Characterisation of extreme air pollution episodes in an urban valley in the Balkan Peninsula

Continuous measurements of SO 2 , NO, NO 2 , O 3 , CO, PM 10 , PM 2.5 and benzene were conducted over a 1-year period at two urban sites in Skopje in the Republic of Macedonia. Extremely high pollutant concentrations were recorded. Most of the limit, target and threshold values set by the European Union for the protection of human health were exceeded. PM 10 and PM 2.5 concentration levels in particular were very high; the annual means were 85 and 56 μg/m 3 , respectively. The PM levels were among the highest in Europe. The most serious pollution episodes occurred during winter months. During pollution episodes, high atmospheric stability prevailed over large areas, and during the daytime, the mixing layer typically was only 700–900 m high. In these circumstances, the mountains surrounding the city of Skopje trapped the pollution inside the valley extremely effectively. As a result, an extremely homogenous pollution field (both spatially and by components) formed in the Skopje Valley. The highest hourly PM 10 and PM 2.5 concentrations during the episodes were 800 and 500 μg/m 3 , respectively, whereas the NO 2 concentration was 440 μg/m 3 . The pollutant concentrations were much lower in summertime (except for ozone). Local emissions and atmospheric chemistry as well as the diurnal cycle of the mixing height controlled the concentrations levels of the photochemically active gases NO x , CO, benzene and O 3 . PM concentrations were further elevated by regional wildfire emissions. The extremely high pollutant concentrations mostly originated from local emissions and were enhanced by the local topography together with poor atmospheric mixing conditions.