A small-footprint Cavity Ring-Down Spectroscopy instrument for in-situ measurements of NO₃ and N₂O₅

Abstract. We present a new, small-footprint instrument for point measurements of NO₃ and N₂O₅. Both molecules play an important role in nocturnal atmospheric chemistry, impacting the NOₓ-budget and the oxidation of biogenic volatile organic compounds. NO₃ and N₂O₅ are often present at concentrations of a few parts per trillion by volume (pptv) and their measurements in remote locations requires instrumentation that is easily transported and lightweight, but maintains high sensitivity and accuracy. We have constructed a relatively compact and light instrument for Cavity Ring-Down Spectroscopy (CRDS) with the dimensions (width × depth × height) of 55 × 55 × 150 cm and a weight of 50 kg that uses two independent cavities to quantify the mixing ratio of NO₃ using an inlet at room temperature and the sum of NO₃ + N₂O₅ via a thermal dissociation inlet. Under laboratory conditions, limits of detection (1σ Allan deviation at 1 s integration) for the NO₃ and (NO₃ + N₂O₅) channel are < 1 pptv and < 2 pptv, respectively. This improves to about 0.1 pptv and 0.2 pptv for 3-minute integration. The total measurement uncertainty for NO₃ is 9.8 % and ≥ 11.5 % for N₂O₅, depending on the NO₃-to-N₂O₅ ratio.

In this publication, we present design details of the instrument, discuss its performance in a controlled environment as well as during a field campaign. Additionally, we present measurements of transmission losses for NO₃ across different filter types and methods to reduce filter reactivity and allow reusability after a cleaning procedure.