Simultaneous measurements of atmospheric HONO and NO2 via absorption spectroscopy using tunable mid-infrared continuous-wave quantum cascade lasers

Abstract

Nitrous acid (HONO) is important as a significant source of hydroxyl radical (OH) in the troposphere and as a potent indoor air pollutant. It is thought to be generated in both environments via heterogeneous reactions involving nitrogen dioxide (NO2). In order to enable fast-response HONO detection suitable for eddy-covariance flux measurements and to provide a direct method that avoids interferences associated with derivatization, we have developed a 2-channel tunable infrared laser differential absorption spectrometer (TILDAS) capable of simultaneous high-frequency measurements of HONO and NO2. Beams from two mid-infrared continuous-wave mode quantum cascade lasers (cw-QCLs) traverse separate 210 m paths through a multi-pass astigmatic sampling cell at reduced pressure for the direct detection of HONO (1660 cm−1) and NO2 (1604 cm−1). The resulting one-second detection limits (S/N=3) are 300 and 30 ppt (pmol/mol) for HONO and NO2, respectively. Our HONO quantification is based on revised line-strengths and peak positions for cis-HONO in the 6-micron spectral region that were derived from laboratory measurements. An essential component of ambient HONO measurements is the inlet system and we demonstrate that heated surfaces and reduced pressure minimize sampling artifacts.