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The Role of Temperature Inversions in Air Pollution Episodes Demonstrated by Examples in the Eastern Alps (Austria)  

Martin Piringer and Kathrin Baumann-Stanzer

Temperature inversions are layers near the Earth’s surface in which air temperature increases with height. They develop predominantly over rural areas during clear nights, caused by radiative cooling at the ground, and can extend a few decimeters to a few hundred meters in the vertical. Over cities, ground-based temperature inversions are rare because a city acts as an anthropogenic heat source creating a well-mixed layer of a few hundred meters depth. Above, an elevated inversion layer is often present during clear nights. Temperature inversions inhibit or strongly reduce vertical exchange because heavy cold air lies below lighter warmer air, a stable situation that can last until the early morning hours when the rising sun starts to warm the ground and radiative heating starts to dissolve the inversion from below. Air pollutants brought into an inversion layer accumulate. In winter, inversions are only partly removed during daytime, and air pollution episodes may last over several days. In addition to radiosoundings conducted twice daily worldwide at selected locations, continuous vertical temperature profiles can be obtained from meteorological stations at different altitudes when a city is surrounded by hills or mountains. Such stations, however, can be subject to local conditions causing the course of air temperature to deviate strongly from that in the free atmosphere. In addition, vertical temperature measurements are undertaken in project-based campaigns—for example, using tethersondes, drones, or microwave radiometers—delivering exemplary profiles in specific settings. Sensors on aircraft and drones can be used to obtain vertical profiles of meteorological parameters (e.g., temperature). Routine meteorological networks usually cannot deliver information about vertical temperature gradients and thus inversions. In dispersion modeling, information on the state of the stability of the boundary layer is therefore obtained by parameters or parameterizations involving more easily obtainable data. Instead of the detailed vertical profile of air temperature, dispersion models use information on the atmospheric stability and the mixing height defining the volume available for the dilution of pollutants on an hourly basis. During the early 21st century, there was a development from purely analog measurements such as tethersondes to the use of ground-based and other remote sensing devices, such as ceilometers, microwave radiometers, and drones, to detect the thermal structure of the atmospheric boundary layer.