Abstract
Water vapor is the most important atmospheric greenhouse gas, but its variability and distribution, particularly the
vertical profile, are not well known due to a lack of reliable long-term observations in the upper troposphere and
stratosphere. Additional design and testing is necessary to extend Water Vapor Sensor System (WVSS) sensitivity to
water vapor from a threshold of 100 ppmv to 2.8 ppmv to support operational and climate applications. Laser
photoacoustic spectroscopy (LPAS) technique can extend the sensitivity to this level without extending absorption
chamber path or using expensive laser emitting at stronger absorption line. A laser photoacoustic spectroscopy sensor
based on inexpensive telecommunication style packaged, fiber-coupled near IR distributed feedback (DFB) laser diodes
was developed to quantify concentrations of water vapor (H2O), CO2, and methane in ambient air. The LPAS sensor
assembled in a compact package was designed for airborne, real-time measurements of atmospheric components. A
resonant photoacoustic cell is used to increase the photoacoustic signal, electrical modulation is applied to replace
mechanical chopper, and wavelength modulation spectroscopy is used to minimize the interfering background signal
from window absorption in the sample cell. The minimum detection sensitivities (1σ) of 5 ppm at 1.39 μm (5 mW) for
water vapor, 6 ppm at 1.6 μm (15 mW) for CO2, and 3 ppm at 1.6 μm (15 mW) for methane, are reported.
