Development and Optimization of Cavity Attenuated Phase-Shift Spectroscopy Technique for Measuring Ambient Nitrogen Dioxide

Nitrogen dioxide
$(\mathbf{NO}_{2})$
stands as a prominent air pollutant known for its detrimental impact on the respiratory system of human beings. Consequently, it becomes imperative to undertake diligent monitoring of ambient
${\text{NO}_{2}}$
concentrations. This research article presents an innovative design, construction, and optimization of a
${\text{NO}_{2}}$
measurement system founded upon the principles of cavity attenuated phase shift spectroscopy (CAPS). The system encompasses distinct components, namely an optical system, a signal acquisition and processing system, and a gas system. The optical system comprises a high-power blue LED
$(\mathbf{central\ wavelength=440}$
nm), a collimating lens, an absorption cell comprised of two high reflectivity mirrors
${(\mathrm{R} > 0.9994)}$
, a confocal optical cavity (252 mm), and a photomultiplier tube (PMT). The signal acquisition and processing system entail a signal generator, lock-in amplifier, and Lab Viewdata acquisition program. Additionally, the gas system encompasses a gas flow control pump, a gas dryer, and a particle filter. Integration of the aforementioned systems within the optical breadboard yields a versatile movable gas online analyzer. The consistency between CAPS and CLD (Chemiluminescence Detection) has been meticulously ascertained through laboratory measurements, thus reinforcing the reliability of the proposed methodology.