The influence of combustion-derived pollutants on limestone deterioration: 1. The dry deposition of pollutant gases

Abstract

This study concerns quantification of the relative chemical degradation effects of dry deposition of combustion-derived atmospheric pollutants, HCl, SO 2 and NO 2, on Portland and Monks Park limestones, employing laboratory exposure chambers for periods of 30 days. Using presentation rates up to 40 × 10 −4 μgcm −2s −1, the pollutants were assessed individually and in various combinations, along with ozone as oxidant, at 84% RH with dry or water-wetted surfaces. The degradation was followed by analysis of exposed stone, for Cl −, SO 4 2− and NO 3 −, and of run-off solution for Ca 2+, in addition to the anions. From these data, the total calcium released from limestone to reaction ions and products and the percentages of each pollutant reacted (and so the deposition velocity ( V D)) in each exposure regime were calculated. HCl acted independently of the presence of other pollutants, showed 40 and 100% reaction (mass transport control) and V D values of 2.2 and 5.3 mm s −1, at 84% RH and on wetted surfaces, respectively. The reactivity of SO 2 rose from 2.5 to 35%, and V D from 1 to 2.9 mm s −1, from conditions of 84% RH to those with the presence of water, NO 2 and O 3. Similarly, the reactivity of NO 2 rose from 3 to 40% at 84% RH in the presence of SO 2 and O 3, with a V D at 3 mm s −1, but decreased in the presence of run-off solution. A limited study was performed on smoke-coated and naturally exposed, reaction-product-coated, samples. In addition to obvious factors such as deposited mass, chemical constitution and transfer, smoke particle size distribution should also be considered in any possible action on limestone. Concerning naturally exposed samples, non-wetted surface reaction products tended to reduce the attack of pollutants, but had no resisting effect when wetted.