Blue light-emitting diode-based, enhanced resonant excitation of longitudinal acoustic modes in a closed pipe with application to NO2

We present a new, very compact resonant photoacoustic system based on a simple closed pipe, transversally illuminated by an array of blue light-emitting diodes uniformly distributed along the tube to produce an acoustic signal from $\mathrm{N}{\mathrm{O}}_2–{\mathrm{N}}_2$ samples. The illumination is modulated in a particular way as to produce amplification of the acoustic resonance corresponding to the second longitudinal mode. The linearity of the system for $\mathrm{N}{\mathrm{O}}_2$ trace detection is studied. This arrangement provides a simple, compact, and cheap setup, useful for both measuring emissions from diesel engines and teaching photoacoustic spectroscopy in gases.