Fast sub-ppb Trace Gas Detection by Infrared Diode-Laser FM-Spectroscopy

Abstract

Sensitivity, specifity, high time resolution and cost effective simultaneous measurements of several components using operational systems are the main requirements for atmospheric research and air pollution monitoring1 Tunable diode lasers absorption spectroscopy (TDLAS) is increasingly being used to measure atmospheric trace gas concentrations down to low ppb-levels (10-9 volume mixing ratio). Semiconductor lead salt diode lasers give access to the mid infrared spectral region, where the most important atmospheric constituents have strong rotational vibrational absorption bands. The application of high frequency modulation (FM) schemes can further improve sensitivity and detection speed of modern instrumentation. Operational instruments are needed for fast airborne measurements and sensitive and reliable field measurements. The great number of gaseous pollutants and their generally low variable concentrations with large local differences pose challenging requirements to analytical techniques. Research in atmospheric chemistry requires highly sensitive techniques for measurement of concentrations of free radicals which determine the rate of photochemical destruction of most atmospheric pollutants.