Yasuhiro Sasano

Light scattering characteristics of various aerosol types derived from multiple wavelength lidar observations

The present study demonstrates the potential of a multiple wavelength lidar for discriminating between several aerosol types such as maritime, continental, stratospheric, and desert aerosols on the basis of wavelength dependence of the aerosol backscatter coefficient. In the analysis of lidar signals, the two-component lidar equation was solved under the assumption of similarity in the derived profiles of backscatter coefficients for each wavelength, and this made it possible to reduce the uncertainty in the extinction/backscatter ratio, which is a key parameter in the lidar solution. It is shown that a three-wavelength lidar system operating at 300, 600, and 1064 nm can provide unique information for discriminating between various aerosol types such as continental, maritime, Saharan dust, stratospheric aerosols in a tropopause fold event, and tropical forest aerosols. Measurement error estimation was also made through numerical simulations. Mie calculations were made using in situ aerosol data and aerosol models to compare with the lidar results. There was disagreement between the theoretical and empirical results, which in some cases was substantial. These differences may be partly due to uncertainties in the lidar data analysis and aerosol characteristics and also due to the conventional assumption of aerosol sphericity for the aerosol Mie calculations.

Improved Limb Atmospheric Spectrometer (ILAS) for stratospheric ozone layer measurements by solar occultation technique

The Improved Limb Atmospheric Spectrometer (ILAS) on board the Advanced Earth Observing Satellite (ADEOS) obtained 8 months of data for trace gas species from November 1996 to June 1997 for studies of the stratospheric ozone layer over the high-latitude regions in both the Southern and Northern Hemispheres. The atmospheric parameters observed by ILAS were vertical profiles of ozone, nitric acid, nitrogen dioxide, nitrous oxide, methane, water vapor, aerosol extinction coefficient (at 780 nm), temperature, and pressure. Version 3.10 of the operational data processing algorithm was used to retrieve the corresponding geophysical profiles, which have been compared whenever possible with independent correlative measurements. Selected examples of ILAS products for ozone, nitric acid, and aerosol extinction coefficient (at 780 nm) are provided. The corresponding data is now publicly available for further analyses by the atmospheric community.

Tropospheric aerosol optical properties derived from lidar, sun photometer, and optical particle counter measurements

Tropospheric aerosols have been observed for the period from November 1990 to April 1992 with a lidar, a sun photometer, and an optical particle counter. Variations of aerosol optical thickness derived from the lidar and the sun photometer data and measurements are presented. The simultaneous measurements of these instruments also allowed us to estimate the extinction-to-backscatter ratio (S(1)), which ranged from 20 to 70. Comparison of optical thicknesses derived from both instruments clearly shows the effect of Mt. Pinatubos eruption and the temporal variation of optical thickness in the stratosphere over 12 km. The possible range of the complex refractive index for the columnar mean aerosols can be deduced from the probable range of S(1) derived by the use of an S(1) diagram as a function of complex refractive index (m). The imaginary part of m can be estimated provided that the real part of m is known.

Observations of OH, HO2, H2O, and O3 in the upper stratosphere: Implications for HOx photochemistry

Balloon-borne observations of concentrations of OH, HO2, H2O, and O3 in the middle and upper stratosphere are used to test our understanding of HOx photochemistry. Assuming our photochemical model is complete, the measured [OH] and [HO2] above 38 km (where HOx partitioning is no longer dependent on [NO]) are modeled best by calculations that use a 25% reduction in the ratio of the reaction rate constants for O+HO2 and O+OH as well as either a 25% reduction of the rate constant of OH+HO2 (the primary HOx sink) or a 25% increase in HOx production. All of these changes are consistent with the uncertainties in the recommended rate constants. The kinetic parameters required to explain our observations of [OH] and [HO2] do not lead to a resolution of the long-standing “ozone deficit problem” above 45 km.

Geometrical form factor in the laser radar equation: an experimental determination.

This paper describes a practical method for determination of the geometrical form factor in the laser radar equation. Based on the laser radar equation and the statistical homogeneity in the spatial aerosol distribution, the factor can be calculated from the field observations by laser radar. Some examples of correction by this factor are also presented.

Tropospheric aerosol extinction coefficient profiles derived from scanning lidar measurements over Tsukuba, Japan, from 1990 to 1993

Mie scattering lidar was used to observe aerosol extinction coefficient profiles in the troposphere over Tsukuba (140 E, 36 N), Japan, for three years from March 1990 to February 1993, and data obtained in fair weather were analyzed. The lidar measurements were made by a vertical scanning mode to generate profiles of extinction coefficients from the lidar level to a 12-km altitude. The extinction coefficients were derived from the lidar signals using a two-component (air molecule and aerosol) lidar equation, in which the ratio of aerosol extinction to backscattering was assumed to be constant. Seasonal average profiles were derived from individual profiles. Three-year average profiles were also calculated and modeled using mathematical expressions. The model profile assumed (1) a constant extinction ratio in the atmospheric boundary layer (ABL), (2) an exponentially decreasing extinction ratio above the ABL, and (3) a constant extinction ratio in the upper troposphere where the extinction ratio can be defined as the ratio of the aerosol extinction coefficient to the air molecule extinction coefficient. The extinction ratios both in the ABL and in the upper troposphere and the scale height that was used to express the exponential decrease were used as three unknown parameters. Seasonal variation of optical thickness that was obtained by integrating extinction coefficients with height was also investigated.

NOy-N2O correlation observed inside the Arctic vortex in February 1997: Dynamical and chemical effects

Simultaneous balloon-borne in situ measurements of total reactive nitrogen (NO ) and nitrous oxide (N 2 O) were made up to 29 km over Kiruna, Sweden (68°N, 21°E) on February 10 and 25, 1997. Kiruna was located at the edge or inside of the Arctic vortex at potential temperatures between 475 (∼19 km) and 675 K (∼26 km). Below 500 K (∼21 km) the N 2 O values were >120 ppbv on both days, and the observed NO mixing ratios agreed well with those calculated using the NO y -N 2 O correlation previously obtained at northern midlatitudes. An exception was a sharp dip in NO at 445 K (18.4 km) observed on February 25. Back trajectory analyses indicate that this layer had experienced cold temperatures close to ice saturation, i.e., favorable conditions for denitrification. Between 500 and 600 K (∼24 km) the N 2 O values were <120 ppbv, and the observed NO y values were some 4-6 ppbv lower than those calculated using the midlatitude NO y -N 2 O correlation, which includes the NO reduction due to the N + NO reaction. The temperatures in the Arctic winter above 55 K were too high to cause extensive denitrification. The combined processes of (1) diabatic descent and (2) quasi-horizontal mixing of vortex air are likely causes of the anomalous NO y -N 2 O correlation. The CH 4 -N 2 O correlation obtained inside the Arctic vortex in February 1997 also supports this hypothesis. A similar anomalous NO y -N 2 O correlation was observed from the ER-2 measurements and from the atmospheric trace molecule spectroscopy ATLAS 2 measurements made inside the vortex in the winters of 1991-1992 and 1992-1993.

Large scale laser radar for measuring aerosol distribution over a wide area

A large scale laser radar was constructed to measure aerosol distribution over a wide area. It is composed of a high-power YAG laser with an average output energy of 30 W (at 1.064 μm) and 10 W (at 532 nm) a 25-pps repetition rate, and a large (effective diameter 1.5 m) receiving telescope. Three problems which degrade the accuracy of the measurement are noted, and a discussion of how to improve the accuracy of the system is included. Noise analysis shows that this system works within theoretical limits.

Odin/SMR limb observations of stratospheric trace gases: Validation of N2O

The Sub-Millimetre Radiometer (Odin/SMR) on board the Odin satellite, launched on 20 February 2001, performs regular measurements of the global distribution of stratospheric nitrous oxide (N2O) using spectral observations of the J = 20R 19 rotational transition centered at 502.296 GHz. We present a quality assessment for the retrieved N2O profiles (level 2 product) by comparison with independent balloonborne and aircraftborne validation measurements as well as by cross-comparing with preliminary results from other satellite instruments. An agreement with the airborne validation experiments within 28 ppbv in terms of the root mean square (RMS) deviation is found for all SMR data versions (v222, v223, and v1.2) under investigation. More precisely, the agreement is within 19 ppbv for N2O volume mixing ratios (VMR) lower than 200 ppbv and within 10% for mixing ratios larger than 150 ppbv. Given the uncertainties due to atmospheric variability inherent to such comparisons, these values should be interpreted as upper limits for the systematic error of the Odin/SMR N2O measurements. Odin/SMR N2O mixing ratios are systematically slightly higher than nonvalidated data obtained from the Improved Limb Atmospheric Spectrometer-II (ILAS-II) on board the Advanced Earth Observing Satellite-II (ADEOS-II). Root mean square deviations are generally within 23 ppbv (or 20% for VMR-N2O > 100 ppbv) for versions 222 and 223. The comparison with data obtained from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on the Envisat satellite yields a good agreement within 9-17 ppbv (or 10% for VMR-N2O > 100 ppbv) for the same data versions. Odin/SMR version 1.2 data show somewhat larger RMS deviations and a higher positive bias.

Optical design of cube-corner retroreflectors having curved mirror surfaces

The characteristics of retroreflectors that include curved mirror surfaces are theoretically investigated. The use of curved mirror surfaces in a cube-corner retroreflector is an effective method for diverging the reflected beam, especially in a large-aperture hollow retroreflector. The effect of curved surfaces is demonstrated for simple examples that give circular and elliptical wave-front patterns. The method was applied in the optical design of a satellite retroreflector in space, which is to be loaded on the Japanese polar orbit Advanced Earth Observing Satellite.