A compact rack-mounted cavity ring-down spectrometer (CRDS) for simultaneous measurements of the nocturnal nitrogen oxides NO<sub>3</sub> and N<sub>2</sub>O<sub>5</sub> in ambient air is described. The instrument uses a red diode laser to quantify mixing ratios of NO<sub>3</sub> (at its absorption maximum at 662 nm) and of N<sub>2</sub>O<sub>5</sub> following its thermal dissociation to NO<sub>3</sub> in a second detection channel. The spectrometer is equipped with an automated zeroing and calibration setup to determine effective NO<sub>3</sub> absorption cross-sections and NO<sub>3</sub> and N<sub>2</sub>O<sub>5</sub> inlet transmission efficiencies. The instrument response was calibrated using simultaneous measurements of NO<sub>2</sub>, generated by thermal dissociation of N<sub>2</sub>O<sub>5</sub> and/or by titration of NO<sub>3</sub> 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 N<sub>2</sub>O<sub>5</sub> concentrations under moderately polluted conditions. Sample N<sub>2</sub>O<sub>5</sub> measurements made on a rooftop on the University of Calgary campus in August 2010 are presented. A maximum N<sub>2</sub>O<sub>5</sub> mixing ratio of 130 pptv was observed, corresponding to a steady-state lifetime of less than 50 min. The NO<sub>3</sub> mixing ratios were below the detection limit, consistent with their predicted values based on equilibrium calculations. During the measurement period, the instrument response for N<sub>2</sub>O<sub>5</sub> was 70% of the theoretical maximum, rationalized by a slight mismatch of the laser diode output with the NO<sub>3</sub> absorption line and a N<sub>2</sub>O<sub>5</sub> inlet transmission efficiency less than unity. Advantages and limitations of the instrument's compact design are discussed.

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