Absolute density measurement of nitrogen dioxide with cavity-enhanced laser-induced fluorescence

The absolute number density of nitrogen dioxide (NO₂) seeded in argon is measured with cavity-enhanced laser-induced fluorescence (CELIF) through using a pulsed laser beam for the first time. The cavity ring down (CRD) signal is acquired simultaneously and used for normalizing the LIF signal and determining the relationship between the measured CELIF signal and the NO₂ number density. The minimum detectable NO₂ density down to (3.6±0.1)×10⁸ cm^-3 is measured in 60 s of acquisition time by the CELIF method. The minimum absorption coefficient is measured to be (2.0±0.1)×10^-10 cm^-1, corresponding to a noise equivalent absorption sensitivity of (2.2±0.1)×10^-9 cm^-1·Hz^-1/2. The experimental system demonstrated here can be further improved in its sensitivity and used for environmental monitoring of outdoor NO₂ pollution.