Preliminary results from the analysis of stone tablets from the National Materials Exposure Programme (NMEP)

The NMEP programme consists of 29 sites at which samples of stone and metals are being exposed for a minimum of 4 years to determine rates of decay in the current pollution climate. The sites were chosen to cover a wide range of environmental conditions, climate and topography. Information on meteorological conditions and atmospheric pollutants is being collected from all the sites. Four of the sites also form part of an international programme that is operating concurrently for the United Nations Economic Commission for Europe (UNECE).

The stone tablets are 50 × 50 × 8 mm and are exposed on freely rotating carousels in sheltered and unsheltered positions which represent the washed and unwashed areas of buildings. Tablets of Portland limestone and White Mansfield dolomitic sandstone are exposed at all sites and in addition Monks Park limestone is exposed at the nine sites. Prior to exposure the tablets were cleaned and weighed, and in some cases the surface roughness was measured using an oblique light source and image analysis system.

Tablets have been retrieved after 1 and 2 years of exposure and re-weighed following drying. Samples of powder have been removed from the surface of pristine, sheltered and unsheltered tablets and analysed using ion-chromatography for soluble ionic species (Ca²⁺, Mg²⁺, Na⁺, NH₄⁺, SO₄⁻⁴, NO₃⁻, Cl⁻). The results showed the expected increases in acid species and soluble calcium in the sheltered tablets and increases in chlorides in tablets exposed near to the coast. Tablet surfaces have also been studied to obtain further diagnostic detail.

The results from the analysis of the stone tablets and from the monitoring of pollutant concentrations and meteorological variables have been used to identify empirical relationships and as a basis for mathematical modelling. Preliminary findings from these studies are presented in this paper.