Salt deposition and soiling of stone facades by traffic-induced immissions

Despite enormous enhancements in air quality, many cities still have serious problems to comply with the legal limits of air pollution. Concentrations of nitrogen oxides and fine particulate matter remain high, originating in relevant proportions from urban traffic. The impact of traffic-induced immissions on our built heritage is the focus of this study. The proportion of historic buildings exposed to elevated traffic emissions was estimated in five German cities of different sizes and different traffic loads. Less than 100 up to more than 1000 historic buildings per city are exposed to increased traffic emissions. For five buildings at heavy-trafficked roads, the near-field air flow and the deposition rate of air pollutants were modelled. Passive samplers were exposed at these buildings to determine the composition and amount of particulate matter, the concentrations of NO2 and HNO3 in the air, as well as the soiling and the recession rate of stone samples. The results clearly demonstrate the deposition of large amounts of particulate matter and the corresponding soiling of stone samples as consequences of road traffic. Despite high concentrations of NO2, the deposition of nitrates on stone surfaces seems to play a limited role. In addition, the deposition of sulphate and at some exposure sites chloride deposition was observed.