Acute episodes of particulate matter pollution: the role of day-night atmospheric vertical stratification

Meteorological conditions favourable to the accumulation of particulate matter in the lower layers of the atmosphere occur mainly in winter. In particular, high-pressure configurations with strong vertical stability, weak circulation and related very low mixed layer thickness may cause acute episodes of PM10 pollution, in the presence of local emission sources. Such episodes may be more severe in valleys or plains, as the local orography may influence the formation and persistence of thermal inversions. The persistence of critical weather conditions caused by blocking mechanisms and the consequent absence of rainfall with its washout effect can lead to an increase in the severity of pollution episodes. The strong day-night thermal excursion due to radiative cooling at night, due to poor or absent cloud cover, is a further critical factor, as can be evidenced by the daily cycle in planetary boundary layer height (HPBL). A method implemented by the LaMMA Consortium for the identification and classification of "weather types" (Huth, 2000) has been applied in this study in order to analyse the correlations between acute episodes of particulate matter pollution and the most recurrent meteorological configurations. The key point here is to put in evidence the role of the day-night atmospheric vertical stratification in relation to the accumulation of pollutants emitted by the main local sources. With this goal, a number of critical areas for PM10 pollution have been identified in Tuscany (central Italy), where hourly PM10 and PM2.5 concentration data are available thanks to the Airqino network of low-cost stations1 and hourly HPBL data from a Weather Research and Forecasting (WRF) model configuration implemented at Lamma Consortium, both for operational forecasts and related studies (https://www2.mmm.ucar.edu/wrf/users/ ). 1 The low-cost Airqino stations, developed at IBE-CNR for detecting the main air pollutants and meteorological parameters are located throughout Italy and in some Finally, a decisive contribution to the characterisation of the emission sources has been given by the compositional analyses of PM10 and PM2.5 samples. These have been performed by applying a range of techniques (ion chromatography, thermo-optical analysis, PIXE, ICP-AES) to characterize the chemical composition of particulate matter and in particular to put in evidence the biomass burning component associated with domestic heating (Nava, 2020).