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| Titel |
LED-CE-DOAS measurements of NO2: intercomparison with CaRDS |
| VerfasserIn |
R. M. Thalman, R. Washenfelder, S. S. Brown, R. Volkamer |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250029603
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| Zusammenfassung |
| The combination of cavity enhanced absorption spectroscopy (CEAS) with Light Emitting
Diode (LED) light sources lends itself to the application of the well established Differential
Optical Absorption Spectroscopy (DOAS) technique (LED-CE-DOAS). In contrast
to other broad band CEAS (BB-CEAS) techniques, CE-DOAS relies only on the
measurement of relative intensity changes, i.e., does not require knowledge of the light
intensity in the absence of trace gases (I0). With CE-DOAS there is no necessity for
sampling lines to supply air samples into a cavity, or filters to remove aerosols
from the airstream, as measurements are possible in a cavity that can be open to the
atmosphere.
A novel LED-CE-DOAS instrument was built at CU Boulder for the sensitive and selective
detection of nitrogen dioxide (NO2), glyoxal (CHOCHO), iodine oxide (IO), water, and
oxygen dimers (O4). CU Boulder’s LED-CE-DOAS instrument was collocated to NOAA’s
NO2 Cavity Ring Down (CaRDS) instrument to test different CE-DOAS data retrieval
algorithms for NO2 and O4. Both instruments were collocated to sample known NO2
concentrations from the same gas manifold, and to sample atmospheric air in a
parking lot. This contribution focuses on the instrument components, challenges and
means to retrieve quantitative concentrations of NO2 by LED-CE-DOAS, i.e., the
distortion of NO2 and O4 absorption features due to different effective path lengths
induced by (1) changes in the mirror reflectivity with wavelength, and (2) changes in
light extinction across the absorption bands due to differential trace gas absorption
features. We demonstrate that simultaneous measurements of O4 and NO2 enable
to characterize the effective pathlength in the absence and presence of NO2 and
perform absolute measurements based only on relative intensity measurements. To our
knowledge these are the first CEAS measurements that rely solely on relative intensity
measurements. |
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