A Compact Diode Laser Cavity Ring-Down Spectrometer for Atmospheric Measurements of NO 3 and N 2 O 5 with Automated Zeroing and Calibration

A compact rack-mounted cavity ring-down spectrometer (CRDS) for simultaneous measurements of the nocturnal nitrogen oxides NO₃ and N₂O₅ in ambient air is described. The instrument uses a red diode laser to quantify mixing ratios of NO₃ (at its absorption maximum at 662 nm) and of N₂O₅ following its thermal dissociation to NO₃ in a second detection channel. The spectrometer is equipped with an automated zeroing and calibration setup to determine effective NO₃ absorption cross-sections and NO₃ and N₂O₅ inlet transmission efficiencies. The instrument response was calibrated using simultaneous measurements of NO₂, generated by thermal dissociation of N₂O₅ and/or by titration of NO₃ with excess NO, using blue diode laser CRDS at 405 nm. When measuring ambient air, the (2σ, 10 s) precision of the red diode CRDS varied between 5 and 8 parts-per-trillion by volume (pptv), which sufficed to quantify N₂O₅ concentrations under moderately polluted conditions. Sample N₂O₅ measurements made on a rooftop on the University of Calgary campus in August 2010 are presented. A maximum N₂O₅ mixing ratio of 130 pptv was observed, corresponding to a steady-state lifetime of less than 50 min. The NO₃ mixing ratios were below the detection limit, consistent with their predicted values based on equilibrium calculations. During the measurement period, the instrument response for N₂O₅ was 70% of the theoretical maximum, rationalized by a slight mismatch of the laser diode output with the NO₃ absorption line and a N₂O₅ inlet transmission efficiency less than unity. Advantages and limitations of the instrument's compact design are discussed.