National Committee on Ecology and Climate Change of the Republic of Uzbekistan

One of the factors contributing to air pollution is atmospheric circulation, or the movement of air masses. Tashkent is located in a foothill area near a valley. Winds from the northeast and southwest play an important role in the region. Wind direction and speed directly affect air exchange and the dispersion of pollutants in the atmosphere.

Uzbek scientists have conducted extensive research into the characteristics of near-surface air circulation across Central Asia. According to the modern scientific classification, 17 types of synoptic processes are identified in the region. Depending on the location of high- and low-pressure systems, air masses move in specific directions under each type of synoptic process.

During the colder half of the year, synoptic processes associated with low wind speeds become more frequent and tend to last longer. Under these conditions, air exchange can slow down not only in Tashkent but across Central Asia as a whole. As a result, pollutants disperse more slowly, remain in the atmosphere for longer and accumulate in the air close to the ground.

Temperature inversions, a phenomenon common in large cities, are also observed in Tashkent. They occur particularly frequently during winter, especially on very cold nights. Under these conditions, a layer of warmer air forms above the colder air near the ground. This restricts vertical air mixing, allowing pollutants to accumulate in the near-surface layer of the atmosphere.

Atmospheric synoptic processes are divided into several groups. One of these is associated with the passage of cyclones, which also occur during the colder half of the year. During this period, warm and cold atmospheric fronts typically pass over Central Asia, including Tashkent. These fronts can bring precipitation, which helps remove some airborne pollutants from the atmosphere and, in turn, improve air quality.

Another important group comprises anticyclonic processes, of which three main types are distinguished. When an anticyclone is located over the European part of Russia, air masses enter Tashkent from the northwest via Kazakhstan. When an anticyclone is positioned over northern Kazakhstan, northerly air currents move towards Tashkent through Kazakhstan. When an anticyclone is situated over Siberia, air masses reach Tashkent from the northeast, again via Kazakhstan. In addition to these processes, high- and low-pressure systems with a weak pressure gradient can also occur. Under the influence of a high-pressure system, calm conditions or very light winds may prevail. Low-pressure systems can also be accompanied by calm conditions or light winds. In such circumstances, air exchange weakens and pollutants disperse less effectively.

Statistical data from recent years indicate an increase in the frequency and duration of anticyclonic processes, particularly periods characterised by high- and low-pressure systems with a weak pressure gradient. Under these conditions, low wind speeds and reduced air exchange create favourable conditions for air pollution to intensify.

Temperature inversions are particularly common at night during winter and can lead to the accumulation of pollutants in the near-surface layer of the atmosphere. Therefore, assessing air quality requires consideration not only of pollution sources but also of meteorological factors, including wind speed and direction, atmospheric pressure, the movement of cyclones and anticyclones, precipitation and temperature inversions.

A thorough understanding of the causes of air pollution and an accurate assessment of the factors involved are essential for developing science-based solutions, planning effective environmental measures and improving air quality.