After 8:00 p.m. on 5 October, PM2.5 concentrations increased significantly. At 9:00 p.m., the concentration reached 28 µg/m³; by 3:00 a.m. on 6 October, it had risen to 38 µg/m³, and by 8:00 a.m., it reached 40 µg/m³. The morning reading was almost three times higher than the daytime level. Between 3:00 a.m. and 8:00 a.m., the citywide average fell within the “Unhealthy for Sensitive Groups” category under the international AQI (AQI 108–111), while remaining at the “Green” level under the national standard.

The deterioration in air quality was largely associated with unfavourable weather conditions that limited the dispersion of pollutants. At the beginning of the week, a cold anticyclone affecting Uzbekistan created conditions for atmospheric stagnation and a stronger temperature inversion during the night. As temperatures fell overnight and wind speeds remained mostly at 0–1 m/s, both horizontal and vertical air circulation weakened. As a result, fine particulate matter accumulated in the air close to the ground.

Vertical monitoring data show that a temperature inversion was observed at 11:00 p.m. on 5 October. It became stronger between 2:00 a.m. and 5:00 a.m. on 6 October and weakened by 8:00 a.m. During a temperature inversion, relatively warmer air forms above colder air near the ground, effectively limiting the upward dispersion of pollutants.

Higher humidity can also affect readings from air-quality monitoring sensors. Some fine particles absorb moisture from the air and increase in size, which can result in higher readings from optical sensors.

Similar conditions may occur temporarily during nighttime and early morning hours through the end of the week.

For reference: Under a joint decision of the Ministry of Health of the Republic of Uzbekistan and the National Committee on Ecology and Climate Change, titled “On the Procedure for Introducing a National Air Quality Indexing System and Preventive Measures to Reduce the Impact of Air Pollution on Public Health,” Uzbekistan has established a national air quality assessment system based on PM2.5 and PM10 concentrations.
Ambient air quality is currently being continuously monitored by the Special Commission.