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Nowcasting of air pollution episodes in megacities: A case study for Athens, GreeceVarotsos, Costas A.; Mazei, Yuri; Saldaev, Damir; Efstathiou, Maria; Voronova, Tatiana; Xue, Yong; University of Athens, Athens, Greece; Lomonosov Moscow State University, Leninskiye Gory, 1, Moscow, Russia; Severtsov Institute of Ecology and Evolution, Russian Academy of Sciences, Moscow, Russia; Shenzhen MSU-BIT University, Shenzhen, China; et al. (Elsevier BV, 2021-06-02)The main objective of the present study is to develop a model for the prediction of the extreme events of air pollution in megacities using the concept of so-called "natural time" instead of the "conventional clock time". In particular, we develop a new nowcasting technique based on a statistically significant fit to the law of Gutenberg-Richter of the surface concentration of ozone (O3), particles of the size fraction less than 10 μm (PM-10) and nitrogen dioxide (NO2). Studying the air pollution over Athens, Greece during the period 2000–2018, we found that the average waiting time between successive extreme concentrations values varied between different atmospheric parameters accounted as 17 days in case of O3, 29 days in case of PM-10 and 28 days in case of NO2. This average waiting time depends on the upper threshold of the maximum extreme concentrations of air pollutants considered. For instance, considering the NO2 concentrations over Athens it was found that the average waiting time is 13 days for 130 μg/m3 and 2.4 years for 200 μg/m3. Remarkably, the same behaviour of obedience to the Guttenberg-Richter law characterizing the extreme values of the air pollution of a megacity was found earlier in other long-term ecological and paleoclimatic variables. It is a sign of self-similarity that is often observed in nature, which can be used in the development of more reliable nowcasting models of extreme events.