Simultaneous in situ detection of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy

Abstract

This article describes the application of cavity ring-down spectroscopy (CaRDS) to the simultaneous concentration measurement of nitrate radical, NO3, and dinitrogen pentoxide, N2O5, in the ambient atmosphere. The sensitivity for detection of both NO3 and N2O5 is 0.5 pptv (2σ) for a 5 s integration, comparable to or better than previous measurements of NO3 (e.g., via DOAS), but with significantly better time resolution. Furthermore, direct measurement of N2O5 represent a previously unavailable capability. Concentrations of both species are measured simultaneously in two separate flow systems and optical cavities pumped by the same pulsed dye laser at 662 nm. One of the flow systems remains at ambient temperature for detection of NO3, while the other is heated to 80 °C to induce thermal decomposition of N2O5 providing a measurement of the sum of the NO3 and N2O5 concentrations. This article outlines a series of laboratory and field tests of the instrument’s performance. Important considerations include signal acquisition, zero measurements, aerosol interference, flow system losses, and the conversion efficiency for N2O5 thermolysis to NO3. We describe the limitations of this method and show how they can be quantified and accounted for in field measurements.