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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Volume 9, issue 1 | Copyright
Atmos. Meas. Tech., 9, 41-52, 2016
https://doi.org/10.5194/amt-9-41-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 15 Jan 2016

Research article | 15 Jan 2016

Broadband cavity-enhanced absorption spectroscopy in the ultraviolet spectral region for measurements of nitrogen dioxide and formaldehyde

R. A. Washenfelder1,2, A. R. Attwood1,2,a, J. M. Flores3, K. J. Zarzana1,2, Y. Rudich3, and S. S. Brown1 R. A. Washenfelder et al.
  • 1Cooperative Institute for Research in Environmental Sciences, University of Colorado, 216 UCB, Boulder, CO 80309, USA
  • 2Chemical Sciences Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration, 325 Broadway, Boulder, CO 80305, USA
  • 3Weizmann Institute of Science, Department for Earth and Planetary Sciences, Rehovot 76100, Israel
  • anow at: Horiba Scientific, Edison, New Jersey, USA

Abstract. Formaldehyde (CH2O) is the most abundant aldehyde in the atmosphere, and it strongly affects photochemistry through its photolysis. We describe simultaneous measurements of CH2O and nitrogen dioxide (NO2) using broadband cavity-enhanced absorption spectroscopy in the ultraviolet spectral region. The light source consists of a continuous-wave diode laser focused into a Xenon bulb to produce a plasma that emits high-intensity, broadband light. The plasma discharge is optically filtered and coupled into a 1m optical cavity. The reflectivity of the cavity mirrors is 0.99930 ± 0.00003 (1−reflectivity = 700ppm loss) at 338nm, as determined from the known Rayleigh scattering of He and zero air. This mirror reflectivity corresponds to an effective path length of 1.43km within the 1m cell. We measure the cavity output over the 315–350nm spectral region using a grating monochromator and charge-coupled device array detector. We use published reference spectra with spectral fitting software to simultaneously retrieve CH2O and NO2 concentrations. Independent measurements of NO2 standard additions by broadband cavity-enhanced absorption spectroscopy and cavity ring-down spectroscopy agree within 2% (slope for linear fit = 1.02 ± 0.03with r2 = 0.998). Standard additions of CH2O measured by broadband cavity-enhanced absorption spectroscopy and calculated based on flow dilution are also well correlated, with r2 = 0.9998. During constant mixed additions of NO2 and CH2O, the 30s measurement precisions (1σ) of the current configuration were 140 and 210pptv, respectively. The current 1min detection limit for extinction measurements at 315–350nm provides sufficient sensitivity for measurement of trace gases in laboratory experiments and ground-based field experiments. Additionally, the instrument provides highly accurate, spectroscopically based trace gas detection that may complement higher precision techniques based on non-absolute detection methods. In addition to trace gases, this approach will be appropriate for measurements of aerosol extinction in ambient air, and this spectral region is important for characterizing the strong ultraviolet absorption by brown carbon aerosol.

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Formaldehyde is the most abundant aldehyde in the atmosphere and plays an important role in photochemistry. Broadband cavity-enhanced absorption spectroscopy uses a high finesse cavity to obtain effective path lengths of kilometers. We use a diode-pumped plasma lamp and custom cavity mirrors to extend this technique further into the ultraviolet spectral region, and we achieve detection limits of hundreds of parts per trillion in 1 min for formaldehyde and nitrogen dioxide.
Formaldehyde is the most abundant aldehyde in the atmosphere and plays an important role in...
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