Characterisation of an event of heatwave and particulate matter enhancement in Bologna

In the context of climate change, the theme of extreme hot temperatures is sparking increasing interest due to their recent increase in terms of frequency and intensity. The WMO defines warm spell as “a persistent period of abnormally warm weather for the time of the year” which can occur at any time of the year. Even though the term heatwave is commonly used to describe the same phenomenon, it is more appropriate for events involving the highest temperature values observed during the year. Furthermore, the identification of such events is complicated by the lack of a common and shared definition: several indices and thresholds have been proposed in literature and are utilized by national alert systems to detect extreme heat events. 

The effects of heatwaves on human health posed by heat stress are often exacerbated by concurrent increases in air pollutant concentrations. Nowadays, a lot of studies focus on heatwave events with concurrent increases in tropospheric ozone concentrations, with significant synergistic effects on human health. So far, however, only a few studies have investigated the association between warm spells and increases in particulate matter concentrations. This study aims at filling this gap, focusing on the longest and most intense event occurred in 2023 in the city of Bologna (44.495 N, 11.345 E). 

To the scope, the event was identified based on the Warm Spell Duration Index (WSDI) and Excess Heat Factor (EHF). Particulate matter enhancements were identified with an originally developed index based on the seasonal variability of the particulate matter. Specifically, seasonal thresholds for the exceedances’ identification were set based on the 80^th percentiles of the seasonal distributions of both PM₁₀ and PM_2.5. The methodology herein developed identifies a total of 7 joint events of warm spells and particulate matter increases throughout the year. 

In particular, the event occurred between 11^th and 20^th of July 2023 is the most interesting one owing to its long duration and intensity. This event is characterized in detail from the meteorological, physical and chemical point of view, by employing different observational datasets. The synoptic analysis pointed out a geopotential pattern which favored the transport of Saharan dusts from Algeria towards North of Italy. Concurrently, the African anticyclone presence extended itself over the Italian Peninsula. The analysis of the particle size distribution highlighted a general increase in particle number concentrations for all sizes while the aerosol chemical composition supported the hypothesis that the air masses arrived at the study site from Sahara Desert, by showing increases in concentrations of typical crustal elements. 

In conclusion, this work has defined a methodology for the analysis of joint warm spell and particulate matter increase events and elucidated the role of the African blocking anticyclonic pattern as responsible for the event. 

This study was carried out within the RETURN Extended Partership and received funding from the European Union Next-GenerationEU (National Recovery and Resilience Plan – NRRP, Mission 4, Component 2, Investment 1.3 - D.D. 1243 2/8/2022, PE0000005).