Author(s)

K. TURE & G. MENGISTU TSIDU

Abstract

Volcanic activity, biological decay, forest fi res, thunderstorm events and stratospheric intrusions are key sources that introduce atmospheric traces to the troposphere. Ozone is one of such traces which can be found in very high concentration than its normal level due to natural factors. Upper and middle tropospheric ozone act as a green house gas while ozone found in high concentration at lower troposphere is a pollutant that harms living organisms. In few ozone and relative humidity data records by measurements of ozone by airbus in service aircraft (MOZAIC), spikes were observed at a fl ying altitude of 250–200 hPa at equatorial Africa. Equatorial Africa is one of the three thunderstorm hot spot regions (South America, Equatorial Africa and Southeast Asia) of the world. This work discusses how ozone of stratospheric origin produced enhanced ozone over the troposphere of equatorial Africa and its redistribution due to dynamical events. To understand the events that attribute to spiky MOZAIC ozone and relative humidity observations, in addition to MOZAIC ozone and relative humidity, data sets from different sources were used for diagnosis. Meridional cross-section of potential vorticity over the region of interest showed high potential vorticity (PV) intrusion below the tropopause level that induced enhanced ozone into upper troposphere. Vertical wind fields in the regions of low outgoing long wave radiation (OLR), very high latent heat and cloud liquid water content transport have indicated presence of convection and thunderstorm events. Enhanced ozone of stratospheric origin, as revealed by GOME satellite data produced during stratospheric intrusion, transported from the stratosphere all the way down to the boundary layer. Convective erosion was found to scavenge ozone concentration mainly at the boundary layers between 1000 and 700 hPa and enhanced ozone volume mixing ratio (VMR) of 60–100 ppbv which exceeds the WHO guideline limit of 60 ppbv were observed. These events have led to ozone pollution.

Keywords

convection, latent heat, OLR, ozone spikes, pollution, PV-intrusion, scavenging, thunderstorm, wind fields.