Fast Response tunable diode laser spectroscopy for trace gas flux measurements

Abstract

A fast tunable diode laser spectrometer was applied for measurements of N2O emissions from a harvested wheat field located in Sealand, Denmark. The dual-channel instrument uses two tone frequency modulation and signal detection at 11 MHz in conjunction with rapid (1 ms) scanning of the laser. A dichroic beam combiner and a mechanical chopper allow time multiplexing. The absorption signals are recorded and analysed on-line. The combination of a software, current and temperature controlled line locking scheme results in high stability of the instrument. The time response (200 ms) of the spectrometer is limited by the measurement-cell gas exchange. For eddy correlation measurements, the trace gas mixing ratio and wind data were sampled synchronously at a rate of 10 Hz using an interrupt-driven algorithm. Mean N2O flux detection limits were 6 pptv m s−, corresponding to a deposition velocity of 2 × 10−5 m s− for a half hour measurement time. In addition, concentration gradients were determined from measurements at four different inlet heights at 0.1 ppbv precision in a shared integration time of 30 min. These measurements resulted in a flux detection limit of 7 pptv m s−1 ror the boundary layer mixing conditions encountered.