Atmospheric stability analysis and its correlation with the concentration of air pollutants: A case study of a critical air pollution episode in Tehran

In the present survey، it has been tried to demonstrate a significant correlation between air pollutant concentrations and meteorological parameters، by the study of the atmospheric conditions in an air pollution episode. Carbon monoxide and nitrogen dioxide were selected as air pollutants. Temperature، wind speed and direction and the dimensionless Richardson number were considered as the meteorological parameters in this study. To this aim، a period of eleven days from 30 November to 10 December 2012 was considered. In this period، a severe temperature inversion and consequently a long-term stable situation was seen in the Tehran weather. The governing stability classes were studied using the Turner algorithm and vertical temperature gradient scheme. For the mentioned period، the sun altitude، wind speed at the height of 10 meters، total cloud cover and ceiling were extracted from the U. S. Naval Observatory web page (http: //aa. usno. navy. mil/data/docs/AltAz. php) to be used in the Turner algorithm. The stability analysis showed that for this time interval، stable conditions prevailed in 89% of cases. The sounding data was collected from the Mehrabad weather station. By the sounding data، the daily maximum mixing depth was calculated and the vertical trend of the temperature was plotted in each day during this period. The morning vertical temperature gradients showed a strong temperature inversion in these eleven days. Also، the maximum mixing depth decreased to the minimum value of 600 meters on the 6th day of the time interval. The maximum mixing height and visibility graphs also confirmed that the atmospheric mixing decreased and consequently more pollutants were trapped in the middle of the period. Once confirming critical conditions by comparing the stability classes، maximum mixing depth and the visibility، the correlation between atmospheric parameters and pollutants concentration was obtained by multiple regression method. Among the meteorological parameters that were considered، temperature، wind speed، wind direction and the Richardson number had a greater correlation with the pollutant concentration. The bulk Richardson number was calculated from the sounding data and used in the regression. The CO and NO2 concentrations were gathered from the pollutant concentration measuring station which is located on the region-10 municipality building، 2500 meters far from the Mehrabad weather station. The coefficients of correlation between the normalized CO concentrations and temperature، West-East component of wind، South-North component of wind and the Richardson number were 0. 707، 0. 078، 0. 028 and -0. 019، respectively. And for NO2 concentration، the correlation coefficients were 0. 353، 0. 016، 0. 015 and -0. 019، respectively. It could be observed that the coefficients of temperature and surface wind for the CO concentration were almost twice the corresponding coefficients for the NO2 concentration. The reason is that the NO2 concentration has been influenced by chemical and photochemical reactions and the mixing depth، while the CO concentration is only affected mainly by the rate of transportation and the mixing depth. In fact، the CO concentration had a stronger functionality to temperature and wind speed than the NO2 concentration. In the most cases، the Richardson number had a positive value and by increasing its value (in the early morning hours or night hours)، the pollutant concentration were reduced. Therefore، as expected، its coefficient were negative in both regressions.