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Abstract. A new instrument for the measurement of atmosphericnitrous acid (HONO) and hydroxyl radicals (OH) has been developed usinglaser photofragmentation (LP) of HONO at 355 nm after expansion into alow-pressure cell, followed by resonant laser-induced fluorescence (LIF) ofthe resulting OH radical fragment at 308 nm similar to the fluorescenceassay by gas expansion technique (FAGE). The LP/LIF instrument is calibratedby determining the photofragmentation efficiency of HONO andcalibrating the instrument sensitivity for detection of the OH fragment. Inthis method, a known concentration of OH from the photo-dissociation ofwater vapor is titrated with nitric oxide to produce a known concentrationof HONO. Measurement of the concentration of the OH radical fragmentrelative to the concentration of HONO provides a measurement of thephotofragmentation efficiency. The LP/LIF instrument has demonstrated a1σ detection limit for HONO of 9 ppt for a 10 min integration time.Ambient measurements of HONO and OH from a forested environment and an urbansetting are presented along with indoor measurements to demonstrate theperformance of the instrument.