Abstract Aerosol concentration and composition were determined in parallel with the measurement of soiling rates of external surfaces, in sheltered and unsheltered conditions, over several years, in the city centre of Oporto, Portugal. Black carbon particles, responsible for surface soiling, originate in about 70% from car emissions. Unsheltered surfaces have an erratic soiling behaviour resulting from the opposite action of aerosol deposition, rainfall, wind and atmospheric corrosion. Surfaces sheltered from rain suffer a continuous decrease in reflectance that obeys to a square root equation on time of exposure. The soiling algorithm, resulting from model fitting to experimental data, previews that a 30% decrease in reflectance, amount usually considered as triggering a need for cleaning/painting action, will be attained within 5.5–8.8 yr in the Oporto urban environment.