Investigating the role of atmospheric circulation patterns in the severe air pollution in Esfahan



Introduction

The atmosphere is a dynamic system in which a large number of physical and chemical processes occur simultaneously. Studying the dynamics and transmission of pollutants in the atmosphere using atmospheric patterns is one of the important topics in this field. Atmospheric patterns simulate the occurrence of different processes within the atmosphere and their interactions. Using an atmospheric model also requires comparing the results of the model with field and laboratory experiments. This helps in understanding the occurrence of chemical and physical processes in the atmosphere as well as evaluating the implementation of a suitable model. Laboratory measurements provide valuable information while at the same time visualizing and describing atmospheric properties and atmospheric composition at specific time and space intervals. An atmospheric model provides a complete picture of the evolution of spatial and temporal variations in atmospheric pollutants at different altitudes. Understanding atmospheric dynamics can be understanded by combining measurements and integrated modeling with using synoptic systems in periods with pollutated air. Therefore, in this study, it has been attempted to analyze the atmospheric factors that cause severe pollution in Esfahan and the relationship and mechanism of the atmosphere at the time of occurrence of this phenomenon.



Data and methods

In this study, three datasets including pollution data recorded at air pollution stations, digital atmospheric data and high atmospheric stations were used. The air pollution data are from three stations of Laleh Square, Azadi and Bozorgmehr which were obtained from Esfahan General Environmental Protection Office. The pollutants include carbon monoxide, nitrogen dioxide, sulfur dioxide, ozone and suspended particulate matter (PM10), which have been prepared and processed daily for a 12-year statistical period (1995-2005). To study atmospheric conditions were used re-analyzed data from the National Center for Environmental Prediction (NCEP / NCAR) include sea level pressure, geopotential height, vertical velocity (Omega), wind orbital components (U), and meridian wind ( V) was used for different levels of atmosphere.

The above atmospheric data were obtained from the University of Wyoming site for the study days, including air temperature, dew point temperature, wind direction and intensity, and atmospheric stability and instability conditions (based on skew-t curves). In this study, a Lagrangian model with the capability of tracking particle backward in different levels of atmosphere called HYSPLIT was used to investigate the days associated with severe pollution.



Results and discussion

The results show that the highly pollutated days of the city of Esfahan can be explained by the four synoptic patterns. The occurrence of days with extremely severe pollution in Esfahan, rather than being rooted in local factors, is due to the interaction of local conditions with atmospheric circulation at the regional scale. In other words, the city of Esfahan will only experience extremely polluted days when the atmospheric circulation of the atmosphere provides conditions for increased concentrations of pollutants.

The main causes of the occurrence of days associated with maximum contamination can be attributed to Subtropical high latitude and its progression to higher latitudes. This circulation system contributes to the occurrence of highly polluted days on most days, either directly or in combination with other atmospheric systems.

The role of local factors such as the formation of inversion layer and the increase of atmospheric thickness due to the dominance of high pressure systems in the region can also be considered to exacerbate the conditions.

The use of suspended particle backward models and the study of atmospheric thermodynamic relationships have provided a deeper and more accurate understanding of the mechanisms dominating the occurrence of pollutants in Esfahan.

The results of this method showed that the occurrence of highly polluted days in the city of Esfahan can not be attributed to urban pollutants such as industrial factories of automobiles and so the influx of particulate matter from different areas has caused higher intensity pollution.



Conclusion

The results showed that four factors and patterns prevailed in the middle of the atmosphere at the time of the most severe days pollution in Esfahan. The results of the PSI values in each pattern showed respectively from pattern of one to four, is 221, 238.6, 203 and 281.

The synoptic conditions can be attributed to the presence of tropical high pressure, which is accompanied by a layer of temperature inversion in the lower levels of the atmosphere and the middle troposphere.

Strength of negative vorticity above 700 hPa and continued surface convergence to this altitude have made the nature of the summer atmosphere clearly observed in the pollution event in the city, which has been enhanced by strong anomalies.

On the other hand, the output of the HYSPLIT model showed that the occurrence of highly polluted days in the city of Esfahan could not be detected in urban pollutants such as automobile industrial plants and. But, the influx of particulate matter from different areas has made the pollution more intense, and the influx of dust particles has exacerbated this hazard.



Keywords: Air Pollution, PSI Index, Atmospheric Regional Circulation Patterns, HYSPLIT Model, Esfahan