Abstract
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 1 m optical cavity. The reflectivity of the cavity mirrors is 0.99930 ± 0.00003 (1− reflectivity = 700 ppm loss) at 338 nm, as determined from the known Rayleigh scattering of He and zero air. This mirror reflectivity corresponds to an effective path length of 1.43 km within the 1 m cell. We measure the cavity output over the 315–350 nm 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 ± 0.03 with 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 30 s measurement precisions (1σ) of the current configuration were 140 and 210 pptv, respectively. The current 1 min detection limit for extinction measurements at 315–350 nm 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.
Highlights
Formaldehyde (CH2O) is the most abundant carbonyl compound in the atmosphere, with typical tropospheric concentrations between 0.1 and 10 parts per billion
To the extent that these challenges can be overcome, BBCEAS measurements in the ultraviolet spectral region may be a powerful technique for atmospheric spectroscopy due to the large number of trace gases with structured absorption in this region and to the importance of aerosol absorption
With a 2σ detection limit of 300 pptv CH2O in 1 min, the current demonstration of this measurement technique is appropriate for laboratory studies of CH2O, ground-based field measurements in regions with high CH2O mixing ratios, and ground-based field measurements of cleaner environments with increased averaging time (>5 min)
Summary
Formaldehyde (CH2O) is the most abundant carbonyl compound in the atmosphere, with typical tropospheric concentrations between 0.1 and 10 parts per billion (ppbv; Grosjean et al, 1996; Fried et al, 2002). Washenfelder et al.: Broadband cavity-enhanced absorption spectroscopy and reaction with OH, with an atmospheric lifetime of a few hours. Despite the distinct advantages of BBCEAS that have made it a powerful technique for trace gas measurements in the visible spectral region, there are multiple challenges in the ultraviolet that have precluded the measurement of atmospheric trace gases or aerosol extinction at ambient levels. Rayleigh scattering by ambient air increases inversely with the fourth power of wavelength, λ−4, and can become a significant loss process that limits light intensity and optical path length for an optical cavity in the ultraviolet spectral region. To the extent that these challenges can be overcome, BBCEAS measurements in the ultraviolet spectral region may be a powerful technique for atmospheric spectroscopy due to the large number of trace gases with structured absorption in this region and to the importance of aerosol absorption. The design requirements for ultraviolet BBCEAS to achieve trace gas detection at background CH2O mixing ratios with 1 min time resolution are discussed in the conclusions
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