An investigation on the effects of column ozone on the variability of UV radiation in a tropical African region

Abstract

Quantifying local and regional Ultra-Violet B (UV-B) radiation levels is important for studies on environmental, biological and human health effects of UV. This is because Total ozone reduction, results in higher amounts of UV-B radiation reaching earth's surface and hence adverse impacts on health. Chronic eye conditions and skin diseases are likely to increase with Ozone depletion. This study is aimed at investigating the relationship of ozone and UV Radiation on selected stations in the Tropical African region namely; Brazzaville (Congo), Bahr Dar (Ethiopia), Nairobi (Kenya), Mahe (Seychelles), Lusaka (Zambia) and Nampula (Mozambique) using Total Ozone Monitoring System (TOMS) satellite data for a period of 24 years from 1979 to 2003. The study was carried out and subdivided into several parts. The first part concentrated on data quality control and homogeneity tests using residual mass curve. The homogenous ozone and UV were then subjected to time series analysis so as to determine their trend, seasonal characteristics and cyclic behavior. Linear correlation, auto correlation and periodogram' analyses were finally used to determine whether the observed UV fluctuations are as a result of' ozone. From the study time series analysis under different sky conditions showed an oscillating characteristic without any significant trend. Occurrences of large and low values were however evident. Total ozone has a seasonal cycle; it reaches a maximum in October (285 DU) and minimum in January (230 DU). Surface UV radiation also exhibits seasonal cycles that have maxima in March (165 w/m'') and minima in December (144 w/m2). Calculations from the anomaly indicated an increasing UV trend of 18.2% per decade against an insignificant 9.9% per decade of ozone decrease but leveling off in the late 90s.The variability of surface UV due to cloud cover are reduced by selecting UV values taken at about 0-100 solar zenith angle (SZA) and under low cloud cover. The UV-B shows a significant negative correlation with total ozone. For low cloud days, this study shows an increase of UV-B. Results show a general increase in surface UV radiation at 310 nrn from the end of the 1970s to the early 1990s during calendar months from December through January. This is followed by generally smaller irradiances through the middle to late 1990s for all months. The long-term variability in monthly irradiation over the time period studied is more complicated than can be described by a simple linear trend. Results from the spectral analysis identified quasi-periodic fluctuations from temporal fluctuations of ozone and UV over the six stations. These fluctuations were centered around the 4-6, 11-12, 23-25 and 34-35 months. TIle dominant 23-25 months cycles could be explained in terms of the Quasi-Biennial Oscillation (QBO) and other general atmospheric systems, while the 11-12 months depicted the annual cycle. The physical reality of the fluctuations which are centered around 4-6 and 34-35 months could however not be identified from the current study.