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| Titel |
Measurements of diurnal variations and eddy covariance (EC) fluxes of glyoxal in the tropical marine boundary layer: description of the Fast LED-CE-DOAS instrument |
| VerfasserIn |
S. Coburn, I. Ortega, R. Thalman, B. Blomquist, C. W. Fairall, 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 ; 7, no. 10 ; Nr. 7, no. 10 (2014-10-28), S.3579-3595 |
| Datensatznummer |
250115937
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| Publikation (Nr.) |
 copernicus.org/amt-7-3579-2014.pdf |
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| Zusammenfassung |
| Here we present first eddy covariance (EC) measurements of fluxes of
glyoxal, the smallest α-dicarbonyl product of hydrocarbon oxidation,
and a precursor for secondary organic aerosol (SOA). The unique physical and
chemical properties of glyoxal – i.e., high solubility in water (effective
Henry's law constant, KH = 4.2 × 105 M atm−1) and short
atmospheric lifetime (~2 h at solar noon) – make it a unique
indicator species for organic carbon oxidation in the marine atmosphere.
Previous reports of elevated glyoxal over oceans remain unexplained by
atmospheric models. Here we describe a Fast Light-Emitting Diode Cavity-Enhanced
Differential Optical Absorption Spectroscopy (Fast LED-CE-DOAS)
instrument to measure diurnal variations and EC fluxes of glyoxal and
inform about its unknown sources. The fast in situ sensor is described, and first
results are presented from a cruise deployment over the eastern tropical
Pacific Ocean (20° N to 10° S; 133 to 85° W) as part of the Tropical Ocean
tRoposphere Exchange of Reactive halogens and Oxygenated VOCs (TORERO) field experiment
(January to March 2012). The Fast LED-CE-DOAS is a multispectral sensor that
selectively and simultaneously measures glyoxal (CHOCHO), nitrogen dioxide
(NO2), oxygen dimers (O4), and water vapor (H2O) with
~2 Hz time resolution (Nyquist frequency ~1 Hz) and a precision of ~40 pptv Hz−0.5 for glyoxal. The
instrument is demonstrated to be a "white-noise" sensor suitable for EC flux
measurements. Fluxes of glyoxal are calculated, along with fluxes of
NO2, H2O, and O4, which are used to aid the interpretation of
the glyoxal fluxes. Further, highly sensitive and inherently calibrated
glyoxal measurements are obtained from temporal averaging of data (e.g.,
detection limit smaller than 2.5 pptv in an hour). The campaign average mixing
ratio in the Southern Hemisphere (SH) is found to be 43 ± 9 pptv
glyoxal, which is higher than the Northern Hemisphere (NH) average of
32 ± 6 pptv (error reflects variability over multiple days). The
diurnal variation of glyoxal in the marine boundary layer (MBL) is measured for the first time, and
mixing ratios vary by ~8 pptv (NH) and ~12 pptv
(SH) over the course of 24 h. Consistently, maxima are observed at sunrise
(NH: 35 ± 5 pptv; SH: 47 ± 7 pptv), and minima at dusk (NH:
27 ± 5 pptv; SH: 35 ± 8 pptv). In both hemispheres, the daytime
flux was directed from the atmosphere into the ocean, indicating that the
ocean is a net sink for glyoxal during the day. After sunset the ocean was a
source for glyoxal to the atmosphere (positive flux) in the SH; this primary
ocean source was operative throughout the night. In the NH, the nighttime
flux was positive only shortly after sunset and negative during most of the
night. Positive EC fluxes of soluble glyoxal over oceans indicate the
presence of an ocean surface organic microlayer (SML) and locate a glyoxal
source within the SML. The origin of most atmospheric glyoxal, and possibly
other oxygenated hydrocarbons over tropical oceans, remains unexplained and
warrants further investigation. |
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