Aircraft instrument for simultaneous, in situ measurement of NO3 and N2O5 via pulsed cavity ring-down spectroscopy

This article describes a cavity ring-down spectrometer (CaRDS) specifically designed and constructed for installation on the NOAA WP-3D Orion (P-3) aircraft for sensitive, rapid in situ measurement of NO3 and N2O5. While similar to our previously described CaRDS instrument, this instrument has significant improvements in the signal-to-noise ratio, the time resolution, and in overall size and weight. Additionally, the instrument utilizes a custom-built, automated filter changer that was designed and constructed to meet the requirement for removal of particulate matter in the airflow while allowing fully autonomous instrument operation. The CaRDS instrument has a laboratory detection sensitivity of 4×10−11cm−1 in absorbance or 0.1pptv (pptv denotes parts per trillion volume) of NO3 in a 1s average, although the typical detection sensitivities encountered in the field were 0.5pptv for NO3 and 1pptv for N2O5. The instrument accuracy is 25% for NO3 and 20%–40% for N2O5, limited mainly by the uncertainty in the inlet transmission. The instrument has been deployed on the P-3 aircraft as part of a major field campaign in the summer of 2004 and during several ground and tower deployments near Boulder, CO.