Cavity ring-down spectroscopy for atmospheric trace gas detection:: application to the nitrate radical (NO3)

Cavity ring-down spectroscopy is a relatively new and quite sensitive technique for the measurement of gas-phase optical extinction. It holds the potential for simple, direct and sensitive measurement of the concentrations of a variety of trace gases in the atmosphere. For example, detection of the nitrate radical, NO3. and its companion, dinitrogen pentoxide, N2O5, has been demonstrated with a sensitivity of 0.25 pptv (1sigma). This paper considers several of the requirements for the application of cavity ring-down spectroscopy to concentration measurements of trace gases in ambient air. These include detection sensitivity, measurement of an accurate zero in the presence of competing absorbers, cavity stability and mirror cleanliness, laser line-width effects, saturation effects, Rayleigh scattering, the influence of atmospheric aerosols and sampling issues for reactive species. Examples drawn from our work on NO3 and N2O5 detection in the field illustrate these considerations.