Understanding the Tropospheric Ozone Photochemistry in Gauteng, South Africa, Using Tropospheric Ultraviolet and Visible (TUV) Radiation Model

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Tropospheric ozone is a secondary pollutant that is formed through a series of complex photochemical reactions in the presence of oxides of nitrogen (NO x ) and volatile organic compounds (VOCs). This chapter identifies the occurrence of elevated ozone episodes over Three Rivers site of Gauteng, South Africa, during January to December 2015. A chemical kinetics-based model is used for understanding the role of basic NO 2 -NO-O 3 photochemical cycle in ozone production and tropospheric ultraviolet and visible (TUV) radiation model (Version 4.1) of NCAR is used for computing the rate constant for the photolysis of NO 2 https://www.w3.org/1998/Math/MathML" display="inline"> ( j NO 2 ) https://www.w3.org/1999/xlink" xlink:href=" https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003638414/15016678-7099-4044-967b-542acbd2751b/content/CH16_Equ_0001.tif "/> . Ozone photochemistry is found to be only NO x driven during morning and evening hours. But some other ozone precursors (benzene, toluene, xylene, and carbon monoxide) also play a vital role in ozone production during the late morning to afternoon hours. Maximum deviation among the observed and model-predicted ozone values is observed around noon. Statistical analysis and computation of Leighton Ratio also support the contribution of other chemical reactions (apart from the basic NO x cycle) in the photochemical production of tropospheric ozone. Long-range horizontal transport of ozone and its precursors from polluted industrial regions is portrayed by HYSPLIT back trajectory simulations. Meteorological variables (temperature, humidity, solar radiation, precipitation, and cloud cover) influence the concentration of surface ozone over the study site.

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