Assessment of the urban pollution island intensity in Rome (Italy) from in-situ PM measurements

The Urban Pollution Island (UPI), describing the temporal and spatial distribution of pollutants’ concentration attributed to the presence of urban features and activities, is one of the major problems affecting urban areas and has become more severe with rapid urbanization. To correctly evaluate the UPI Intensity (UPII), i.e., the difference in pollution concentration between the urban agglomeration and its rural surroundings, it is crucial to carefully select rural and, above all, urban reference stations, as local factors such as orography, location of the air quality monitoring stations, and street orientation can significantly impact UPII values. In this work, the UPII in Rome (Italy) is determined using daily-averaged concentrations of PM10 and PM2.5 collected by in-situ stations over the period 2018–2023. Three different methods for the assessment of UPII are tested and compared by varying the sub-set of selected urban stations, according to their environmental classification. The approach proposed will have significant implications on the management of urban environment and on the tailored design of urban air quality improvement strategies. Results show slight differences in the monthly-averaged concentrations of both PM10 and PM 2.5 between the “urban traffic” and “urban background” stations, suggesting that the proximity of the emission sources to the monitoring stations moderately influences the concentrations, potentially due to limited ventilation within street canyons, which can inhibit mixing processes. The annual variation of UPII reveals that PM10 is more sensitive to the selection of the stations particularly during winter, when the differences between the three assessment methods reach 100%. Our findings also indicate that, in the case of Rome, using the largest number of stations available in the urban area could enhance the UPII evaluation, taking into consideration the urban structure and the specific characteristics of local emission sources. The results presented here, although related to a single city, demonstrate that the selection of urban stations for the evaluation of UPII is not straightforward and requires further investigation