High surface ozone concentrations are of major concern in many regions of the world. Despite major efforts to reduce emissions, some areas (e.g Asia) regularly experience a large number of ozone pollution events in the summer. In addition, high ozone formation rates have also been observed in wintertime. Photochemical transformation of organic compounds in the presence of nitrogen oxides is responsible for ozone production. Due to the strong dependence of ozone production on nitric oxide concentrations, emission reduction of nitrogen oxides affects also ozone concentrations in the troposphere. The session aims for contributions reporting observations of ozone pollution and/or production for example in field campaigns and investigating reasons for high ozone pollution using observations and model calculations. In addition, contributions analysing trends of ozone concentrations and making suggestions for possible mitigation strategies are welcome.