements on mobile
platforms like cars give the opportunity to create a map of the NO2 for a whole city area with
high spatial resolution. The CE-DOAS instrument is distinguished by its compact and robust
setup. It directly measures NO2 by its optical absorption and therefore it does not have
interferences with other trace gas species like O3 or NOy. This is a great advantage over other
NO2 instruments (e.g. solid state detectors or chemiluminescence instruments which have
such known problems). For our measurements a van travels along carefully selected tracks to
give a representative coverage of the area of interest. The mobile measurements are corrected
for the varying meteorological conditions and traffic variations during the different
measurements by comparing them to a fixed measurement of NO2 to derive a representative
distribution map. This is done by a simultaneously performed Long Path DOAS
measurement which gives the average NO2 concentration along a few kilometer long light
path over the area of interest. Our measurements show that NO2 concentrations in
Hong Kong vary by more than an order of magnitude on scales of only few 100m.
Concentrations up to several ppmv are observed at various locations, clearly showing that
environmental measurement stations with their fixed point measurements do not sufficiently
capture these variations and the given results may not be correct for a location close
by.
Besides the determination of NO2 maps the Long Path DOAS measurements since
December 2009 are used to validate DOAS based OMI satellite measurements. This is done
to assess the usability of satellite based measurements for air quality monitoring in urban
areas. Monthly averaged LP-DOAS and OMI measurements correlate well but the satellite
measurements significantly underestimate the NO2 concentration. Furthermore, the
mean weekly NO2 cycles, display different characteristics which will be discussed. |
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