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Broadband cavity-enhanced absorption spectroscopy in the ultravioletspectral region for measurements of nitrogen dioxide andformaldehyde
[摘要] Formaldehyde (CH2O) is the most abundant aldehyde in the atmosphere,and it strongly affects photochemistry through its photolysis. We describesimultaneous measurements of CH2O and nitrogen dioxide (NO2) usingbroadband cavity-enhanced absorption spectroscopy in the ultravioletspectral region. The light source consists of a continuous-wave diode laserfocused into a Xenon bulb to produce a plasma that emits high-intensity,broadband light. The plasma discharge is optically filtered and coupled intoa 1 m optical cavity. The reflectivity of the cavity mirrors is 0.99930 ± 0.00003 (1− reflectivity  =  700 ppm loss) at 338 nm, as determinedfrom the known Rayleigh scattering of He and zero air. This mirrorreflectivity corresponds to an effective path length of 1.43 km within the1 m cell. We measure the cavity output over the 315–350 nm spectral regionusing a grating monochromator and charge-coupled device arraydetector. We use published reference spectra with spectral fitting softwareto 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 measurementsat 315–350 nm provides sufficient sensitivity for measurement of tracegases in laboratory experiments and ground-based field experiments.Additionally, the instrument provides highly accurate,spectroscopically based trace gas detection that may complement higherprecision techniques based on non-absolute detection methods. In addition totrace gases, this approach will be appropriate for measurements of aerosolextinction in ambient air, and this spectral region is important forcharacterizing the strong ultraviolet absorption by brown carbon aerosol.
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