The relationship between PM2.5 concentrations and atmospheric conditions in severe and persistent urban pollution in Tabriz, northwest of Iran

The study is aimed at investigating the effects of atmospheric conditions on the concentration of PM 2.5 in one of the persistent and severe urban pollutions in Tabriz city, Iran. For this, 3 types of meteorological data were used: (i) observational data, (ii) sounding data and information derived from Skew-T diagram, and (iii) ERA-interim reanalyzed data. HYSPLIT backward trajectories were run for 3 days to obtain air masses reaching Tabriz. The results showed certain daily variation in PM 2.5 concentration related to human activities and daily weather conditions. Major atmospheric conditions that increased PM 2.5 concentrations in Tabriz include the sequential deployment of stable synoptic systems on the surface and upper levels, mainly omega blocking, formation of the inversion layer, and the establishment of cold air with slow wind blowing in the layers near the surface. Time-height sections confirmed the very cold air formation in the near-surface layers, which caused a significant difference in temperature between the near-surface layers with layers of about 2000 geo-potential meters which caused the intensification of inversion. Establishment of calm or slow wind flows in the near-surface layers to about 1–3 m/s especially on December 30 and 31, 2013, was identified as other main cause of highest concentration of PM 2.5 . The results indicated that when the height of the inversion layer is less than 1000 m above the ground, there is a significant impact on the accumulation of pollutants. The shorter length of HYSPLIT trajectories especially at near-surface levels in most days confirmed persistence of stable atmospheric systems in northwest of Iran.