A
comparison of the total ozone measurements done with |
date of original version 2.0: 28 December 2004 |
Summary | A series of year-long comparisons between
total ozone column measurements done
with a Brewer at the RMI at Uccle (Belgium) and the Microtops II
sunphotometer at Diekirch (Luxembourg) shows a good linear relationship between both
instruments. From 1997 to April 2003 the Microtops #3012 has been used; from
April 24th 2003 it has been replaced by Microtops #5375 as the main
instrument; however both spectrometers were operational during the
remainder of 2003. A previous report suggests to apply a multiplier of 0.93 to 0.95 to the Microtops #3012 readings which are most of the time in excess to the Brewer (and the #5375); the agreement of the newer #5375 instrument and Brewer is much better and practically does not need any adjustment. The Microtops II #5375 was last calibrated at Solar Light Co. in December 2002. This paper also compares the Brewer and Microtops readings with those of the TOMS and the GOME satellites for 2003. |
INDEX: |
1. Reminder on Uccle and Diekirch locations and previous findings |
3. Comparison between Microtops # 5375 and Uccle Brewer | |
5. Conclusion | |
6. Acknowledgments | |
7. References |
1. Reminder on Uccle and Diekirch locations and previous findings
Uccle (Belgium) station 53 of the WOUDC |
50°48' North 4°21' East altitude = 100m a.s.l. |
Brewer spectrophotometer automatic measurements |
zenith sky measurements and direct sun (DS) measurements |
Diekirch (Luxembourg) station 412 of the WOUDC |
49°52' North 6°10' East, altitude = 218m a.s.l. |
Microtops II Sunphotometer manual measurements |
only direct sun measurements (weather permitting) |
The conclusion from the years 1997 to 2002 comparison between Microtops #3012 and the supposed correct Brewer DS measurement is to apply multiplier of 0.93 - 0.95 to the Microtops readings to give a good agreement with the Brewer.
2. A comparison between the Microtops #3012 and #5375
During the whole year 2003 all total ozone column measurements were done at Diekirch with both Microtops instruments. The main differences between the 2 instruments are in slightly different wavelenghts used to measure the ozone thickness and minor firmware modifications. Instrument #3012 uses the 3 wavelengths 300, 305.5 and 312.5nm, whereas #5375 uses 305.5, 312.5 and 320.5nm.
3. Comparison between Microtops # 5375 and Uccle Brewer
There are 84 readings allowing a direct comparison between Brewer and Microtops #5375; please note that only DIRECT SUN readings taken at times less than 1 hour apart have been retained (these will be called "common" readings)!
year | Brewer direct sun | Microtops II #5375 |
2003 | N = 84 319.1 +/- 33.3 |
N = 84 317.3 +/- 30.2 |
differences with Brewer DS means: 1.8 DU |
The mean values are practically the same!
4. Comparing Brewer, Microtops, Toms and assimilated Gome data
Uccle and Diekirch readings will now be compared with those of 2 satellites: Nasa's TOMS [1] which passes Uccle every day at approx. 10:00 UTC (+/- 30 minutes), and the now defunct GOME's instrument assimilated data [2] on the European ESR-2 (assimilated means the actual readings are interpolated or computed by applying a model to the same hour as the Uccle/Diekirch measurements); it should be noted that the TOMS data are taken up to 2 hours earlier to the Uccle and Diekirch readings.
The greatest differences exist between the Uccle and the satellite data; the Microtops mean values are in-between; even if some Toms values are much lower than Uccle's at the start of May, they are not systematically lower for the full period from 24th April to 11th May (which could suggest a Brewer problem). The Gome data are practically always the lowest of the 4 sensors.
5 Conclusions The Microtops II #5375 measured mean total ozone column is virtually the same as the mean value from the Brewer direct sun measurements. In this comparison the "corrected O3" data from the Microtops have been used. Most of the time the absolute differences are in a [-10, +10] DU interval. The new Microtops (#5375) gives usually much better agreement with the Brewer than the previous one (#3012), there is no need to apply a corrective multiplier. The Microtops mean values are closer to the Brewer than are those of the TOMS and GOME satellites. It should be noted that good operational practice of the Microtops requires careful pointing to the sun [3], and is best done by the same operator. |
Special thanks to Dr. De Backer of the Royal Meteorological Institute of Belgium (RMI) for providing the Uccle Brewer data and his continuing help.
1. | TOM's data are available at: ftp://jwocky.gsfc.nasa.gov/pub/eptoms/data/overpass/OVP053_ept.txt |
2. | GOME's data are from: http://www.knmi.nl/gome_fd/tm3/ascii_cal2003.html |
3. | Francis Massen : Influence of intentionally mis-pointing on Microtops II readings [2004] |
4. | Dr. Hugo De Backer, RMI: personal communication |
5. | NASA Toms calibration problems: http://toms.gsfc.nasa.gov/news/news.html#nov18 |
History of versions:
2.0 original version 28 Dec 2004
2.01 added comment on recalibration of Brewer 016
in May 2003 and [ref.4] + [ref.5]
Please send all mail to francis.massen@education.lu |