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

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Abstract
This article describes the application of cavity ring-down spectroscopy (CaRDS) to the simultaneous concentration measurement of nitrate radical, NO 3 , and dinitrogen pentoxide, N 2 O 5 , in the ambient atmosphere. The sensitivity for detection of both NO 3 and N 2 O 5 is 0.5 pptv (2σ) for a 5 s integration, comparable to or better than previous measurements of NO 3 (e.g., via DOAS), but with significantly better time resolution. Furthermore, direct measurement of N 2 O 5 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 NO 3 , while the other is heated to 80 °C to induce thermal decomposition of N 2 O 5 providing a measurement of the sum of the NO 3 and N 2 O 5 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 N 2 O 5 thermolysis to NO 3 . We describe the limitations of this method and show how they can be quantified and accounted for in field measurements.