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.
Highlights
The photolysis of ozone followed by the reaction of excited oxygen atoms with water vapor has been recognized as an important source of hydroxyl radicals (OH) in the troposphere (Rohrer and Berresheim, 2006), several studies have indicated that the photolysis of nitrous acid (HONO) (Reaction R1) is a significant, if not dominant, source of OH in several environments (Kleffmann et al, 2005; Acker et al, 2006; Dusanter et al, 2009b; Volkamer et al, 2010; Ren et al, 2013; Griffith et al, 2016)
The overall calibration uncertainty is estimated to be 35 % (1σ ), including the uncertainty associated with the OH calibration (18 %, 1σ ), and depends on the precision of the photofragmentation efficiency measurement
The laser photofragmentation (LP)/laser-induced fluorescence (LIF) instrument was deployed in two locations on the Indiana University campus in Bloomington to measure outdoor concentrations of HONO as a test of the instrument
Summary
The photolysis of ozone followed by the reaction of excited oxygen atoms with water vapor has been recognized as an important source of hydroxyl radicals (OH) in the troposphere (Rohrer and Berresheim, 2006), several studies have indicated that the photolysis of nitrous acid (HONO) (Reaction R1) is a significant, if not dominant, source of OH in several environments (Kleffmann et al, 2005; Acker et al, 2006; Dusanter et al, 2009b; Volkamer et al, 2010; Ren et al, 2013; Griffith et al, 2016). This instrument exhibited a low detection limit of 15 ppt for a 1 min integration time, the wavelength of 282 nm used for excitation of OH made it less suitable for environments with higher ozone and water-vapor mixing ratios due to the potential for laser-generated OH inside the detection cell from the photolysis of ozone followed by reaction of O(1D) with water vapor (Reactions R4 and R5) (Wennberg et al, 1994) This interference can impact the detection limit of HONO by increasing the measured OH background signal. In addition to a description of the instrument, a calibration method for HONO based on a measurement of the photofragmentation efficiency is described, and examples of measurements of HONO concentrations by this instrument in both outdoor and indoor environments are presented
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