The effect of NAO and MJO on Tehran metropolitan air pollution

Volume 2, Issue 2 - Serial Number 1
Summer 2019
Pages 93-113

Document Type : Original Article

Authors

1 ِDepartment of Earth Sciences, Science and Research branch, Islamic Azad University, Tehran, Iran

2 Department of Earth Sciences, Science and Research branch, Islamic Azad University, Tehran, Iran.

3 Department of Earth Sciences, Science and Research branch, Islamic Azaad University, Tehran, Iran

4 Atmospheric Science and Meteorological Research Center, Tehran, Iran.

Abstract
The most important factors controlling the distribution of pollutants emitted from different sources and in different season are atmospheric circulations.This study dedicated to the investigation of the impact of two North Atlantic and Maden-Julian Oscillations (NAO and MJO) on Tehran air pollution during the period of 1984-2013. To show those oscillations relationship with the latter said event their correlations with the event were studied. As part of the correlation study the impact of time lag in the relation between those oscillations and the city air pollution was studied, too. Then meteorological fields and their anomalies that were considered effective on the formation of acute periods of air pollution, as well as the significant features and characteristics of atmospheric circulation at different levels were determined by using bootstrap and wavelet statistical methods. Afterwards, correlations between those identified anomalies and characteristics with different phases of the NAO and MJO oscillation was investigated. Accordingly, it was found that the accumulation of pollutants from local sources into the city atmosphere was mainly affected by the positive phase of NAO, which caused the warm and humid westerly air flows departed from the middle east and positive MJO phase especially the fourth phase and departure of convective cell to the eastern part of Indian ocean, during the cold and hot seasons. It was also found that the city atmosphere contamination with pollutants from regional sources mainly take place in both positive and negative phases of NAO. However it was more pronounced with shifts from positive to negative phase of NAO. The negative phases of MJO were also found effective in the formation of such circumstances especially the first phase and when the convective cell was at its closest distance from the deserts of the western and south western neighboring countries. Collectively, it may be stated that warm season contamination of Tehran’s atmosphere from local or regional sources is associated with weak MJO and negative phases of the oscillation (Phases 1, 2, 7 and 8), while contamination occurring during cold seasons is associated with MJO positive phases (phases 3, 4, 5 and 6).

Keywords