Factors influencing sandstone response to changing environmental conditions in Northern Ireland

The built environment will respond to climate change. Evidence already suggests that the ‘greening’ of sandstone masonry reflects recent atmospheric changes in moisture and pollution. This emphasises the importance of changing environments as a key controller on stone decay processes. As such, there is a need to understand decay, not just in a dynamic environment, but in a world where the nature of the dynamics themselves are changing. The current study investigates how changing future meteorological conditions impact upon the underlying drivers of stone decay – specifically, the thermal and moisture cycles experienced at and below the stone surface. To evaluate the nature and scale of future damage to masonry knowledge of the current interplay of water, materials and surroundings is required. Environmental monitoring of both meteorological and internal sandstone conditions will satisfy this need. The construction of test-walls embedded with sensors will record temperature and wetness profiles with depth from the surface. This is relevant for identifying internal moisture cycles which have influence the deliquescence, movement and precipitation of hygroscopic salts, the swelling of clay minerals and on associated stress gradients. Both heat and moisture are monitored in real-time, an approach that will consider the synergies between the two variables. Logging stone moisture contents will allow the ‘time-of-wetness’, a variable of importance for biological colonisation, to be quantified. The presence of a weather station mounted to the test walls permits measurement of the ‘perturbed’ situation. The observed microclimate will be linked to conditions recorded within the stone. Matching stone response to changing meteorological parameters will provide an understanding into the scale interaction between, and lag-structures associated with, seasonal, daily and sub-hourly cycles. It will also allow the identification of potential feedbacks (such as stones with high surface wetness will have a lower albedo, therefore altering the thermal regime, and perhaps resulting in less marked temperature ranges between the surface and sub-surface) between atmospheric and stone decay processes. Few studies consider the influence of climatic change on stone decay processes. Where studies exist, they all neglect the uncertainty inherent in climate modelling. This research employs a modelling structure that allows the development of multiple, equally plausible futures and at a finer resolution than previous stone decayclimate change studies. Future projections for; temperature, precipitation, wind speed, relative humidity, potential evapotranspiration, and solar radiation, will be made using the Statistical DownScaling Model 4.2. This involves establishing relationships between observed surface stations and large-scale atmospheric variables. The model is then forced under future emissions scenarios to produce a daily time series for a 30-year time-slice. Uncertainty is catered for by using multiple climate models and emission scenarios and also an ensemble of model runs. It is necessary for model outputs to be made relevant to factors affecting decay processes. Therefore, rather than investigating annual sums of rainfall, the intensity, duration and frequency of fall events are of more importance. Future climate scenarios are yet to be downscaled, however, expected results are the increased contrast between winter and summer rainfall, and also increasing night-time temperatures (due to increased atmospheric moisture, hence greater cloud cover). This research into stone response to environmental condition willfeed into the development of a new model of sandstone decay that considers increased winter wetness and the implications this has for deeper-penetrating moisture (and therefore salts) and increased algal colonisation.