Deposition of atmospheric pollutants to monuments, statues, and buildings

The impact of atmospheric pollutants on ∼ 100-year old marble monuments was measured at Gettysburg National Military Park, Pennsylvania, USA, from 1986–1988. Dry deposition of pollutants, at increased spatial scale and surface complexity, was studied at the Merchants Exchange Building in Philadelphia from 1988 through 1994. At Gettysburg, runoff from small, well-defined catchment areas was collected from two Carrara marble statues, a Carrara marble obelisk and a Pennsylvania Blue marble obelisk. The catchment areas were chosen to be surfaces that were washed almost entirely by direct precipitation, to minimize the long-term accumulation of gypsum crust. Chemical differences between incident rain and monument runoff were observed for individual rain storms. Concurrent meteorological and air quality data were collected so that estimates of dry deposition of particulate sulfate and nitrate, and gaseous sulfur dioxide and nitric acid could be computed for between-rain intervals. Thus, both the wet and dry deposition of atmospheric pollutants could be examined. The data indicate that marble loss from complex, sculptural marble surfaces is several times greater than from flat, vertical surfaces of comparable petrography and exposure history. Weathering rates based on simple geometries must be adjusted when applied to monuments and statues with more complex surface geometry. The Merchants Exchange was built of Pennsylvania Blue marble and Carrara marble in central Philadelphia in 1835–1839. After 150 years of exposure to an urban environment, deterioration, in the form of surface erosion, discoloration, gypsum crust development and cracking, varies with location around the building for both types of marble. From 1988 through 1994, stone surface wetness cycles and airborne pollutant concentrations at two locations on the buildingsʹ exterior were monitored to determine the potential variation of dry deposition rate with exposure orientation. Near-surface relative humidity, air temperature, wind speed and solar radiation, as well as the surface wetness and temperature of the marble were continuously measured. An air sampling system, controlled by measured surface wetness, collected separate pollutant samples for conditions of wet and dry marble surface. The measured microclimate parameters, dry deposition rates derived from the Gettysburg data and measured pollutant concentrations were used to estimate sulfur deposition to the marble building exterior. These estimates of deposition were compared with relative measurements of soluble surface deposits sampled by deionized water washdowns. The concentration of Ca++, SO4= and NO3− in these surface samples agreed closely with the calculated deposition regime and the observed condition of the surface.