Challenges and guidelines for artificial ageing testing of cultural heritage materials exposed to a changing troposphere

Outdoor Cultural Heritage (CH) suffers severe damage due to the interaction triggered by physical and chemical atmospheric factors. To study the mechanisms of degradation of materials induced and to test the effectiveness of experimental protective treatments, accelerated ageing tests and artificial rains are widely used in the field of material science and outdoor CH conservation. In a scenario where climate and tropospheric composition are changing, the compositional variation of atmospheric deposition is likely to strongly contribute to the gap between the results obtained in laboratory with respect to those from real outdoor exposure. In order to reduce this gap, wet and dry formulations, representative of changed wet and dry depositions, seem necessary for material testing. Currently, no ready-to-use updated formulations are available on account of the mentioned atmospheric changes. The study herein presented aims at providing updated environmental parameters for the formulation of synthetic depositions to fine-tune accelerated ageing test and so to set up better conservative and preventive strategies. Specifically, this work intends to provide a suitable range of particulate matter composition and hydrometeor ion concentrations to formulate both synthetic PM and precipitation, following their long-term evolution. Concerning the synthetic PM, a formulation is proposed based on the statistical elaboration of current urban PM compositional data from a wide bibliographical survey. The synthetic rain formulation, instead, is based on the analysis of trends and sources of ions in atmospheric bulk depositions (1997-2019 period) collected in a typical highly polluted southern European location, Po Valley. Results are compared with other Mediterranean locations, to provide an effective compositional range for each chemical parameter valid for the southern European region.