The impact of heat waves on the level of air pollution in Kyiv

b) dependence between formaldehyde and air temperature on MS № 20 during the HW 16-21.08.2021

Fig. 3 Sea level pressure (hPa) during heat waves in Kyiv

Larger horizontal pressure gradients are observed over pressure centers over the ocean, and less significant gradients over the continent. On the 500 hPa constantpressure surface, it was found that during heat waves, this surface has bigger heights along the line from the Iberian Peninsula, along the coast of the Mediterranean Sea, Ukraine, and the western territories of russia bordering Ukraine. Gradually, the 500 hPa constant-pressure surface slopes northwest toward the North Atlantic. The air temperature on the 850 hPa constant-pressure surface decreased from > 25°C over the Mediterranean Sea to the northwest < 0°C. One such example is the case of the heat wave in Kyiv on August 3, 2017 (fig. 4).

a) height of 500 hPa constant-pressure surface (gpm) b) temperature on the 850 hPa constant-pressure surface (°C)

Fig. 4 The maps of the height of 500 hPa constant-pressure surface and temperature on the 850 hPa constant-pressure surface on August 3, 2017

This distribution of atmospheric pressure causes circulation, which determines the conditions that lead to heat waves over Southern, Central, and Eastern Europe.

The occurrence of heat waves in Eastern Europe during the studied period was associated with a ridge of high pressure passing through Europe from the Iberian Peninsula to the northern regions of russia. Closed areas of high pressure (up to 1020-1030 hPa) are formed within this ridge, which determine the weather conditions over the territory where they are stationed. The surface slope of 500 hPa with maximum heights over the eastern part of the continent and a slope to the northwest further confirms the presence of warm air masses over the study area, since temperature and air density are closely related: warm air masses are less dense than cold masses, and therefore the pressure drop with height in warm air masses occurs more slowly.

The circulation system described above provided a powerful influx of solar radiation, due to little or zero cloud cover. Such conditions are characteristic of anticyclonic weather. In almost all investigated cases of heat waves, except for one, the sea-level pressure, which determined the weather conditions over the studied territory, had the same structure and shape. And in the case that was different, in general, the main regularities of the distribution of the pressure field within the studied territory were the same as in the other cases of heat waves.

Thus, during the studied period of 2017-2021, during the heat waves that occurred in Kyiv, the sea-level pressure field was determined by the same circulation processes and the distribution of the main baric centers over the Atlantic, Europe, and the neighboring territories of russia. The baric centers that determined the conditions for the formation of heat waves in Kyiv were distributed as follows: a baric ridge stretched along the coast of the Mediterranean Sea from the Azores, the Iberian Peninsula, through Italy, the Balkans, Ukraine, belarus, and the northwestern regions of russia. Within this area of high atmospheric pressure, closed centers with a pressure of 1020-1030 hPa were formed. Most often, these closed areas are formed over the Azores Islands and the territory of belarus. In turn, the large ridge of high pressure is blocked by deep closed areas of low pressure (9951000 hPa), which are formed over Great Britain, the Scandinavian Peninsula, and northwestern regions of russia and Turkey. Such circulation conditions in summer lead to the emergence of such a phenomenon as heat waves over Southern, Central, and Eastern Europe. After all, stable anticyclonic weather with low or zero cloud cover is formed over these regions. This contributes to additional overheating of the air, which is stationary over one area for a long period. The situation may be further aggravated by the advection of hot air from the territory of North Africa.

Discussion and conclusions

The atmospheric circulation conditions during heat wave events in Kyiv for the period 2017-2021 were similar to the atmospheric conditions during HW events observed in Eastern Europe during 1973-2010 (Tomczyk, 2017), and in lowland Germany over the period 1966-2015 (Tomczyk, Sulikowska, 2018). Anticyclonic conditions over Western Europe, due to the subsidence of air masses, provided cloudless weather, which intensified solar radiation flux as shown in the research study of Tomczyk, Potrolniczak & Bednorz (2017). There are also several studies of the circulation conditions that lead to the appearance of heat waves in some regions of Europe. These papers show that this occurs in most cases under anticyclonic weather conditions that have the effect of enhancing insolation (Fink et al. 2004; Rebetez et al., 2006; Founda, Giannakopoulos, 2009).

A synoptic pattern was identified in which the northern regions of Africa were under the combined influence of an anticyclonic area to the north and a trough over the Sahara extending the depression centered in the south. This was shown for two cities in Morocco: Casablanca and Marrakech, between 2010 and 2019. (Khomsi et al., 2022).

The occurrence of heat waves in Kyiv was related to a ridge of high-pressure lying over Europe, within which there was a local high-pressure area formed with its center over the belarus and the northwestern regions of russia. A similar circulation type causing the occurrence of heat waves in Central Europe is a low-pressure area with its center to the west of Ireland (Tomczyk, Bednorz, 2016).

The effect of heat waves on the concentrations of nitrogen dioxide, formaldehyde, and sulfur dioxide in the air of Kyiv was shown. An increase in the concentrations of these pollutants during the heat wave events at all studied monitoring stations (regardless of the initial level of air pollution) and a gradual decrease in concentrations after the heat wave is completed is determined. The influence of air temperature on the concentrations of formaldehyde is explained by the fact that during the heat waves, there are very favorable conditions for the course of photochemical reactions from precursor substances, which leads to an increase in the concentration of this pollutant. The relationship between air temperature and concentrations of sulfur dioxide and nitrogen dioxide is caused by the fact that heat waves are characterized not only by high air temperature and sunny weather (which are favorable conditions for photochemical reactions) but also by all features of anticyclonic weather (like low wind speeds (or calm), inversions, etc.) which are favorable conditions for the accumulation of pollutants in the atmosphere.

In order to confirm the influence of stable anticyclonic weather during heat waves on the air pollution level, the circulation processes cause to the formation of HWs over the territory of Ukraine were analyzed. It was shown that the same circulation conditions for the formation of heat waves were found in all researched cases, except for one. Namely, an area of high atmospheric pressure stretched along the coast of the Mediterranean Sea from the Azores, the Iberian Peninsula, through Italy, the Balkans, Ukraine, belarus, and the northwestern regions of russia. Closed centers with a pressure of 1020-1030 hPa were formed within this area. Most often, they are formed over the Azores Islands and the territory of belarus. The large ridge of high pressure is blocked by deep closed areas of low pressure (9951000 hPa), which are formed over Great Britain, the Scandinavian Peninsula, and the northwestern regions of russia and Turkey. Such circulation conditions lead to the emergence of heat waves in summer not only over Kyiv, but also over Southern, Central, and Eastern Europe. Thus, it was confirmed that the formations of heat waves in Kyiv are connected with anticyclonic formations in the atmosphere which will contribute to the accumulation of pollutants in the atmospheric air and, therefore, during periods of HWs, the population of Kyiv experiences both a strong health negative impact of heat and high concentrations of air pollutants.

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