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Transport of Po Valley aerosol pollution to the northwestern Alps. Part 1: phenomenology

Abstract. Mountainous regions are often considered pristine environments, however they can be affected by pollutants emitted in more populated and industrialised areas, transported by regional winds. Based on experimental evidence, further supported by modelling tools, we demonstrate and quantify here the impact of air masses transported from the Po Valley, a European atmospheric pollution hotspot, to the northwestern Alps. This is achieved through a detailed investigation of the phenomenology of near-range (few hundreds km), trans-regional transport, exploiting synergies of multi-sensor observations mainly focussed on particulate matter. The explored dataset includes vertically-resolved data from atmospheric profiling techniques (Automated LiDAR-Ceilometers, ALC), vertically-integrated aerosol properties from ground (sun photometer) and space, and in situ measurements (PM10 and PM2.5, relevant chemical analyses, and aerosol size distribution). During the frequent advection episodes from the Po basin, all the physical quantities observed by the instrumental setup are found to significantly increase: the scattering ratio from ALC reaches values > 30, AOD triplicates, surface PM10 reaches concentrations > 100 µg/m3 even in rural areas, secondary inorganic compounds such as nitrate, ammonium and sulfate increase up to 28 %, 8 % and 17 % of the total PM10 mass, respectively. Results also indicate that the advected aerosol is smaller in size and less light-absorbing compared to the aerosol type locally-emitted in the northwestern Italian Alps, and hygroscopic. In this work, the phenomenon is exemplified through detailed analysis and discussion of three case studies, selected for their clarity and relevance within the wider dataset, the latter being fully exploited in a companion paper quantifying the impact of this phenomenology over the long-term (Diémoz et al., 2018). For the three case studies investigated, a high-resolution numerical weather prediction model (COSMO) and a lagrangian tool (LAGRANTO) are employed to understand the meteorological mechanisms favouring the transport and to demonstrate the Po Valley origin of the air masses. In addition, a chemical transport model (FARM) is used to further support the observations and to partition the contributions of local and non-local sources. Results show that the simulations are not able to adequately reproduce the measurements (with modelled PM10 concentrations 4–5 times lower than the ones retrieved from the ALC, and maxima anticipated by 6–7 hours), likely owing to deficiencies in the emission inventory and particle water uptake not fully taken into account. The advected aerosol is shown to remarkably degrade the air quality of the Alpine region, with potential negative effects on human health, climate and ecosystems, as well as on the touristic development of the investigated area. The findings of the present study could also help design mitigation strategies at the trans–regional scale in the Po basin, and suggest an observations-based approach to evaluate the outcome of their implementation.
