Differential Absorption Lidar Techniques in the Determination of Trace Pollutants and Physical Parameters of the Atmosphere

Line-pair selections for remote sensing of atmospheric ammonia by use of a coherent CO2 differential absorption lidar system

Research on wavelength selection of CO2 laser lines for range-resolved remote sensing of atmospheric ammonia by use of a coherent differential absorption lidar system is described. Four laser line pairs are suggested for different levels of ammonia concentrations from approximately a few parts per billion to 1 part per million in a polluted atmosphere. The most suitable line for measuring ambient ammonia concentrations is 9R(30), because it has the highest absorption coefficient. 10R(14) has the lowest absorption coefficient, making it suitable for strong source mapping. 10R(8) and 10P(32) are best for intermediate levels of ammonia concentration. Absorption coefficients of ammonia calculated from the HITRAN96 database are in good agreement (mostly within +/-10% )with other experimental results.Sensitivity of measurement, interference from water-vapor lines with typical humidity in the summer,and sensitivity of ammonia absorption cross section to temperature and pressure are analyzed and calculated for the four wavelength pairs. The results show that the interference from water-vapor lines is easily correctable to a negligible amount, and errors caused by uncertainties in temperature and pressure are insignificant.

Quantitative chemical identification of four gases in remote infrared (9-11 mum) differential absorption lidar experiments

A combined experimental and computational approach utilizing tunable CO(2) lasers and chemometric analysis was employed to detect chemicals and their concentrations in the field under controlled release conditions. We collected absorption spectra for four organic gases in the laboratory by lasing 40 lines of the laser in the 9.3-10.8-mum range. The ability to predict properly the chemicals and their respective concentrations depends on the nature of the target, the atmospheric conditions, and the round-trip distance. In 39 of the 45 field experiments, the identities of the released chemicals were identified correctly without predictions of false positives or false negatives.

Temperature dependence of water vapor absorption coefficients for CO(2) differential absorption lidars

A temperature correction of water vapor differential absorption coefficients for the CO(2) transition line pairs (10R20, 10R18) and (10R20, 10R22) for temperatures between -0.5 °C and 20 °C is computed, with a reference temperature of 27 °C, from medium-range CO(2) lidar field measurements. The empirical temperature correction, X(T), is fitted with the polynomial X(T) = α(0) + α(1) × T + α(2) × T(2). For the transition line pair (10R20, 10R18) the temperature dependence ranges from 1.62%/°C to 3.47%/°C, and the temperature correction for the transition line pair (10R20, 10R22) ranges from 1.32%/°C to 2.43%/°C.

Measurement of the NH(3), CCl(2)F(2), CHClF(2), CFCl(3), and CClF(3) absorption coefficients at isotopic (13)C(16)O(2) laser wavelengths by photoacoustic spectroscopy

Owing to the linearity of the photoacoustic signal, the absorption coefficients of Freons were measured on isotopic (13)C(16)O(2) laser lines.

High pulse repetition frequency, multiple wavelength, pulsed CO(2) lidar system for atmospheric transmission and target reflectance measurements

A multiple wavelength, pulsed CO(2) lidar system operating at a pulse repetition frequency of 200 Hz and permitting the random selection of CO(2) laser wavelengths for each laser pulse is presented. This system was employed to measure target reflectance and atmospheric transmission by using laser pulse bursts consisting of groups with as many as 16 different wavelengths at a repetition rate of 12 Hz. The wavelength tuning mechanism of the transversely excited atmospheric laser consists of a stationary grating and a flat mirror controlled by a galvanometer. Multiple wavelength, differential absorption lidar (DIAL) measurements reduce the effects of differential target reflectance and molecular absorption interference. Examples of multiwavelength DIAL detection for ammonia and water vapor show the dynamic interaction between these two trace gases. Target reflectance measurements for maple trees in winter and autumn are presented.