Development of a Laser-Photofragmentation Laser-Induced Fluorescence instrument for the detection of nitrous acid and hydroxyl radicals in the atmosphere

Abstract. A new instrument for the measurement of atmospheric nitrous acid (HONO) and hydroxyl radicals (OH) has been developed using laser photofragmentation (LP) of HONO at 355 nm after expansion into a low-pressure cell, followed by resonant laser-induced fluorescence (LIF) of the resulting OH radical fragment at 308 nm similar to the fluorescence assay by gas expansion technique (FAGE). The LP/LIF instrument is calibrated by determining the photo-fragmentation efficiency of HONO. In this method, a known concentration of OH from the photo-dissociation of water vapor is titrated with nitric oxide to produce a known concentration of HONO. Measurement of the concentration of the OH radical fragment relative to the concentration of HONO provides a measurement of the photo-fragmentation efficiency. The LP/LIF instrument has demonstrated a 1σ detection limit of 9 ppt for a 10-min integration time. Ambient measurements of HONO and OH from a forested environment and an urban setting are presented along with indoor measurements to demonstrate the performance of the instrument.