Two-Channel, Fast Time-Response CAPS-Based NOx Monitor

We have developed a turnkey, fast response two-channel NO_x monitor (NO_x =NO + NO₂) which provides  simultaneous measurements of both NO_x and NO₂ and thus NO by subtraction.  The NO_x channel employs a carefully controlled concentration of photolytically-produced ozone (O₃) to convert NO into NO₂.  The monitor can measure NO₂ and NO_x with an accuracy of better than 5% and precision of < 0.2 ppb (1s, 1s) with <1 second physical time constant in each measurement channel.  The monitor is based on Aerodyne Research’s patented CAPS (Cavity Attenuated Phase Shift) technology previously employed to measure NO₂concentrations, aerosol optical extinction and aerosol single scattering albedo. 

A CAPS-based NO₂ monitor utilizes a light-emitting diode (LED) as a light source (450 nm for the NO₂ channel and 405 nm for the total NO_x channel) and a sample cell incorporating two high reflectivity mirrors; a vacuum photodiode is used to detect the light emitted from the cell. The square wave modulated light from the LED passes through the absorption cell and is detected as a distorted waveform which is characterized by a phase shift with respect to the initial modulation.   The amount of that phase shift is a function of fixed instrument properties - cell length, mirror reflectivity, and modulation frequency– and of the presence of variable concentrations of nitrogen dioxide.  The mixing ratio is calculated from the value of the cotangent of the phase shift, the speed of light, LED modulation frequency and the absorption coefficient of NO₂.  The use of 405 nm detection in the total NOx channel greatly reduces the possible interference caused by the presence of several ppm of O3 in the sample flow.

We present data from an intercomparison of the NOx monitor with Aerodyne TILDAS monitors measuring both NO and NO2.  The TILDAS monitors utilize infrared diode lasers to probe individual ro-vibrational absorption lines and are considered the “gold standard” for measuring concentrations of small molecules.  The monitors were deployed on the Aerodyne Mobile Laboratory during the MOOSE (Michigan Ontario Ozone Source Experiment) campaign in late 2021.  The figure presented below compares the measurement of both NO₂ and the sum of NO+NO₂ measured by the TILDAS monitors compared with that obtained by the CAPS NO monitor at 1 second resolution.  Note the excellent agreement with respect to both magnitude and time resolution.  Long term comparisons indicate that correlation coefficients at 1 second  resolution approach 1.