Influence of photolysis on multispectral photoacoustic measurement of nitrogen dioxide concentration

Multispectral photoacoustic instruments are commonly used to measure aerosol and nitrogen dioxide (NO₂) light absorption coefficients to determine the radiation budget of the atmosphere. Here a new photoacoustic system is developed to explore the effect of photolysis on the measured signal in a multispectral photoacoustic spectrometer. In this system, a 405-nm laser is used primarily as light source for photolysis. Additionally, a well-overlapped 532-nm laser, modulated at the resonant frequency of the photoacoustic instrument, is used to probe the NO₂ concentration. As a result, the photolysis effect at 405 nm can be observed by the photoacoustic instrument through the 532-nm laser. This work determines an 11% reduction of the photoacoustic signal caused by the photolysis effect for typical conditions, which needs to be taken into account when calibrating multispectral photoacoustic spectrometers with NO₂.

Implications: Multispectral photoacoustic instruments are commonly used to measure aerosol and nitrogen dioxide (NO₂) light absorption coefficients to determine the radiation budget of the atmosphere. A 405-nm laser is often used in these multispectral photoacoustic instruments. Although NO₂ absorbs strongly at 405 nm, it also has a strong photolysis pathway that is accessible by light of the same wavelength. Photolysis reduces the photoacoustic signal, necessitating special care when interpreting photoacoustic measurements. This paper offers a method for the multispectral photoacoustic instrument user to quantify the influence of the 405-nm NO₂ photolysis effect on the photoacoustic signal.