An ultrasensitive detection technique for tunable diode laser spectrometers: Application to detection of NO₂ and H₂0

Recent advances in room-temperature tunable diode lasers and ultrasensitive electronic noise quieting detection techniques now enable a new generation of compact, optoelectronic, ultrasensitive trace gas sensors. These advances are key to producing sensors capable of routine and extended field use. We achieve near shot noise-limited signal detection using a novel, balanced ratiometric detector (BRD) which permits measurements of absorbances of 1 : 10⁶. High sensitivity is achieved by coupling this technology with an extended optical pathlength. The BRD is characterized by a wide linear dynamic range. A 10 Hz measurement rate enables ground level flux measurements or airborne concentration measurements. We will present an overview of two applications of our ultrasensitive detection technology to in situ atmospheric sensing. The first sensor is being developed to monitor boundary layer NO₂ fluxes. This sensor operates at 670 nm, utilizes an open multipass optical cell, and has a sub-ppbv detection sensitivity. The second sensor is an airborne, near JR (1 .39 μm) diode laser hygrometer. The sensor uses an in-situ air measurement probe housing a 50 cm, open optical path to circumvent problems inherent in extractive sampling. The sensor is capable of measuring water vapor throughout the troposphere and has a sensitivity of 0.5 ppmv at the tropopause.