Nianwen Cao

Trace atmospheric SO2 measurement by multiwavelength curve-fitting and wavelength-optimized dual differential absorption lidar

We present a new differential absorption lidar (DIAL) method for atmospheric trace SO2 using multi-wavelength curve fitting. With this method we use five wavelengths around a SO2 absorption peak and obtain SO2 and O3 concentrations by fitting their absorption cross sections to measured DIAL and null results. A SO, concentration of 6 parts in 10(9) (ppb) was obtained for an altitude of 1050 m with 150-m range resolution. In addition, we optimized the wavelengths for dual-DIAL SO2 measurement and demonstrated a high sensitivity of <0.5 ppb with 300-m range resolution. Comparison of these two methods is also presented.

Sum-frequency-generation system for differential absorption lidar measurement of atmospheric nitrogen dioxide

A sum-frequency-generation system for differential absorption lidar measurement of atmospheric nitrogen dioxide in the lower troposphere was developed. The system uses a combination of a pair of KD*P crystals and a tunable dye laser with LDS 765 dye pumped by the second harmonic of a Nd:YAG laser to generate lambdaon and lambdaoff alternatively. Compared with the conventional system that uses Coumarin 445 dye pumped by the third harmonic, the output energy and long-term stability were improved. By use of this system, atmospheric NO2 concentrations of approximately 10-50 ppb were measured, with an instrumental error of approximately 7 ppb.

Differential absorption lidar system for simultaneous measurement of O3 and NO2: system development and measurement error estimation

A differential absorption lidar system for simultaneous measurement of O 3 and NO 2 in the lower troposphere is developed. The system is based on a pair of Nd:YAG pumped dye lasers each capable of emitting two wavelengths on alternate pulses. Sum frequency generation is used to generate laser pulses of wavelengths 446.8 and 448.1 nm used for NO 2 measurement. O 3 concentrations of approximately 30 to 60 ppb and NO 2 concentrations of approximately 4 to 45 ppb are obtained for a vertical range of 1000 to 1500 m. Changes in O 3 and NO 2 concentration over a time interval of a few hours suggests an anticorrelation between these quantities. The measurement error, estimated from statistical error and instrumental error, is 2.5 ppb for O 3 concentration and 5.5 ppb for NO 2 concentration. In the case of NO 2 measurement, the error due to differential backscatter gradient is found to be a major source of error, but does not fully account for the measurement error.

Estimation of differential absorption lidar measurement error for NO2 profiling in the lower troposphere

Measurement error for NO2 profiling in the lower troposphere, including the effects of aerosol backscatter, molecular backscatter, and extinction due to inhomogeneous aerosol distribution, is estimated for vertical NO2 concentration profiles measured by differential absorption lidar (DIAL). Vertical NO2 concentration profiles of 0 to 40 ppb and null profiles are obtained for an altitude of 900 to 2250 m with a 150-m range resolution. The deviation of the null profiles from zero, which provides a measure of how precisely the laser beams were aligned, was ,2.0 ppb. The mean statistical error, systematic error from aerosol, and absorption cross section error were 3.4, ,3.0, and ,1.2 ppb, respectively.

Tropospheric O3 measurement by simultaneous differential absorption lidar and null profiling and comparison with sonde measurement

A differential absorption lidar (DIAL) system consisting of two identical tunable laser systems and a single optical receiver is applied to measurement of O3 concentration profiles in the lower troposphere. Each laser is capable of emitting two wavelengths on alternate pulses, so the system is capable of simultaneous measurement of two species in the same wavelength region. We set the two lasers to emit at identical wavelength pairs consisting of on wavelength 285.0 nm and off wavelength 290.1 nm for simultaneous measurement of two null profiles, one at each wavelength, and two DIAL profiles, or O3 concentration profiles. Null profiles are useful in estimating instrumental error and checking the vertical range interval in which the DIAL profiles are accurate. Null and DIAL profiles are obtained for vertical range 1000 to 4000 m using neutral density filters of different transmissions to prevent the strong return signals from close range from saturating the photodetector. The obtained O3 concentration profiles agree with simultaneous O3 sonde measurements. An evaluation of the measurement error shows that the average O3 measurement error for vertical range 1000 to 4000 m was 3.4 ppb, or 8% relative to the average O3 concentration of 42.3 ppb, most of which is due to statistical error. The error due to differential Mie attenuation and differential backscatter gradient was found to be 0.5 ppb.

Tropospheric profiling of SO2 and O3 by multiwavelength DIAL

A multiwavelength differential absorption lidar (DIAL) system for measurement of trace atmospheric substances was developed. Vertical concentration profiles of SO 2 in the lower troposphere were measured by three wavelength dual-DIAL (on wavelength=300.05 nm, off wavelengths=299.35 nm, 300.90 nm, with null profiles obtained at 300.05 nm). The measurement wavelengths were chosen to minimize the effect of O 3 . Measurement results showed an average SO 2 concentration of 0.6 ppb for height 2000-4000 m, and the SO 2 measurement error was estimated to be about 1 ppb for 150 m range resolution. Although the measured SO 2 concentration was below the resolution of our system, the results show that the system is capable of measuring trace SO 2 with approximately 0.15 ppm-m resolution. In addition, vertical O 3 concentration profiles in the lower troposphere were measured using two identical DIAL pairs (on wavelength=285.0 nm, off wavelength=290.1 nm), from which two null profiles and two DIAL profiles were obtained simultaneously. The DIAL profiles, which showed an average O 3 concentration of 42 ppb for height 1000-4000 m, agree with simultaneous O 3 sonde measurements. The effect of backscatter gradients was shown to be small. The O 3 measurement error was estimated to be 3.4 ppb for 150 m range resolution, or about 0.5 ppm-m.

Simultaneous profiling of O3 and NO2 in the lower troposphere by multiwavelength differential absorption lidar

Vertical concentration profiles of O3 and NO2 in the lower troposphere were measured simultaneously using a multi wavelength differential absorption lidar (DIAL) system based on a pair of Nd:YAG pumped dye lasers each capable of emitting two wavelength on alternate pulses. A mixture of Rhodamine 590 and Rhodamine 610 dyes and second harmonic generation was used to generate wavelengths 288.2 nm and 293. 5 nm for O3 measurement, and sum frequency mixing of LDS765 dye laser radiation with Nd:YAG fundamental was used to generate wavelengths 448.1 nm and 446.8 nm for NO2 measurement. O3 profiles of approximately 30-50 ppb and NO2 profiles of 0-20 ppb were obtained for vertical range 1500-2500 m. The measurement error was estimated to be < 6 ppb for 150 m range resolution, or < 0.9 ppm-m, for both O3 and NO2.

DIAL measurement of daytime variations of vertical tropospheric O/sub 3/ concentration profiles

DIAL measurement of vertical daytime O/sub 3/ concentration profiles was performed. The DIAL system linearity was checked using O/sub 3/ S-value profiles. In the winter season, O/sub 3/ concentration is variable between 50-270 ppb.