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| Titel |
The CU ground MAX-DOAS instrument: characterization of RMS noise limitations and first measurements near Pensacola, FL of BrO, IO, and CHOCHO |
| VerfasserIn |
S. Coburn, B. Dix, R. Sinreich, R. Volkamer |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1867-1381
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Measurement Techniques ; 4, no. 11 ; Nr. 4, no. 11 (2011-11-10), S.2421-2439 |
| Datensatznummer |
250002133
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| Publikation (Nr.) |
 copernicus.org/amt-4-2421-2011.pdf |
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| Zusammenfassung |
We designed and assembled the University of Colorado Ground Multi AXis
Differential Optical Absorption Spectroscopy (CU GMAX-DOAS) instrument to
retrieve bromine oxide (BrO), iodine oxide (IO), formaldehyde (HCHO),
glyoxal (CHOCHO), nitrogen dioxide (NO2) and the oxygen dimer (O4)
in the coastal atmosphere of the Gulf of Mexico. The detection sensitivity
of DOAS measurements is proportional to the root mean square (RMS) of the
residual spectrum that remains after all absorbers have been subtracted.
Here we describe the CU GMAX-DOAS instrument and demonstrate that the
hardware is capable of attaining RMS of ∼6 × 10−6 from
solar stray light noise tests using high photon count spectra (compatible
within a factor of two with photon shot noise).
Laboratory tests revealed two critical instrument properties that, in
practice, can limit the RMS: (1) detector non-linearity noise, RMSNLin,
and (2) temperature fluctuations that cause variations in optical resolution
(full width at half the maximum, FWHM, of atomic emission lines) and give
rise to optical resolution noise, RMSFWHM. The non-linearity of our
detector is low (∼10−2) yet – unless actively controlled – is
sufficiently large to create RMSNLin of up to 2 × 10−4. The
optical resolution is sensitive to temperature changes (0.03 detector
pixels °C−1 at 334 nm), and temperature variations of 0.1°C can cause
RMSFWHM of ~1 × 10−4. Both factors were actively
addressed in the design of the CU GMAX-DOAS instrument. With an integration
time of 60 s the instrument can reach RMS noise of 3 × 10−5, and
typical RMS in field measurements ranged from 6 × 10−5 to 1.4 × 10−4.
The CU GMAX-DOAS was set up at a coastal site near Pensacola, Florida, where
we detected BrO, IO and CHOCHO in the marine boundary layer (MBL), with
daytime average tropospheric vertical column densities (average of data
above the detection limit), VCDs, of ∼2 × 1013 molec cm−2,
8 × 1012 molec cm−2 and 4 × 1014 molec cm−2, respectively.
HCHO and NO2 were also detected with typical MBL VCDs of 1 × 1016
and 3 × 1015 molec cm−2. These are the first measurements of BrO, IO and CHOCHO over
the Gulf of Mexico. The atmospheric implications of these observations for
elevated mercury wet deposition rates in this area are briefly discussed.
The CU GMAX-DOAS has great potential to investigate RMS-limited problems,
like the abundance and variability of trace gases in the MBL and possibly
the free troposphere (FT). |
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