Abstract
Malta is known for its limestone megalithic temples of which many are inscribed on the UNESCO World Heritage List. A variation of this limestone was historically, and until very few years ago, a primary building material in Malta. The temples are subject to various environmental influences which until recently have led to several collapses due in part to serious stone surface and infill loss. As a protection measure, open-sided shelters have been built over three of these temples. This work assesses the degrading influence of air pollution (nitrogen dioxide, ozone, particle matter, sulfur dioxide, and acidity in rain) on the temples, in combination and comparison with the influence of other environmental factors (relative humidity, temperature, precipitation, moisture, sea salt, wind) and in this respect evaluates the potential protective effect of the shelters. The variation in air pollution weathering of limestone exposed outdoor in Malta was calculated by exposure–response functions from the ICP-materials programme and compared with measured values, and its contribution to the deterioration of the temples was evaluated. The difference between urban and rural locations in Malta, in the first year of atmospheric chemical weathering of limestone due to air pollution, was found to be about one micrometer loss of stone surface. This is probably less than the annual variations due to the influence of natural climatic factors, and small compared to the present annual variations in continental Europe. The deposition of sea salt and presence of salts on and in the limestone megaliths and changes in salt-crystallization events due to relative humidity fluctuations, inside and outside the shelters, will account for more of the variations in the first year of weathering of Globigerina limestone than variations in air pollution. The deterioration will also be related to temperature (including condensation events), wind parameters and rainfall, as well as ground water replenished from areas beyond the shelter.
Highlights
This paper compares fresh limestone atmospheric weathering rates with regards to the presence of air pollution in different locations in Malta, and discusses the importance of this weathering, in relation to other environmental influences such as humidity and salts, and the importance of airThe paper compares the measured recession of Globigerina and Portland limestone, with that expected in different locations in Malta with different amounts of air pollution
The comparisons are based on results from exposures of experimental samples, and on calculations by an air pollution exposure–response function (ERF) for the recession of Portland limestone in continental Europe, which are reported in the literature
That, it would be useful to apply the existing ERF for Portland limestone to assess the situation in Malta, and evaluate the results based on a reported experimental comparison of the recession of Globigerina and Portland limestone in the UK, and on the well documented differences in the climate and air pollution situation in Malta and continental Europe
Summary
This paper compares fresh limestone atmospheric weathering rates with regards to the presence of air pollution in different locations in Malta, and discusses the importance of this weathering, in relation to other environmental influences such as humidity and salts, and the importance of airThe paper compares the measured recession of Globigerina and Portland limestone, with that expected in different locations in Malta with different amounts of air pollution. To assess the relevance of the recession values of Portland limestone in Malta calculated by the ERF, for the recession of Maltese Globigerina limestone, the recession values calculated by the ERF were compared with observed reported values for Maltese Globigerina limestone samples exposed inside and outside the protective shelters at the megalithic temple site of Ħaġar Qim (Cabello-Briones and Viles 2017). This information was further used to assess differences in limestone weathering in Malta due to anthropogenic air pollution. More complex long-term deterioration of limestone is not discussed
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