Home > Guidance > Journal of the Aerological Observatory > Vol.64
| Title | Author | |
|---|---|---|
| Foreword | in Japanese | Hamada Tadaaki |
| Cases of Forced Descent by Icing in Rawin-Sonde Observation | in Japanese | Yasutomo KIKUCHI |
| The Trial for Secular Change Consideration of Radioactivity Sonde Observation Data | in Japanese | Shogo INOUE, Shohei IWATSUBO, Masashi SATO, Tatsuru FUJITA and Yasuyuki TAKEI |
| Reflected Spectral UVB Observation on the Ground Surface using Modified Brewer Spectrophotometer | [Abstract] | Mahito ITO |
| Development of the Calibration System for Brewer DS O3 and DS SO2 Measurements and their Observation Results in the JMA Brewer MKIII UV Network | [Abstract] | Mahito ITO and Koji MIYAGAWA |
| Some Tests Results of a New UV Spectrophotometer | [Abstract] | Mahito ITO |
| The Correction of Global Ultraviolet Radiation by Brewer Spectrophotometers over Japan for the Directional (Cosine/Azimuth) Responses | in Japanese | Atsushi SAITO |
| On the Grating Sunphotometer PGS-100 (Part II) | in Japanese | Osamu IJIMA |
| On the Net-Radiometer CNR1 (Part II) | in Japanese | Katsue NAGAI and Hiroshi NAGANUMA |
| Introduction of Precision Pyranometers with Low Zero Offset Errors in Diffused Solar Radiation Measurement | in Japanese | Hiroshi NAGANUMA and Katsue NAGAI |
A modified Brewer spectrophotometer and the calibration method for observing reflected spectral UVB on the ground surface were developed at the Aerological Observatory, Tsukuba. In this paper, the details of the Brewer modification for use at reverse position, the calibration procedure to ensure the instrument factors of modified Brewer at reverse position, and the results of spectral UVB reflectivity measurement of the ground under some typical surface conditions in winter season are presented. They are summarized as follows;
1) Brewer spectrophotometer MKII #059 was converted for accurate reflected spectral UVB measurement on the ground equipping with adjust foot #1 and #2, plates #1 and #2, level adjusters, and the cover. The spectrophotometer was carefully tuned by the tension adjustment of micrometer spring, the adjustment and fixing of mirror, the adjustments of zenith prism and iris of fore-optics, and full tightening of all screws in the instrument. The program of routine observation, skd.rtn, was modified for this purpose.
2) NIST lamp tests using the new calibration apparatus (Ito: 2002) showed that the difference between the calibration factors at the normal and the reverse position of modified #059 was within 0.1%. The modified #059 at reverse position was proved to work normally in the tests of b2-ci, b1-ci, sl, hg, dt, rs, and others.
3) The daily responsivity trend of modified #059 in reflected UVB observation were traceable as accurately as in the normal operation of Brewers through regular external and internal lamp tests.
4) Global UVB irradiances by modified #059 at normal position and the Japanese standard #113 agreed within 1%.
5) Spectral UVB reflectivity measurements on the ground were carried out by modified #059 at reverse position and #113 (temporarily replaced by #052) at normal position under some typical surface conditions. Both of the instruments were run by the time-controlled scan of uz.rtn which is a special routine prepared for instrument intercomparison. The results clarified (a), (b), and (c) as stated below;
(a) Dry concrete surface on the rooftop indicated the UVB reflectivity of about 0.07. It reduced to 0.05 when the surface was wet/pooled after rainfall. Linear wavelength-dependence of the spectral reflectivity was recognized, namely it showed higher values at longer wavelengths.
(b) The UVB reflectivity of dried grass in winter season was about 0.018 and 0.015 under clear/cloudy and rainy sky conditions, respectively. In these cases, no significant wavelength-dependence of spectral UVB reflectivity was observed for all weather conditions. Frost-covered grass surface in the morning showed the reflectivity of about 0.07.
(c) On the fresh snow surface, the maximum UVB reflectivity reached 0.94 under clear sky. The reflectivity decreased gradually as the snow melting proceeded, but it was kept above 0.60 until the grass surface appeared. Spectral UVB reflectivity on the snow surface showed no significant wavelength-dependence as a whole, but a weak declination at longer wavelength was shown during the time interval around the maximum reflectivity.
In atmospheric ozone monitoring careful data quality control plays an essential role for correct analysis and evaluation of the long-term ozone trend. In this connection the first WMO summary report on the comparison of total ozone measurements of Dobson and Brewer spectrophotometers (WMO:2003) says that the simultaneous operation of Dobson and Brewer instruments at the same station is highly recommended to improve the reliability of the total ozone measurements of the station. A problem in this measurement redundancy, however, is the present situation that no officially authorized calibration mechanism exists for Brewer, while Dobson is standardized in the solid calibration hierarchy established by the GAW/WMO.
As an approach to this problem in the Brewer MKIII UV observation network of the Japan Meteorological Agency (JMA), we have developed the calibration system for Brewer DS O3 measurement on the basis of the total ozone comparison between Dobson and Brewer performed under the most appropriate clear atmospheric conditions. For Brewer DS SO2 measurement, which is needed to evaluate the influence of SO2 on the Dobson total ozone measurement, a field Brewer MKIII traceable to one of the Brewer standards maintained by the Meteorological Service of Canada (MSC)/ Kipp & Zonen was used as the working calibration reference for other field Brewers.
The Japanese Brewer MKIII UV observation network is run concurrently with the Dobson ozone network comprised of Sapporo, Tsukuba, Kagoshima, Naha, and Syowa Antarctica. These overlapped networks continue to store quasi-simultaneous total ozone comparison data as well as SO2 since 2002. Long-range data accumulation into the future with these networks is expected to offer valuable data set to further enrich the understanding of the difference between Dobson and Brewer, which will lead to more accurate and sensitive ozone trend monitoring at these stations.
The summary of the calibration procedures and observation results in the latest few years are given below.
1) The instrument parameters of Brewer MKIII for DS O3 measurement were calibrated by comparing with any of the Dobson Asian standard D#116, field Dobson at local station, or traveling standard D#129. The calibration accuracy was shown to be within about 1 per cent.
2) For Brewer DS SO2 measurement, calibration was done by comparing with Brewer MKII B#113 which was traceable to the Canadian Brewer MKIII B#158 via Brewer MKIII B#174. The SO2 calibration was difficult to carry out because the chance of having enough amount of SO2 for calibration was rather sporadic to happen.
3) On a daily basis the total ozone measurements by Dobson and Brewer sometimes indicated large discrepancies when ozone changed violently. This was due to the difference of the calculation method of daily value (Brewer: daily mean, Dobson: daily representative). On yearly time scale, both of the measurements agreed very well.
4) Observation results in the latest two years showed that the difference between Brewer MKIII DS O3 and the total ozone by Dobson stayed within about 1 per cent except for Syowa Antarctica. Excellent agreement was observed at Sapporo, Tsukuba, Kagoshima after 2003, and Naha where the total ozone variation is small through seasons.
5) The difference between Brewer DS O3 and the total ozone by Dobson showed similar seasonal variation as stated in the WMO summary report. The variation was small at Sapporo, Tsukuba, and Naha, and rather large at Kagoshima. The variation at Syowa Antarctica was the most distinctive.
6) Through Brewer DS SO2 measurement some characteristic behaviors of the SO2 at these stations were revealed. The SO2 at Sapporo kept nearly constant values of 2 to 3 m atm-cm through seasons. Whether it was caused by calibration error or urban background air pollution is still uncertain. Abrupt increase of SO2 was sometimes observed at Tsukuba. These phenomena could be the influence of Miyakejima volcano, about 220 km southwest of Tsukuba, but not yet confirmed. Sudden high level of SO2 lasting for few days often occurred at Kagoshima, which could be ascribable to the eruption of Sakurajima volcano located about 10 km from the station. The frequency of the sudden increase at Kagoshima seemed to have seasonal variation. The SO2 at Naha sometimes showed sudden but a small amount of increase. It was noticeable that the SO2 level over 5 m atm-cm was rarely observed even at Syowa Antarctica.
A new grating UV spectrophotometer is currently under development aiming to measure ultraviolet irradiance in the wavelength region from 200 to 400 nm. The new instrument has Si-CCD array detector and capable of measuring 960 channels simultaneously in the wavelength region. To evaluate the performance of the new spectrophotometer, some tests using spectrum lamps and NIST lamps were carried out. The summary of the results is shown below.
1) Scanning tests with spectrum lamps showed that the scanning accuracy was within 0.7 nm. The wavelength resolution was about 1.6 nm.
2) NIST lamp tests were repeated by changing the distance between the lamp and the detector surface to find the optimum distance for calibration. The result was that the calibration of this instrument could not be properly performed at the distance other than 50 cm due to stray light and dark counts. Even at the distance of 50 cm, the calibration in the wavelength region shorter than 280 nm proved to be extremely difficult because of dark counts.
3) Relative responsivity fluctuations during scans were about ±2% throughout the wavelength region from 200 to 400 nm.
4) Dark counts of the instrument increased as the irradiance level became higher by shortening the distance between the NIST lamp and the detector surface.
5) The lamp tests with and without the UV dome at the distance of 20, 30, and 50 cm clarified that the transmissivity of the dome was about 98%.
6) As practical operational modes for outdoor solar irradiance measurement with this instrument, the exposure time should be longer than 50 ms and the scanning over 50 times in each measurement. The UVA and UVB observations can be carried out every minute with this mode.
The problems caused by stray light and dark counts are expected to improve by furnishing with a cut-off filter and a thinner diffuser plate. Another improvement needed for practical use of this instrument may be the software to process and convert the raw data into user-friendly form depending on the purpose.