Development of a laser-photofragmentation laser-induced fluorescence instrument for the detection of nitrous acid and hydroxyl radicals in the atmosphere

Abstract

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 photofragmentation efficiency of HONO and calibrating the instrument sensitivity for detection of the OH fragment. 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 photofragmentation efficiency. The LP/LIF instrument has demonstrated a 1σ 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 urban setting are presented along with indoor measurements to demonstrate the performance of the instrument.

Detection of nitrous acid in the atmospheric simulation chamber SAPHIR using open-path incoherent broadband cavity-enhanced absorption spectroscopy and extractive long-path absorption photometry

“…The mid-IR was targeted using Fourier transform (Hanst et al, 1982) and tuneable diode laser spectroscopy (TDLAS) (Schiller et al, 2001). Laser-induced fluorescence (LIF) can also be used for sensitive HONO detection but only through the emission of OH radicals that are formed after HONO photolysis (Rodgers and Davis, 1989;Liao et al, 2006;Bottorff et al, 2021).…”

Comparison of Simultaneous Measurements of Indoor Nitrous Acid: Implications for the Spatial Distribution of Indoor HONO Emissions

“…Fluorescence from the OH radical fragment is collected at right angles to both the sampled air stream and the laser emissions and detected using a micro-channel plate (MCP) photomultiplier tube (Photek PMT325) and a time-gated detection scheme. 36 …”

“…With a known sensitivity toward OH, measurements of the photolysis efficiency (PE) of the 355 nm laser allow the determination of the instrumental sensitivity toward HONO. 36 A known amount of OH and HO 2 is produced within the calibration source and an excess of NO is added to convert OH and HO 2 in the calibrator to HONO. When the 355 nm photolysis laser is turned on, a portion of HONO is converted back to OH in the sampling cell.…”

“…The limit of detection for HONO was approximately 9 ppt (S/N = 1, 10 min average) based on the standard deviation of the background signal. 36 , 38 The overall calibration uncertainty during HOMEChem was approximately ±35% primarily due to the precision of the measurement of the photofragmentation efficiency (±25%). As discussed below, the precision of the measurement varied between approximately 20 and 30% during the campaign due to variations in laser power, alignment, and wavelength that impacted the on-line signal.…”

Comparison of nitrous acid detection using open-path incoherent broadband cavity-enhanced absorption spectroscopy and extractive long-path absorption photometry

“…The mid-IR was targeted using Fourier transform (Hanst et al 1982) and tuneable diode laser spectroscopy (TDLAS) (Schiller et al 2001). Laser-induced fluorescence (LIF) can also be used for sensitive HONO detection, but only through the emission of OH radicals that are formed after HONO photolysis (Rodgers and Davis 1989, Liao et al 2006, Bottorff et al 2021.…”