Tropospheric Ozone Climatology in Eastern Equatorial Africa

Abstract

Tropospheric ozone climatology in eastern Equatorial Africa has been at the core of this study. Seasonal and annual tropospheric ozone distribution and variation have been investigated using SHADOZ network data from Nairobi for the period 1998-2013. Meteorological parameters including air temperature, relative humidity, atmospheric pressure as well as ozone partial pressure have permitted to provide the first comprehensible tropospheric ozone climatology over this region. Mean seasonal tropospheric ozone distribution displays two distinct peaks occurring in winter with 43 DU (July) and 46.8 DU in spring (October). Comparison of mean seasonal ozone partial pressure with relative humidity profiles shows a logarithmic trend with strong regression coefficient for ozone partial pressure (0.81<R2<0.92). Conversely relative humidity variation displays a linear trend with a weak seasonal regression (0.57<R2<0.74). Seasonal vertical tropospheric ozone variation displays two ozone peaks of 121 ppbv in JJA and 126 ppbv in SON at 100 hPa respectively. A minimum photochemical source contribution from local and neighboring countries has been noted at surface to mid latitude. Investigation of individual profiles chosen as case studies for JJA (25/07/2001) and SON (09/10/2002), to assess the origin of high tropospheric ozone concentration has been performed by using back trajectory HYSPLIT model. The strong role played by the Indian Ocean in the long range transport of easterly air mass into eastern Africa at mid and upper troposphere in both seasons has been confirmed. However mean monthly NCEP/NCAR Reanalysis model used to assess the contribution of STE (Stratospheric Tropospheric Exchange), shows a positive mean eddy divergence values during the months of June to July 2001, with evidence of high vorticity 1 sigma values occurring over the same period. In summer higher positive 1 sigma divergence as well as 1 sigma vorticity values have been noted showing stronger STE activity occurring in SON period. These parameters suggest that STE contribution to ozone enhancement in equatorial east Africa is stronger during SON than in JJA. Given the complexity of climate patterns over the equatorial region, in which Nairobi is located, the influence of ENSO (El Nino-Southern Oscillation) and QBO (Quasi Biennial Oscillation) in the STE occurrence cannot be excluded as noted by previous studies.