Field survey of the relationship between environmental conditions and algal growth on exterior walls

Building facades often become black and/or green because of soiling caused by airborne algae. The algal growth is influenced by available water supply including rain, condensation, and water vapor. The influence of other environmental factors, such as solar radiation, temperature, and relative humidity, on algal growth has not been quantitatively investigated. This study aimed to quantify the relationship between wall surface algal growth and environmental conditions. To identify sites where algal soiling is likely to occur, a field survey of soiling was conducted on building facades in Kyoto, Japan. Soiled areas were divided into two classes depending on the source of moisture contributing to algal growth. The algae were identified by the observation of soiling by microscopy. In addition, to clarify the relationship between algal growth and environmental factors, soiling, temperature, and humidity were measured at Kusyo Myojin Shrine, Ninna-ji Temple, Kyoto. Algal growth was influenced by local microclimates, with a strong correlation between algal growth and death with ambient temperature and humidity. For example, algae grew under conditions of high ambient humidity at the shrine. However, parts of the walls exposed to direct solar radiation were not discolored because the algae were more likely to die at high temperatures. Based on these measurements, a predictive model was proposed to calculate the population of algae using the solar radiation, temperature, and humidity as inputs. This model could reproduce the observed trend of wall surface brightness of algal soiling.