| Abstract | "Visibility" is meant as the greatest distance at which an observer can see a distant object in
contrast with the horizon and, when referred to a landscape, it depends on the optical characteristics
of the atmosphere, which are also linked to the presence of gaseous or particulate pollutants that
contribute to generating haze that obstructs clear vision.
Therefore, visibility can become a useful indicator of air quality in natural areas with a significant
fruition vocation such as, in the case of the "Visibility" project, the Italian National Parks, where the
possibility of enjoying a clear panorama immersed in a natural landscape represents a precious
recreational value, a real ecosystem service, as well as an asset protected by the Constitution (Article
9) and by Italian laws preserving the beauty of the landscape.
For the "pilot" action, representative of the first case of visibility monitoring in Europe, the Circeo
National Park (LT) was selected, with the positioning of the measuring instruments for atmospheric
pollution evaluation at the Lago dei Monaci site. The goal is to test the American protocol
I.M.PRO.V.E. (Interagency Monitoring of PROtected Visual Environment), based on the algorithm
developed by Malm et al. (1994, Spatial and seasonal trends in particle concentration and optical
extinction in the United States. J. Geophys. Res. 99: 1347–1370), for the quantification of a
coefficient that describes the light extinction (Bext) as a function of different chemical-physical
parameters associated with molecules dispersed in the air, elements and compounds in the particulate
and gaseous phase of both anthropic and natural origin.
During spring-summer 2021 and winter 2021/2022, 24-hours samples were collected (with a
frequency of one every three days) to measure the air concentration of PM2.5 and PM10 particulate
matter, sulphates and nitrates, elemental carbon and organic carbon, metals and trace elements and
the gaseous species nitrogen dioxide (NO2); relative atmospheric humidity was also measured. The
visual detection of the degree of air transparency was performed by a panoramic camera pointed in
the direction of the "landmark" Monte Circeo, taken as a reference for the definition of long-distance
visibility.
A first visibility index (Bext) was therefore calculated by applying the formula proposed by the
I.M.PRO.V.E. methodology, and preliminary results show that the parameters that seem to contribute
most to extinguishing visibility and hence to the increase of Bext are the Inorganic Secondaries
(Ammonium Sulphate and Ammonium Nitrate) closely linked to atmospheric humidity. The good
agreement between qualitative assessment of visibility (on camera images) and the quantitative
estimation of Bext seems to encourage further tests on different sites.
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