Tropospheric ozone (O3) pollution in Johannesburg, South Africa: Exceedances, diurnal cycles, seasonality, Ox chemistry and O3 production rates

Abstract

Ground-level ozone (O₃) is an air pollutant of major health and environmental concern. The Johannesburg-Pretoria megacity in South Africa is the industrial and economical capital of the country with more than 10 million inhabitants experiencing poor air quality. In 2004, the City of Johannesburg (CoJ) began monitoring trace gases to assess ground-level O₃ pollution. Here, we use CoJ’s publicly available air quality data, and present the first long-term data analysis of O₃, nitric oxide (NO), nitrogen dioxide (NO₂), NO_x and carbon monoxide (CO) in the City from 2004 to 2011 at three air quality monitoring sites: Buccleuch, Delta Park and Newtown. We quantified CoJ’s South African National Ambient Air Quality Standards (NAAQS) exceedances for O₃ and NO₂, and demonstrate the City’s substantial O₃ and NO₂ air pollution problem. O₃ mixing ratios peak in the early afternoon as expected due to photochemical production. To estimate O₃ production rates, we summed O₃ and NO₂ diurnal profiles to obtain O_x mixing ratios at each site. This analysis provided insight into missing volatile organic compound (VOC) reactivity as well as primary NO₂ emissions information necessary for developing tropospheric O₃ pollution mitigation strategies. Furthermore, CoJ experiences high O₃ mixing ratios on weekends due to lower NO_x traffic emissions titrating the O₃, thereby providing evidence of a VOC-limited regime for O₃ production. Seasonal peak O₃ occurs in the austral spring, a maximum that we link to increases in water (H₂O) concentrations which in turn increases radical chemistry leading to O₃. In addition, wintertime VOC and aerosol emissions from biomass burning over the winter add important precursors for O₃ formation once radical chemistry is initiated during the first rain events in early spring. In all, this study will help inform air quality modelling and policy work on air pollutants in the City of Johannesburg, South Africa.