THE EFFECT OF NATIONAL EVENTS an‎d HOLIDAYS ON AMBIENT PM2.5 CONCENTRATIONS

Introduction: The aim of this study was to evaluate the effect of national events and holidays on PM2.5 concentrations in Tehran megacity. Materials and methods: Air quality data were collected from Tehran Air Quality Control Company (TAQCC) over a period of four years, 2012 – 2015, then data from 10 air quality monitoring stations were processed using R- 3.3.3. The Iranian New year, Ashura and Eid al - Fitr were the national events that considered to study. To compare the daily mean PM2.5 concentrations in Norooz holidays and other days of spring, the Mann - Whitney test and difference in PM2.5 concentrations in each season were investigated with Kruskal-Wallis test. Results: In Norooz holidays, the daily mean PM2.5 concentrations in comparison with other spring days were decreased significantly (Pvalue < 0.001). The same decreasing occurred for 10 fi rst days of Muharram too (Pvalue < 0.01) but the daily PM2.5 concentrations during Ramadan days were signifi cantly higher than that of summer days (Pvalue < 0.001). Based on Kruskal-Wallis test the difference in PM2.5 concentrations between seasons was signifi cant and only between summer-autumn (Pvalue > 0.05) and winter-summer (Pvalue > 0.05) was not. Conclusions: The effect of holidays on PM2.5 concentration was not observed in summer, however it was an effect in autumn. In addition the concentration of PM2.5 was not influenced by holidays in winter regarding the thermal inversion and stability of air.