RESUMO
A new lidar instrument has been developed to measure tropospheric ozone and water vapor at low altitude. The lidar uses Raman scattering of an UV beam from atmospheric nitrogen, oxygen, and water vapor to retrieve ozone and water-vapor vertical profiles. By numerical simulation we investigate the sensitivity of the method to both atmospheric and device perturbations. The aerosol optical effect in the planetary boundary layer, ozone interference in water-vapor retrieval, statistical error, optical cross talk between Raman-shifted channels, and optical cross talk between an elastically backscattered signal in Raman-shifted signals and an afterpulse effect are studied in detail. In support of the main conclusions of this model study, time series of ozone and water vapor obtained at the Swiss Federal Institute of Technology in Lausanne and during a field campaign in Crete are presented. They are compared with point monitor and balloon sounding measurements for daytime and nighttime conditions.
RESUMO
Measurements of the spatial uniformity of Hamamatsu H5783-06 photosensor modules were performed by the flying spot method. The results were used to simulate the influence of the photomultiplier tube on a lidar signal. A simple method for improving the spatial uniformity is proposed.
RESUMO
Mixtures of methane, hydrogen, and argon (CH(4):H(2):Ar) were studied as UV Raman shifters for ozone differential absorption lidar application. They have higher photochemical stability than pure CH(4) and the capability to produce, with high enough efficiency, either first CH(4) Stokes or, simultaneously, CH(4) and H(2) first Stokes with equal energies. These mixtures can be used as an inexpensive replacement for D(2) or a more stable substitute for pure CH(4) in single-pass high-power Raman shifters.
RESUMO
Single-pass Raman cells pumped by either a quadrupled Nd:YAG (266-nm) laser or a KrF excimer laser are studied. The Raman-active gases comprise H(2), D(2), or CH(4), as well as a mixture of them, with the addition of He, Ne, or Ar. A parametric study, in which the Stokes conversion efficiency and the beam quality (M(2)) were measured, was made. The first Stokes efficiency increases and all the Stokes thresholds decrease with an increase in the lens focal length or the M(2) parameter of the pump beam. The quality of the Stokes beams deteriorates when the active-gas pressure increases but is improved by the addition of an inert gas. Laser-induced breakdown is shown to be a factor that limits the conversion efficiency and the quality of the Stokes beams. With a mixture of D(2), H(2), and Ar, a 10-15-mJ pulse energy is obtained (depending on the pump M(2) parameter) in the first Stokes beam of D(2) (289 nm) and H(2) (299 nm), with a full-angle divergence of 0.5 mrad (at 86% power).
RESUMO
From lidar signals detected with a shot per shot differential absorption lidar instrument tuned for tropospheric ozone measurements and recording each individual return, we reconstruct histograms of their sampled values for each channel of our digitizers. The analysis of their shape permits the correction of our measurements for experimental biases. In particular, a negative correlation is found between the skew of the histograms and the intensity of the backscattered light. The skew comes from a tail at high sampled values, interpreted as due to a raise of the relative contribution to the signal of a signal-induced noise when this intensity diminishes. By fitting a Gaussian function to the histograms without considering their tails, we calculate average signals unbiased by the corresponding noise. This approach allows us to increase the range of our ozone profiles, up to as much as double it in some cases.
