## 4 Dependence of Air Pollution at the Listvyanka Station on Meteorological Parameters  
To systematize the meteorological data and gain insight on how the wind speed and direction affect changes in the concentrations of acid gases in the air of the southern basin of Lake Baikal, we used a nonparametric wind regression (NWR) model, described in more detail in [70]. As input data, we used the 20-min average concentrations of SO\({}_{2}\), NO\({}_{2}\), and NO, as well as the resulting wind speed and direction, measured at the Listvyanka station. Figure 8 shows the results of the NWR analysis for each season. This figure indicates that the highest concentrations of all the investigated gases were observed with northwesterly winds. As can be seen in Figure 1, this direction corresponds to the locations of large industrial centers in the Irkutsk region. Figure 8: NWR analysis for the 20-min concentrations of SO\({}_{2}\), NO\({}_{2}\), and NO at the Listvyanka station in the polar coordinate system from January 2020 to December 2023. The highest concentrations of acid gases (>100 ug/m\({}^{3}\) for SO\({}_{2}\) and >25 ug/m\({}^{3}\) for NO\({}_{2}\)) were recorded in air mass transport from two directions. Azimuths of the first wind direction range from 310 to 340 degrees, with a wind speed from 10 to 12 m per second. The second direction has azimuths ranging from 280 to 295 degrees, with a wind speed from 6 to 9 m per second. The direction of the first peak corresponds to the location of the air pollution sources in the Irkutsk-Cheremkhovo industrial hub, the largest centralization of anthropogenic objects in the Irkutsk region (Figure 1). The overall air emissions from these sources exceed 289,000 tons per year, of which 52% are sulfur dioxide (SO\({}_{2}\)) emissions and 14% are nitrogen dioxide (NO\({}_{x}\)) emissions. The direction of the second peak also corresponds to the location of the emission sources in the Irkutsk agglomeration. However, the shift of the peak to the western azimuth may indicate that, in winter, emissions from a municipal boiler facility in the Listvyanka settlement can contribute to emissions from large remote air pollution sources. This increases the level of air pollution at the station. With winds of the southerly and souteasterly sectors (from the water surface of the lake), the concentrations of gaseous pollutants decreased to regional background values. The distribution of nitrogen monoxide (NO) concentrations differed from SO\({}_{2}\) and NO\({}_{2}\). Figure 8 demonstrates that, in spring, summer, and autumn, the concentrations of nitrogen oxide increase in air masses brought to the station from the lake. This may be due to the onset of navigation and the influx in the emissions to the air above the station from numerous ships in Listvennichny Bay. Figure 9 shows an example of severe air pollution at the rural station resulting from air mass transport from anthropogenic sources in the Irkutsk agglomeration. This episode occurred in the second decade of December 2023. At that time, there was anticyclonic weather characterized by weak winds, no precipitation, and an air temperature of 11-17 \({}^{\circ}\)C below the climatic norm for December. Such conditions led to an increase in the emissions from thermal power facilities and a decrease in the atmospheric dispersion potential. Figure 9a-c shows the temperature stratification of the atmosphere above the city of Angarsk on December 13 and 14, 2023. Based on radiosonde data, a temperature inversion formed and persisted above Angarsk at an altitude from 138 to 349 m above ground level. To confirm the area where the polluted air masses form, we used the HYSPLIT model, which is commonly applied to analyze air mass pathways [71; 72; 73]. The calculation revealed that air masses from the pollution sources of Irkutsk, Angarsk, and Shelekhov moved to the southeast (Figure 10).