Volatile organic compounds at two urban areas in the Italian Po Valley: Milan and Bologna

Volatile organic compounds (VOCs) released into the atmosphere by natural and anthropogenic sources play a key role in atmospheric processes. They can react with atmospheric oxidants leading to secondary organic aerosols and tropospheric ozone, with effects on air pollution, human health and climate.

In Europe, the improvement of air quality policies in the last decades has caused some pollutants’ concentrations to decrease. This is the case for nitrogen dioxide and particulate matter concentrations that decreased between 30-50% during 2000-2010, leading to a decreasing trend of the associated health effects on people exposed to them. Yet, 70% of EU citizens lives in urban areas, and 99-97% of this population is exposed to concentrations of ozone and fine particulate matter above the guidelines recommended by WHO in 2021 for protecting public health (EEA, 2021). The Po Valley, located in the North of Italy is one of the areas in Europe suffering the worst air pollution, with several air quality threshold exceedances throughout the years. A recent example was recorded in the city of Milan in winter 2024, when, during several days of high atmospheric pressure conditions, PM 2.5 concentration was above 100 μg/m³, while the WHO recommended threshold is 15 μg/m³ on a 24-hour averaging time.

Within the EU-funded H2020 project RI-URBANS and ACTRIS, we conducted two field campaigns at two urban areas of different size, 200 km apart in the Italian Po Valley: Milano and Bologna. In both cases, VOCs were measured with a Vocus CI-ToF (chemical ionization time of flight) 2R mass spectrometer (Tofwerk, Switzerland) that was deployed first in Milan from January 2023 during one year, then in Bologna in September 2024 for one month. The analysis of our study focuses on sixteen VOCs common to both measuring sites, identified and quantified with a certified VOC mixture and covering the measured mass range 42-371 amu. The absolute concentration, the diel and seasonal variability of the VOC measured in Milan and Bologna are discussed and compared. In particular, we determined the effect of atmospheric dilution and atmospheric reactivity on the measured concentrations. We also discuss the results in terms of potential formation of ozone and secondary organic aerosols.