A Compact Diode Laser Cavity Ring-Down Spectrometer for Atmospheric Measurements of NO3 and N2O5 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 (2 sigma, 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.