Abstract. An inexpensive, compact instrument for the sensitive measurement of NO₃ and N₂O₅ in ambient air at high time resolution is described. Light from a red-emitting laser diode (≈662 nm) is coupled off-axis into an optical resonator defined by two highly reflective mirrors to achieve effective absorption paths exceeding 20 km. Light leaking from the cavity is detected either as single ring-down events (time constant of ≈100 μs) following rapid switching of the laser intensity at 200 Hz (Cavity Ring Down mode), or as an integrated intensity (Cavity Enhanced Absorption mode). The operational conditions, detection limits (2 pptv in 5 s) and total uncertainty (<15% for NO₃<10 pptv) for the prototype device for NO₃ and N₂O₅ detection/monitoring are assessed and the first measurements in ambient air and from an environmental chamber are described.