An investigation of high ozone episodes in the City of Johannesburg

Abstract

A study of ozone monitoring data in Johannesburg highlighted that the city is frequently affected by high ozone episodes. There is limited knowledge about the chemical and meteorological drivers of these high ozone episodes in Johannesburg. In this paper, a case study of the Delta Park air quality monitoring station is used to describe the process used to identify ozone episodes and the drivers thereof in the city. In 2005 alone, it was found that there were 483 exceedances of the 8-hourly South African National Ambient Air Quality Standard for ozone at Delta Park. The bulk of these episodes occurred in the summer (DJF) and spring (SON) months. Examples of the pollution events during these seasons occurring on 3 February 2005 and 16 September 2005, as well as an event on 31 July 2005 occurring during winter (JJA) are presented. Concentrations of ozone precursor gases at the station were not high for days prior to and on the day of the three ozone episodes. It was unlikely that local sources of pollution contributed to the high ozone concentrations measured at Delta Park station. To determine the origin of the air masses which were transported over Delta Park for the 3 episodes, the Hybrid Single Particle Lagrangian Integrated Trajectory Model (HYSPLIT) model was used. The results of the case study are used to make recommendations for air quality improvements in the city.