Impacts of NAO on the meteorological conditions of air quality in Tehran during 2006-2017

In this research, the possible effects of NAO, on Tehran air quality during 2007-2016 were investigated. First, the daily indices of air quality of Tehran in the autumn and winter seasons for the study period were applied to identify polluted and unpolluted periods. The 1th ,5th and 9th deciles of the air quality indices were chosen as the indicator for good, middle and bad air qualities. Based on these indices, five polluted and eight unpolluted periods were identified. Then, the daily indices of NAO and air quality of Tehran were used to calculate the Pearson correlation coefficients and discuss the relationships between air quality and the NAO indices. In this regard, we attempted to find out the optimum time lag for each case by examining different times that there was maximum correlation between air quality index and the NAO indices. For dynamical study, the European Centre for Medium-Range Weather Forecasts (ECMWF) (ERA- Interim) reanalysis data set including mean sea level pressure, wind, temperature, geopotential height, and specific humidity in a time period from 1979 to 2017 were used. The horizontal resolution of the initial data is 0.75°×0.75° in longitudinal and latitudinal directions prepared operationally every six hours at 60 levels.
     The statistical analysis of the NAO indices shows that in the polluted periods, the positive phase of NAO is dominant, while there is no significant statistical difference between the positive and negative phases of NAO in the unpolluted periods. In general, the unpolluted periods (five cases) are associated mostly with the negative phase of NAO. Because of this limitation, we decided to analyze the synoptic-dynamic situations of all cases using the anomaly maps of the quantities relative to their long means. Thus, it is possible to improve the reliability of the results concerning the the effects of NAO on air quality in Tehran.   
    Synoptic-dynamic analysis of the cases with the highest correlation between Tehran air quality index and NAO daily indices indicates that in the positive phase of NAO, there was advection of warm and humid air to the study area, while the Middle East region had a cold and dry conditions. On the other hand, when NAO was in the negative phase, advection of moist and colder air from the Atlantic Ocean toward the east occurred, thereby existing better air quality in the region. Besides, when polluted air period coincided with the negative phase of NAO, in contrast to the normal situation, the deviation of the Siberian high-pressure axis into the meridian and adhering to the Azores high-pressure created a barrier against the westerly winds causing their meridional deviations. In the unpolluted air periods associated with the positive phase of NAO, in contrast to the normal situation, the positioning of the Atlantic high-pressure at higher latitudes, with respect to the mean state, and joining with the Siberian high-pressure make them act as a barrier in the middle latitudes, thereby existing zonal winds in the study area.