Measurements and analysis of vertical distribution, surface fluxes, and chemical composition of atmospheric aerosol in two Italian Alpine valleys

The results of research activities performed in two Italian Alpine valleys (Chiese Valley, Trentino; Camonica Valley, Lombardy) are presented. The four intensive field campaigns held during summer 2019 and winter 2020 covered different topics: wintertime black carbon (BC) concentrations, techniques for measuring PM10 and temperature vertical profiles, surface size-resolved aerosol fluxes, aerosol concentrations, and chemical composition. Firstly, the contribution of two significant PM sources (traffic and biomass burning) to wintertime total black carbon concentrations was estimated, and the effect of meteorological factors on BC levels was assessed. In both pilot areas, traffic resulted as the predominant BC source during the daytime, while biomass burning weighed more than 50% at night. Atmospheric mixing and strong winds contributed to the removal of BC from the atmosphere, while wet scavenging was not effective if accompanied by low wind and friction velocities along with a significant increase in emission sources. Other aerosol sources, such as secondary particulate matter formation in the atmosphere, manure, fertilizers, or lithospheric erosion, were instead appointed in both seasons through the chemical speciation of the inorganic aerosol fraction, which had deposited on the filters of a multi-stage Electrical Low-Pressure Impactor (Elpi+, Dekati, FI) during an entire campaign. Results showed that SIA (secondary inorganic aerosol) components were the most abundant inorganic water-soluble ions in the collected samples. Secondly, three techniques for measuring PM10 and temperature vertical distributions were applied and compared. The first method enabled continuous monitoring by positioning 5 battery-powered stations, equipped with low-cost sensors, on the mountain slope overlooking the valleys. These measurements extended up to about 1000 m above the valley floor and were accompanied by drone profiles in summer and tethered balloon soundings in winter, both of them equipped with the same sensors installed in the slope stations. The research aimed at evaluating the effectiveness of the temperature and PM10 slope pseudo-vertical profiles in reproducing soundings measured in the valley centre. Slope stations successfully replicated the vertical profiles, especially in the morning/evening hours, thus representing a good and inexpensive alternative for long-lasting campaigns or even excellent support to traditional methods. Finally, the role of a typical alpine agro-economic ecosystem (pasture/grass field) in the atmosphere-Earth surface aerosol exchange was evaluated, studying aerosol size-segregated fluxes (9 classes, 10 nm ≤ GMD ≤ 0.76 m) with the eddy covariance technique, employing the aforementioned Dekati (FI) Elpi+ multi-stage impactor. Surprisingly, the pasture did not behave as an aerosol sink, favouring aerosol removal from the atmosphere, but rather contributed to the formation of secondary particulate matter through ammonia, NOX, and organic sulphides emissions from soil and vegetation. Deposition phenomena were registered under atmospheric stability or low turbulence conditions, but emission phenomena were very frequent, especially during winter. Thanks to the ion chromatography analysis of the inorganic particulate soluble fraction deposited on the impactor filters, aerosol fluxes were also linked to aerosol chemical composition and sources, thus hypothesizing nucleation, growth, and coagulation processes as responsible for the formation of concentration gradients in the atmosphere and the observation of deposition fluxes in the ultrafine range. The data collected and described in the present thesis had an interesting follow-up within the EU Alpine Space project BB-CLEAN, within which the activities developed. In particular, the experimental data were used by modellists to calibrate a meteorological and dispersion modelling chain that provided 48-hour PM concentration forecasts to a smartphone app, indicating when the activation of biomass burning heating systems might be sustainable. The researchers of the BB-CLEAN project also employed the model to evaluate some scenarios that envisaged the reduction of PM emissions from biomass burning appliances (e.g., through system upgrades, app use, and realization of a district heating network). Simulations showed that some of these scenarios could lead to a significant decrease in PM concentrations. However, no scenario can be elected as an absolute best, as policymakers should consider the characteristics of their respective municipalities when faced with the need to decide which scenario to implement.