Effect of dry deposition of NOx and SO2 gaseous pollutants on the degradation of calcareous building stones

Abstract A laboratory-based atmospheric flow chamber, using realistic presentation rates of SO 2 , NO and NO 2 pollutants directed to various dry and wetted surfaces, has been employed to quantify the effects of the individual pollutants and the role of ozone as an oxidant. For the individual pollutant gases reacting with stone surfaces coming to equilibrium with 84% relative humidity (r.h.), chemical reaction in the presence of a moisture film proceeds and the extent of this reaction is related to pollutant gas solubility in the moisture film, i.e. SO 2 > NO 2 > NO. After dissolution in the moisture film, the pollutant gases are oxidized in the presence of catalysts associated with the stones. The additional presence of ozone promotes oxidation of the pollutant gases and thus their reaction with the stones. For SO 2 pollutant, oxidation in the gas phase is not significant compared with that in the moisture film, with enhanced oxidation in the presence of catalysts. Ozone increases oxidation of NO and NO 2 pollutant gases in the gas phase and moisture film; however, the oxidation of SO 2 in the moisture film is more significant than that of NO or NO 2 . Wetting of the stone surfaces, in the absence of ozone, reveals the consistently greatest chemical reaction with SO 2 compared with NO and NO 2 , which is related to SO 2 solubility, oxidation in the presence of catalysts and production of sulphuric acid. Generally similar behaviour is evident of NO and NO 2 , but NO shows a reduced extent of chemical reaction, implying that its oxidation in surface water, in the presence of catalytic species, is slow and hence the reactants are lost in the form of run-off. In the additional presence of ozone, the SO 2 pollutant gas gives rise to enhanced chemical reaction, whereas both NO and NO 2 show lower extents of chemical reaction than for the dry stones. This arises from the relatively slow conversion of N 2 O 5 in the liquid phase to nitric acid, allowing loss of reactants in run-off.

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