The effects of ozone and NOx on the deterioration of calcareous stone

This paper looks at the changes in urban air quality and the effect on building materials with particular regard to their deterioration arising from pollution due to vehicles. The paper aims to review basic information on the likely effect of NOx and ozone on calcareous building materials (limestone, marble, dolomite, calcareous cemented sandstone) by examining experimental and fieldwork relating to the principle reaction of the oxidation of the sulphite to sulphate. NO2 is only one of many possible oxidants present in the urban and rural environment. A brief synopsis of the effects of other oxidants and inhibitors is given along with a review of the processes involved in aqueous phase oxidation, which seems to be the phase where the reactions take place. Controlling factors in the reaction and oxidation of sulphite to sulphate are the type and porosity of the stone, the level of humidity, the nature of the oxidants and to a lesser extent the presence of other extraneous reactants, e.g. metal oxides, chlorine and ammonia salts. For freshly cut stone unpolished marble is much less reactive than porous limestone and the degree of activity has different relative humidity thresholds. Reaction of the stone with the gaseous compounds occurs during the process of absorption onto the surface. The oxidation is accelerated at the gas–water interface by the presence of NOx, and seems to depend on the NO2 component but it is clear that NOx is only one of many possible oxidants in the urban and rural environments. Correlation studies of the damage to stone materials are unlikely to pick up the effects of NOx due to the nature of their duration and the rather transient nature of the appearance and removal of nitrates on the stone. Presence of other oxidants and catalysts such as H2O2, O2 (catalysed by Fe3+ and Mn2+), OH, HSO5−, SO42−, O3, HO2, NO2−, NO3, O2− and PAN, plus the fact that the reactions are pH and ionic strength sensitive, add greatly to the complications of determining what affects the oxidation rate. These difficulties are further compounded by the presence of trace elements in the chemical reagents, particulate matter and the stone itself (such as Mn2+ or Vn2+). In conclusion there is sufficient evidence that NOx has a role to play in the oxidation of sulphite. However, the magnitude of influence NOx and related compounds have in the environment is far from clear due to the prevalence of other stronger oxidants (ozone and hydrogen peroxide), the intricately linked NOx and ozone chemistry in relation to their diurnal cycle and the diurnal humidity variations, seasonal variations, the presence of metal oxide catalysts (Fe and Mn) in the stone and environment, and the possibility of organic oxidation inhibitors.