## 3 Results and Discussion
### Seasonal Variability of Gaseous Pollutants under Urban and Rural Conditions

Table 2: Number of episodes with elevated SO2 and NO2 concentrations at the “Listvyanka” station, n—total numbers of measurements. is also confirmed by data from the Acid Deposition Monitoring Network in East Asia (EANET) (Table 3). Low air temperatures lead to an increase in the volume of emissions from heat sources, and the minimum precipitation (7-8 mm on month) weakens the mechanism for washing out anthropogenic pollutants from the air [51, 52, 53]. _ABL thickness._ According to [54], in the winter months, the ABL height decreases to 500-700 m at the Listvyanka station, which is 1.5 times lower than in the transitional seasons (spring and autumn). A thinner ABL at this time is associated with an increase in the thermal stability of the lower atmospheric layers, which also weakens the processes of the natural dispersion of pollutants, increasing the concentrations of the latter in the air [55, 56, 57]. _Wind regime character._ In addition to the amount of emissions from the thermal power plant, the frequency of air mass transport from large pollution sources and the wind regime character affect the state of the air basin above the investigated areas. During the cold season of the year, with a high-pressure area above most of East Siberia, weak winds prevail in the lower 500 m of the atmospheric layer. As the anticyclone disintegrates, the character of the weather conditions changes. In summer, due to the development of the cyclonic activity, wind speeds increase dramatically. On the contrary, in the higher atmospheric layers, minimum wind speeds are recorded in July, and the maximum recorded in November and December [58]. Although the northwesterly direction is predominant for air mass transport in the region (on average 60-70% per year) [59], orographic features determine the wind direction in the ground-level atmospheric layer. As shown in [60], the number of cases of transport from high-altitude air pollution sources situated in the cities of the Irkutsk agglomeration varies significantly from month to month. In January 2022, we recorded the maximum number (84%) of plume transport from the regional thermal power plants to the southern basin of Lake Baikal. This, together with other factors, explains the frequent occurrence of the winter concentration maxima of sulfur and nitrogen dioxides at the rural Listvyanka station. In spring, the wind regime changes in the region: the ABL grows (to 800-1000 m) [54] and the amount of precipitation increases (more than by a factor of 2-4) [50]. Noteworthy is that the frequency of northwesterly transport does not always lead to an increase in the concentrations of gaseous pollutants at the Listvyanka station. In certain winter months and in summer, the frequency of northwesterly transport may be the same, but an increase \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline
**Month** & **I** & **II** & **III** & **IV** & **V** & **VI** & **VII** & **VIII** & **IX** & **X** & **XI** & **XII** \\ \hline \multicolumn{11}{c}{Listvyanka station (rural)} \\ \hline T, \({}^{\circ}\)C & \(-14.4\) & \(-15.1\) & \(-6.9\) & \(1.0\) & \(7.0\) & \(10.6\) & \(13.5\) & \(13.1\) & \(8.2\) & \(2.5\) & \(-4.8\) & \(-11.7\) \\ \hline \multicolumn{11}{c}{Precipitation amount, mm/month} \\ \hline Mean wind speed, m/s & 3.2 & 3.8 & 3.5 & 3.4 & 3.3 & 2.8 & 2.6 & 2.3 & 3.0 & 3.3 & 3.7 & 4.3 \\ \hline \multicolumn{11}{c}{Irkutsk station (urban)} \\ \hline T, \({}^{\circ}\)C & \(-16.2\) & \(-14.8\) & \(-4.9\) & 3.8 & 11.0 & 16.4 & 18.3 & 16.0 & 9.4 & 2.5 & \(-6.2\) & \(-14.4\) \\ \hline \multicolumn{11}{c}{Precipitation amount, mm/month} \\ \hline Mean wind speed, m/s & 1.6 & 1.7 & 2.0 & 2.0 & 2.0 & 1.6 & 1.5 & 1.4 & 1.7 & 1.8 & 1.7 & 1.4 \\ \hline \end{tabular}
\end{table}