Amplitude modulated multimode-diode-laser-based cavity enhanced absorption spectroscopy with a phase-sensitive detection for high-sensitivity NO2 detection

Accurate and sensitive measurements of NO₂ play an extremely important role in atmospheric studies. In the past 20 years, high sensitivity, precision, and accurate NO₂ detection technology has developed rapidly, especially optical methods based on high-finesse cavities. In combination with chemical conversion, NO₂ detector is used to measure a range of other important reactive atmospheric species, such as total reactive nitrogen (total peroxy nitrate (ΣPNs), total alkyl nitrate (ΣANs), and nitric acid (HNO₃)), total peroxy radicals, O₃ and nitric oxide (NO), which greatly enhanced our understanding of nitrogen chemistry, free radical chemistry, and atmospheric oxidation capacity.

In this presentation, we will report the development of an Amplitude Modulated multimode-diode-laser-based Cavity Enhanced Absorption Spectroscopy (AM-CEAS) system operating at 406 nm that uses phase-sensitive detection for extremely sensitive NO₂ detection. A detection limit of 35 pptv (1σ, 1s) was achieved with reflectivity R ~ 99.985% (ring-down time τ₀ ~ 10.87 μs). When the integration time was extended to 30 s, the precision can be further improved to 8 pptv. The reported AM-CEAS method provides a powerful, straightforward, and general method for ultra-sensitive absorption and extinction measurements.