نبذة مختصرة : Ground-level ozone (O 3) 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 O 3 pollution. Here, we use CoJ's publicly available air quality data, and present the first long-term data analysis of O 3 , nitric oxide (NO), nitrogen dioxide (NO 2), 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 O 3 and NO 2 , and demonstrate the City's substantial O 3 and NO 2 air pollution problem. O 3 mixing ratios peak in the early afternoon as expected due to photochemical production. To estimate O 3 production rates, we summed O 3 and NO 2 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 NO 2 emissions information necessary for developing tropospheric O 3 pollution mitigation strategies. Furthermore, CoJ experiences high O 3 mixing ratios on weekends due to lower NO x traffic emissions titrating the O 3 , thereby providing evidence of a VOC-limited regime for O 3 production. Seasonal peak O 3 occurs in the austral spring, a maximum that we link to increases in water (H 2 O) concentrations which in turn increases radical chemistry leading to O 3. In addition, wintertime VOC and aerosol emissions from biomass burning over the winter add important precursors for O 3 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.
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