Identification of PM10 episodes in Tehran, Iran: Regional versus local causes

Middle Eastern sand and dust storms (SDSs) can deteriorate air quality and human health of millions of inhabitants in downwind areas such as Tehran, Iran. Limited information is available about severity, frequency, magnitude, and duration of PM10 episodes in Tehran. To identify PM10 episodes between 2015 and 2021, the Seasonal-Trend decomposition procedure based on Loess (STL) method was applied to measured PM10 concentrations at 15 sites across Tehran. The PM10 episodes were separated into 3 duration categories (PE1, PE2, and PE3). The PM10 concentrations and related meteorological characteristics of the episodes were analyzed to assess the differences. All sites had higher PM10 concentrations compared to AQG levels for daily (24-h) PM10 (45 μg m−3). PM10 episode frequency, magnitude, and duration were 2.60/year, 35.0 percent, and 1 day/episode over the 7-years study period, respectively. The overall frequency of PM10 PE1, PE2, and PE3 events were 205, 54, and 14, respectively. Events dissipated under strong westerly winds. PM10 accumulation occurred periods of weakening of the westerly winds to stagnant conditions. Site-specific bivariate polar plots (BBPs) also supported this finding. The increase of regional dust during high wind speeds on dusty days led to lower PM2.5/PM10 ratios at most sites. Back trajectory analyses of PM10 concentrations showed that the transboundary transport from westerly directions produced the high PM10 concentrations. Understanding the PM10 pollution episode characteristics could permit planning of practical anthropogenic emissions reductions and support the joint air pollution control strategies for regional air pollution.