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Air pollution in developing countries has recently become a serious environmental problem, which needs more active air quality monitoring and analyses. To assess air quality characteristics over the city of Zahedan, southeast Iran, airborne particulate matter (PM) concentrations with aerodynamic diameters of < 10, < 2.5, and < 1.0 mu m were measured during the period July 2008 to March 2010 using an Environmental Dust Monitor (EDM-180). The data were analyzed on a daily, monthly, and seasonal basis. The highest monthly mean PM10 levels (172 mu g m(-3)) were recorded during the summer period (June-August), presumably due to frequent dust storms from the nearby Sistan desert located to the north, while less PM10 concentrations are recorded in winter (December-February; 101 mu g m(-3)). Linear regression analysis between the PM2.5 and PM10 time series reveals high correlation coefficients (r > 0.82) for all seasons, implying that PM10 and PM2.5 may have the same source regions or that they are influenced by the same local conditions. In contrast, neutral correlation is found between PM10 and PM1.0 in autumn and winter. Taking into account that the annual variation of PM1.0 exhibits a clear pattern of peaking in winter and dropping in summer (in contrast to PM10), it is suspected that PM1.0 is of different origin than PM10 and mainly influenced by local anthropogenic emissions. The daily PM10 variation is strongly seasonally defined. The maximum PM10 concentrations occur in the morning hours during winter, autumn (September-November), and early spring (March), while in summer, PM10 concentrations increase significantly in the afternoon, closely associated with the intense northerly winds blowing from the desert. As far as the Air Quality Index (AQI) is concerned, its highest monthly values occur in summer, while they are reduced in winter. Desert dust aerosols are found to be the major component in determining the AQI in Zahedan. The analysis shows that 15.3% of the days are unhealthy for sensitive people, while 2% are considered as hazardous.