A Practical Alternative to Chemiluminescence-Based Detection of Nitrogen Dioxide: Cavity Attenuated Phase Shift Spectroscopy

Abstract

We present results obtained from a greatly improved version of a previously reported nitrogen dioxide monitor (Anal Chem. 2005, 77, 724-728) that utilizes cavity attenuated phase shift spectroscopy (CAPS). The sensor, which detects the optical absorption of nitrogen dioxide within a 20 nm bandpass centered at 440 nm, comprises a blue light emitting diode, an enclosed stainless steel measurement cell (26 cm length) incorporating a resonant optical cavity of near-confocal design and a vacuum photodiode detector. An analog heterodyne detection scheme is used to measure the phase shift in the waveform of the modulated light transmitted through the cell induced by the presence of nitrogen dioxide within the cell. The sensor, which operates at atmospheric pressure, fits into a 19 in.-rack-mounted instrumentation box, weighs 10 kg, and utilizes 70 W of electrical power with pump included. The sensor response to nitrogen dioxide (calculated as the cotangent of the phase shift) is demonstrated to be linear (r2 > 0.9999) within +/- 1 ppb over a range of 0-320 ppb (by volume). The device exhibits a detection limit (3sigma precision) of less than 60 parts per trillion (0.060 ppb) with 10 s integration, a value derived from measurements at NO2 concentration levels of both 0 and 20 ppb; the detection limit improves as the integration time is increased to several hundred seconds. The observed baseline drift is less than +/- 0.5 ppb overthe course of a month. An intercomparison of measurements of ambient NO2 concentrations over several days using this sensor with a quantum cascade laser-based infrared absorption spectrometer and a standard chemiluminescence-based NOx analyzer is presented. The data from the CAPS sensor are highly correlated (r2 > 0.99) with the other two instruments. The absolute agreement between the CAPS and each of the two other instruments is within the expected statistical noise associated with the infrared laser-based absorption spectrometer (+/- 0.3 ppb with 10 s sampling) and chemiluminescence analyzer (+/- 0.4 ppb with 60 s averaging). The major limitation concerning accuracy is a direct spectral interference with phototchemically produced 1,2-dicarbonyl species (e.g., glyoxal, methylglyoxal). However, this interference can be readily removed by shifting the detection band to a slightly longer wavelength and ensuring that the lower edge of the detection band is greater than 455 nm.

Vertically-resolved Characteristics of Air Pollution during Two Severe Winter Haze Episodes in Urban Beijing, China

“…The instruments including a Cavity Attenuated Phase Shift Particulate Matter extinction monitor (CAPS-PM ext , Aerodyne Research Inc.) (Kebabian et al, 2007), a CAPS-NO 2 monitor (Kebabian et al, 2008;Ge et al, 2013), and a 7-wavelength Aethalometer 30 (model: AE33; Magee Scientific Corp.) (Drinovec et al, 2015) were equipped in the container for real-time measurements of light extinction coefficient (b ext , λ = 630 nm) of dry fine particles, gaseous NO 2 , and BC with a time resolution of 1 s, 1 s and Atmos. Chem.…”

Validation of in situ Measurements of Atmospheric Nitrous Acid Using Incoherent Broadband Cavity-enhanced Absorption Spectroscopy

“…The detection limit of the NOx analyzer was measured using a standard deviation of the blank signals (σ) and estimated to be 42 pptv for a 1-min integration time (3σ). To measure NO2 independently, an NO2 monitor based on a CAPS method 25 was employed. The detection limit of NO2 by the CAPS-NO2 analyzer was estimated to be 17 pptv for a 1-min integration time (3σ).…”

Aerosol Light Extinction Measurements by Cavity Attenuated Phase Shift (CAPS) Spectroscopy: Laboratory Validation and Field Deployment of a Compact Aerosol Particle Extinction Monitor

“…A more detailed description of the CAPS technology is given elsewhere (Kebabian et al , 2008. Regular switching between aerosol sample flow and particlefree air is performed to ensure good instrument performance; in fact, variations in the instrument response (i.e., cotθ • ) can occur via baseline drift and because of the contribution of gas phase absorption to the total extinction.…”

“…This method has already been already successfully employed for measurements of ambient NO 2 concentrations Kebabian et al 2008). The CAPS instrument uses a square wave modulated light emitting diode (LED) as a light source and measures the change in the phase shift of the distorted waveform of the modulated light leaving a highly reflective optical cell.…”