Detection of nitrogen dioxide by cavity attenuated phase shift spectroscopy

Area of Science:

• Atmospheric chemistry
• Optical sensing technologies
• Spectroscopy

Background:

• Accurate monitoring of atmospheric nitrogen dioxide (NO2) is crucial for understanding air quality and environmental impacts.
• Existing methods for NO2 measurement can be complex or expensive, necessitating the development of more accessible technologies.

Purpose of the Study:

• To present initial results from a novel optical absorption sensor for ambient atmospheric nitrogen dioxide (NO2) monitoring.
• To evaluate the sensitivity and reliability of the developed sensor for detecting NO2 concentrations ranging from 0 to 200 ppb.

Main Methods:

• Utilized cavity attenuated phase shift spectroscopy (CAPS), a technique related to cavity ringdown spectroscopy.
• Employed a modulated broadband incoherent light source (430-nm LED) coupled to an optically resonant cavity.
• Measured the phase shift in the photodetector signal, which is proportional to NO2 concentration.

Main Results:

• Achieved a sensitivity of 0.3 ppb in the photon (shot) noise limit using a 0.5-m cell.
• Demonstrated reliable and quantitative measurement of both large and small ambient NO2 concentration fluctuations over a 2-day monitoring period.
• Showed potential for integration times of 10 s or less with improved optical coupling, reaching a noise equivalent absorption coefficient of <1 x 10(-8) cm(-1) Hz(-1/2).

Conclusions:

• The developed CAPS-based optical absorption sensor is a reliable instrument for quantitative monitoring of ambient atmospheric nitrogen dioxide.
• The sensor shows promise for widespread environmental monitoring applications due to its sensitivity and ability to detect concentration fluctuations.