A cavity ring down/cavity enhanced absorption device for measurement of ambient NOsub3/sub and Nsub2/subOsub5/sub

摘要

An inexpensive, compact instrument for the sensitive measurement of NO₃and N₂O₅ in ambient air at high time resolution is described.Light from a red-emitting laser diode (≈662 nm) is coupledoff-axis into an optical resonator defined by two highly reflective mirrorsto achieve effective absorption paths exceeding 20 km. Light leaking fromthe cavity is detected either as single ring-down events (time constant of≈100 μs) following rapid switching of the laser intensity at200 Hz (Cavity Ring Down mode), or as an integrated intensity (CavityEnhanced Absorption mode). The operational conditions, detection limits (2 pptv in 5 s) and total uncertainty (<15% for NO₃<10 pptv) forthe prototype device for NO₃ and N₂O₅ detection/monitoringare assessed and the first measurements in ambient air and from anenvironmental chamber are described.