A small-footprint Cavity Ring-Down Spectroscopy instrument for in situ measurements of NO ₃ and N ₂ O ₅

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 cm × 55 cm × 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 and < 2 pptv, respectively. This improves to about 0.1 and 0.2 pptv for 3 min 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.