Laser spectroscopic analysis of atmospheric gases in scattering media

Sammanfattning: Laser spectroscopic techniques for gas analysis under various influences of light scattering are explored in a wide spectral range from the ultraviolet to the infrared. The techniques rely on spectrally narrow-band light sources that are used in differential optical absorption measurements. A DIfferential Absorption Light detection and ranging (DIAL)application for the monitoring of atomic gaseous mercury, utilizing ultraviolet light from an Optical Parametric Oscillator (OPO), is introduced. Results from six two-week field campaigns at chlor alkali plants in Sweden, Italy, and Poland, performed within a cross disciplinary European Union funded project concerning European Mercury Emissions from Chlor-Alkali Plants (EMECAP) are presented. The resulting time series of the mercury flux and the animations showing the spatial concentration distributions constitute important input for the environmental community. In addition, by employing a multiple-wavelength DIAL technique in the infrared spectral region, simultaneous remote measurement of mixtures of gaseous substances with overlapping spectra is achieved. A diode-laser-based spectrometer comprising nonlinear frequency mixing for the detection of sulfur dioxide in the ultraviolet region is presented, as well as a temporal gas correlation procedure that does not require any knowledge of the exact spectrum of either the gas or the light. Finally, a novel concept of non intrusive, real-time tunable-laserbased GAs in Scattering Media Absorption Spectroscopy (GASMAS) measurements is introduced. Sensitive detection of absorption is obtained by utilizing wavelength modulation spectroscopy. Combined with time resolved measurements employing Time-Correlated Single-Photon Counting (TCSPC), the gas concentration in various porous materials is estimated. Applications related to the food packaging industry, e.g., Modified Atmosphere Packaging (MAP), are explored by studies of gas exchange in horticultural produce. Furthermore, applications related to the building industry are explored in studies on gas transport and drying processes in wood by the detection of molecular oxygen and water vapor in the near infrared spectral region.