Dual channel photoacoustic hygrometer for airborne measurements: background, calibration, laboratory and in-flight inter-comparison tests

Abstract

Abstract. This paper describes a tunable diode laser based dual channel photoacoustic (PA) humidity measuring system called WaSul-Hygro primarily designed for aircraft based environment research. It is calibrated for total pressures and water vapor (WV) volume mixing ratios (VMRs) possible during airborne applications. WV VMR is calculated by using pressure dependent calibration curves and a cubic spline interpolation method. Coverage of the entire atmospheric humidity concentration range which might be encountered during airborne measurements is facilitated by applying an automated sensitivity mode switching algorithm. The calibrated PA system was validated through laboratory and airborne inter-comparisons, which proved that the repeatability, the estimated accuracy and the response time of the system is 0.5 ppmV or 0.5% of the actual reading (whichever value is the greater), 5% of the actual reading within the VMR range of 1–12 000 ppmV and 2 s, respectively. The upper detection limit of the system is about 85 000 ppmV, limited only by condensation of water vapor on the walls of the 318 K heated PA cells and inlet lines. The unique advantage of the presented system is its applicability for simultaneous water vapor and total water volume mixing ratio measurements.