Ground-based measurements of total ozone and UV radiation by the Brewer spectrophotometer #115 at Hong Kong

Ground-based total ozone (O3) and spectral solar ultraviolet (UV) radiation measurements recorded by a Brewer #115 at Hong Kong (Cape D'Aguilar, 22°13′N, 114°15′E, 60m mean sea level) are presented. The ground-based data show good agreement with the satellite data (the correlation coefficient r is 0.942) but there is a systematic difference between the two data sets. In general, the TOMS ozone is about 3% higher than the Brewer ozone. The seasonal variations of total ozone and UV radiation from January 1995 to April 2001 are discussed. Total ozone has a seasonal cycle, it reaches a maximum in May and minimum in January. A quasi-biennial oscillation signal is clearly identified in the total ozone time series. Surface UV radiation also exhibits seasonal cycles that have maxima in July and minima in January. The solar zenith angle (SZA) is confirmed as a crucial factor governing both the seasonal change and magnitude of the surface UV radiation. Clouds and total ozone are also important. The relationship between total ozone and surface UV is discussed. The variability of surface UV due to SZA and cloud are eliminated by selecting UV measurements taken at 45° SZA and under a clear sky. The filtered UV-B shows an anti-correlation with total ozone. It is estimated that 1% increase of total column ozone can cause a 1% decrease of surface UV-B. The analysis has shown that within the period January 1995-April 2001, the total ozone over Hong Kong increased and surface UV decreased.