Osamu Uchino

Trends of ozone in the troposphere

Using a set of selected surface ozone (nine stations) and ozone vertical profile measurements (from six stations), we have documented changes in tropospheric ozone at a number of locations. From two stations at high northern hemisphere (NH) latitudes there has been a significant decline in ozone amounts throughout the troposphere since the early 1980s. At midlatitudes of the NH where data are the most abundant, on the other hand, important regional differences prevail. The two stations in the eastern United States show that changes in ozone concentrations since the early 1970s have been relatively small. At the two sites in Europe, however, ozone amounts increased rapidly into the mid-1980s, but have increased less rapidly (or in some places not at all) since then. Increases at the Japanese ozonesonde station have been largest in the lower troposphere, but have slowed in the recent decade. The tropics are sparsely sampled but do not show significant changes. Small increases are suggested at southern hemisphere (SH) midlatitudes by the two surface data records. In Antarctica large declines in the ozone concentration are noted in the South Pole data, and like those at high latitudes of the NH, seem to parallel the large decreases in the stratosphere.

Ozone differential absorption lidar algorithm intercomparison

An intercomparison of ozone differential absorption lidar algorithms was performed in 1996 within the framework of the Network for the Detection of Stratospheric Changes (NDSC) lidar working group. The objective of this research was mainly to test the differentiating techniques used by the various lidar teams involved in the NDSC for the calculation of the ozone number density from the lidar signals. The exercise consisted of processing synthetic lidar signals computed from simple Rayleigh scattering and three initial ozone profiles. Two of these profiles contained perturbations in the low and the high stratosphere to test the vertical resolution of the various algorithms. For the unperturbed profiles the results of the simulations show the correct behavior of the lidar processing methods in the low and the middle stratosphere with biases of less than 1% with respect to the initial profile to as high as 30 km in most cases. In the upper stratosphere, significant biases reaching 10% at 45 km for most of the algorithms are obtained. This bias is due to the decrease in the signal-to-noise ratio with altitude, which makes it necessary to increase the number of points of the derivative low-pass filter used for data processing. As a consequence the response of the various retrieval algorithms to perturbations in the ozone profile is much better in the lower stratosphere than in the higher range. These results show the necessity of limiting the vertical smoothing in the ozone lidar retrieval algorithm and questions the ability of current lidar systems to detect long-term ozone trends above 40 km. Otherwise the simulations show in general a correct estimation of the ozone profile random error and, as shown by the tests involving the perturbed ozone profiles, some inconsistency in the estimation of the vertical resolution among the lidar teams involved in this experiment.

Differential-absorption-lidar measurement of tropospheric ozone with excimer-Raman hybrid laser.

The vertical distribution of tropospheric ozone was measured by a two-wavelength differential absorption lidar based on excimer lasers. Emission of a XeCl laser (308 nm) and stimulated Raman scattering of methane (290.4 nm) pumped by a KrF laser were used. Ozone concentrations in an altitude range of 4-12 km were determined with a resolution of 750 m. The proposed method can be used to measure the ozone distribution over a wide altitude range (2-28 km) with high reliability because the excimer-Raman hybrid laser is simple and has high average power and a high repetition rate, in comparison with frequency-doubled two-wavelength dye lasers.

Random modulation cw lidar using new random sequence.

New modulation codes are presented for a random modulation cw lidar. One characteristic of these modulation codes is that for very noisy background conditions, the signal-to-noise ratio is improved by using these new sequences, and is better than for the maximum-length sequence (the M-sequence) which is commonly used as the modulation code. Another characteristic of these modulation codes is that there is no correlation between them. This fact will be useful for the simultaneous multitransmitter of the differential absorption lidar. These two characteristics of the new modulation codes were confirmed experimentally.

Observation of stratospheric ozone layer by a XeCl laser radar

We observed the stratospheric ozone layer by a newly designed powerful laser radar using a differential‐absorption technique. A discharge‐pumped XeCl excimer laser with a 308‐nm wavelength was used as an emitter in this system. The observed ozone concentrations in an altitude range 16–25 km are in good agreement with those measured by radiosonde. The present system is promising for the continuous monitoring of the stratospheric ozone concentration because of the simplicity of the construction.

Influence of volcanic sulfur dioxide on spectral UV irradiance as measured by Brewer Spectrophotometers

Spectra of UV irradiance measured by Brewer spectrophotometers at 13 stations in Japan and Canada have been examined to determine the effect of absorption by sulfur dioxide (SO2). A simple algorithm to estimate the total column amount of SO2 from global UV irradiance measurements has been developed. The algorithm which overestimates SO2 amounts at low altitudes is useful for identifying large SO2 absorption and for establishing upper limits of the SO2 values. Except at one station, the overall occurrence of days with SO2 amounts greater than 10 Dobson Units (DU) was about 0.22%. The exception was the Kagoshima observatory located 10 km from the volcano, Sakurajima. Volcanic activity resulted in frequent observation of column SO2 amounts of more than 20 DU over Kagoshima and in extreme cases up to 100 DU. The reduction in the long-term erythemally weighted UV irradiation due to absorption by SO2 is between one and two percent at Kagoshima and negligible at the twelve other stations.

Essential characteristics of the Antarctic-Spring Ozone Decline: Update to 1998

Using both ground-based and satellite data up to December 1998, the intensity, duration, appearance at both horizontal and vertical scales and ozone-mass deficiency (O3MD) of the severe ozone decline observed over the southern polar region are presented. Over the last six years: the depletion within the ozone hole area (i.e. total ozone amounts <220 m atm-cm) is almost three times more severe than in the early 1980s, the ozone hole (O3h) area has exceeded 22 million km² at its maximum, and the lowest total ozone was ∼100 m atm-cm. However, during the 1998 season for the first time for nearly 20 days the O3h area exceeded 25 million km², and the integrated O3MD during September-October, poleward of 60°S, exceeded the average value of the 1990s by ∼25%. At the beginning of the 1980s, ozone hole values were observed mostly in October, but more recently, they frequently continue until mid-December. The depletion was most severe in the 12–20 km layer over Antarctica: ∼70% in September, ∼90% in October and ∼80% in November. Related to this, the monthly mean lower stratospheric temperatures between 70–90°S have declined by ∼10°C in October, ∼12°C in November and by ∼5°C in December. Taking the last six years seasonally integrated O3MD over the area poleward of 60°S (average ∼6.300 Mt) is about a third greater than the O3MD over the northern polar region winter-springs. Over the southern mid-latitudes O3MD is about a third less than the deficiency observed between 35–55°N.

Applications of excimer lasers to laser-radar observations of the upper atmosphere

Development of efficient ultraviolet (UV) rare-gas halide excimer lasers has added new possibilities for laser radar (lidar) technique for monitoring atmospheric constituents. An experimental result of the observation of the stratospheric ozone layer by a XeCl laser (308 nm) based on the differential-absorption-lidar (DIAL) technique is described. The obtained ozone profile in an altitude range of 15- 25 km are in good agreement with those measured by radiosonde. The measurement error is analyzed for one-wavelength lidar. The accuracy is estimated to be 10-30 percent within an altitude range of 10-30 km and at a range resolution of 1 km or less. The accuracy and the resolution are higher than the Umkehr method. Recent progress of rare-gas halide lasers and their frequency conversion techniques as a transmitter for lidar are reviewed. Many powerful and reliable new sources are available in almost every wavelength over the near UV region. New applications of these UV sources to the lidar system are also briefly discussed, especially from the scientific field on the middle atmosphere.

UV picosecond pulse amplification by a XeCl laser

A UV‐preionized discharge‐pumped XeCl excimer laser effectively amplified the frequency‐doubled picosecond (3–4 ps) pulses generated by a passively mode‐locked flashlamp‐pumped rhodamine 6G dye laser tuned to the 308‐nm band. The peak power of the amplified pulse is 710 MW, and the net power gain is 151. The proposed scheme is promising for the construction of a scalable high‐power ultrashort‐pulse generator in the UV region.

Influence of differences in current GOSAT XCO2 retrievals on surface flux estimation

We investigated differences in the five currently-available datasets of column-integrated CO2 concentrations (XCO2) retrieved from spectral soundings collected by Greenhouse gases Observing SATellite (GOSAT) and assessed their impact on regional CO2 flux estimates. We did so by estimating the fluxes from each of the five XCO2 datasets combined with surface-based CO2 data, using a single inversion system. The five XCO2 datasets are available in raw and bias-corrected versions, and we found that the bias corrections diminish the range of the five coincident values by ~30% on average. The departures of the five individual inversion results (annual-mean regional fluxes based on XCO2-surface combined data) from the surface-data-only results were close to one another in some terrestrial regions where spatial coverage by each XCO2 dataset was similar. The mean of the five annual global land uptakes was 1.7 ± 0.3 GtC yr−1, and they were all smaller than the value estimated from the surface-based data alone.