A broadband absorption spectrometer using light emitting diodes for ultrasensitive, in situ trace gas detection

Abstract

A broadband absorption spectrometer has been developed for highly sensitive and target-selective in situ trace gas measurements. The instrument employs two distinct modes of operation: (i) broadband cavity enhanced absorption spectroscopy (BBCEAS) is used to quantify the concentration of gases in sample mixtures from their characteristic absorption features, and (ii) periodic measurements of the cavity mirrors' reflectivity are made using step-scan phase shift cavity ringdown spectroscopy (PSCRDS). The latter PSCRDS method provides a stand-alone alternative to the more usual method of determining mirror reflectivities by measuring BBCEAS absorption spectra for calibration samples of known composition. Moreover, the instrument's two modes of operation use light from the same light emitting diode transmitted through the cavity in the same optical alignment, hence minimizing the potential for systematic errors between mirror reflectivity determinations and concentration measurements. The ability of the instrument to quantify absorber concentrations is tested in instrument intercomparison exercises for NO2 (versus a laser broadband cavity ringdown spectrometer) and for H2O (versus a commercial hygrometer). A method is also proposed for calculating effective absorption cross sections for fitting the differential structure in BBCEAS spectra due to strong, narrow absorption lines that are under-resolved and hence exhibit non-Beer-Lambert law behavior at the resolution of the BBCEAS measurements. This approach is tested on BBCEAS spectra of water vapor's 4v+δ absorption bands around 650 nm. The most immediate analytical application of the present instrument is in quantifying the concentration of reactive trace gases in the ambient atmosphere. The instrument's detection limits for NO3 as a function of integration time are considered in detail using an Allan variance analysis. Experiments under laboratory conditions produce a 1 detection limit of 0.25 pptv for a 10 s acquisition time, which improves with further signal averaging to 0.09 pptv in 400 s. Finally, an example of the instrument's performance under field work conditions is presented, in this case of measurements of the sum of NO3 + N2O5 concentrations in the marine boundary layer acquired during the Reactive Halogens in the Marine Boundary Layer field campaign. © 2008 American Institute of Physics.