The Role of Regional Scale Atmospheric Circulation on the Occurrence of Critical Air Pollution Episodes in Mashhad, Iran

Air pollution is one of the most important environmental issues in Megacities which is seriously threatening human health. High concentration of air pollution is frequently seen over large cities of Iran in recent years. It is difficult to estimate the exact loss of lives and properties resulting by air pollution because of the lack of data، however there are reports that show the vast destructive effects of this phenomenon over the polluted cities. Air quality over the cities is determined with respect to both the total air pollution in a given area as it interacts with meteorological conditions such as humidity، temperature، and wind characteristics to produce an overall atmospheric condition. Air quality is determined by emission sources and weather situations; however، serious pollution episodes in the urban environment mainly result from certain weather conditions which pose difficulty for pollutant dispersion. Air pollution is influenced by meteorological factors through transport، chemical transformations، and removal via wet and dry processes. In this research، the role of regional atmospheric circulation on the occurrence of critical air pollution episodes in the Mashhad metropolitan is investigated. Also the synoptic and thermodynamic mechanisms which are dominated during the most polluted days over the city is identified. Study Area: The study area is located in the valley of Kashaf-Rood River between two mountain ranges of Binalood and Hezar-masjed. Its geographical coordinates are 59º، 27''، 0«E to 59º، 40''، 30'' E and 36º، 22''، 0'' N. The city benefits from the proximity of the mountains، having cool winters، pleasant springs، rather hot summers، and beautiful autumns. It is only about 250 km (160 mi) from Ashgabat، Turkmenistan. Mashhad is 999 m higher than the sea level. According to meteorological records، the average maximum temperature for the four seasons in Mashhad are: winter 8. 3°C (52°F)، spring 21° C (88°F)، summer 33. 2°C (92°F)، and autumn 22. 2°C (72°F). The average minimum temperature for the four seasons also is: winter -3. 6°C (18. 9)، spring 7. 3°C (45. 2°F)، summer 13°C (61. 3°F)، and autumn 5. 7°C (42°F). Mashhad has a semi-arid climate with occasional rainfall in spring and autumn and light snow in the winter. The city only sees about 250 mm of precipitation per year، some of which occasionally falls in the form of snow. Mashhad also has wetter and drier periods with the bulk of the annual precipitation falling between the months of December and May. Summers are typically hot and dry، with high temperatures sometimes exceeding 35 °C (95 °F). Winters are typically cool to cold and somewhat damper، with overnight lows routinely dropping below freezing. Mashhad enjoys on average just less than 2900 hours of sunshine per year. Its population was 2،772،287 at the 2011 population census; so، the city is ranked as the second most populous city of Iran (http: //en. wikipedia. org/ wiki/Mashhad).

In this study، a combined synoptic analysis approach is employed to identify the role of regional atmospheric circulation pattern on the occurrence of high-polluted days over Mashhad. Four types of dataset including air-quality monitoring station data، reanalysis gridded data، upper air soundings data and a lograngian backward trajectory model (HYSPLIT) outputs are used to do this research. The air-quality monitoring station data is obtained from Khorasan-e-Razavi Environmental Protection Administration، while the upper air soundings data and reanalysis gridded data are obtained from National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR). and Wyoming Weather Web، respectively. Also، the Hybrid Single-Particle lograngian integrated Trajectory (HYSPLIT) model is employed to determine the main sources of air parcels by using a backward trajectory approach. Using the Pollutant Standards Index (PSI). daily، seasonal and annual variations of Mashhad air quality is investigated for a 7-year (2005-2011) time period. Using PSI index، we first detected and extracted all very high-polluted days (PSI>200) over Mashhad. Then، a combined synoptic classification approach has been applied by using a set of manual synoptic classification، lograngian backward trajectory model outputs and thermodynamic diagrams.

Investigating the temporal variations of five air pollutants over Mashhad، it is seen that Carbon monoxide is apparently shown a decreasing trend among all air pollutants. While Ozone and Sulfur dioxide experience an increasing trend during the study period. Also، it’s seen that the PM10 shows the most monthly، seasonal and annual values among the others. Moreover، it is revealed that those pollutants with low concentration are showing more seasonal variations in Mashhad. Using combined synoptic classification approach it is indicated that the days with highest level of air pollution in Mashhad can be classified into four main atmospheric circulation patterns including: combined Siberian high-subtropical high، migrating anticyclone، subtropical high and extra-tropical cyclone patterns. In combined Siberian high-subtropical high pattern، a simultaneous domination of Siberian high in lower atmosphere and subtropical high in middle troposphere has been caused an atmospheric stability and high levels of air pollution during cold period of the year. In this pattern، formation and coexistence of a shallow inversion layer in the lower atmosphere and another upper inversion layer in mid-troposphere show coincident roles of Siberian high and subtropical high in increasing the concentration of air pollution to higher level. In mitigating anticyclone pattern، by crossing a Rossby wave over the area، a strong mid-tropospheric ridge over the Oral Mountains and the Caspian Sea will dominate. The eastward inclination of this ridge has been caused an anticylonic circulation in a region from Caspian Sea to northern Khorasan in lower atmosphere. In this pattern، a few thin inversion layers form at the same time below 400hPa level. Rapid temperature drop between the inversion layers indicate the strength and weakness of air subsidence in this pattern. The subtropical high pattern has caused a high level of air pollution over Mashhad only in warm period of the year and it shows a distinct pattern of subsidence inversion type in the vertical profile of atmosphere. The Low-pressure pattern has identified by passing of a rather deep trough over the northern Iran in mid-troposphere and the formation and eastward movement of a cyclone in the lower atmosphere. In general، this pattern is not associated with a distinct and strong inversion layer in vertical profile of the atmosphere، while the thermodynamic diagrams show baroclinic instability through the atmosphere. In this pattern the days with very high levels of air pollution have been caused by lifting، transportation and distribution of dust and particles to the study area due to instability and vast ascend of air as well as lack of suitable moisture feeding into the low pressure.

The results indicate that the middle atmospheric circulations play the most important role in creating extremely high air pollution episodes in Mashhad. The results also demonstrate that the importance of each atmospheric circulation pattern differs from one season to another season. In other words، while in cold period of the year the occurrence of extremely high air pollution episodes has strongly related to extra-tropical atmospheric circulation patterns (i. e. mitigating anticyclone and low-pressure patterns). in warm period، all of high polluted days have mainly related to subtropical atmospheric circulation pattern. However، the subtropical ridge is the most important and also the most frequent pattern which should be consider among 4 main atmospheric circulation patterns. Implication of a combined synoptic classification approach by combining manual synoptic classification، backward trajectory analysis and investigating atmospheric thermodynamic conditions provide a better understanding of dominant mechanisms in extreme air pollution episodes over northeast of Iran. This approach was successful and effective and it has created reasonable and reliable results.