## 3 Results and Discussion
### Seasonal Variability of Gaseous Pollutants under Urban and Rural Conditions
Moreover, the data analysis (Table 1) identified the maximum values exceeding the average gas concentrations by one or two orders of magnitude, which was likely due to the influx of smoke plumes from the thermal power plant when the weather conditions affecting the dispersion of the pollutants changed. After selection, we attempted to estimate the frequency and seasonal distribution of these cases (Table 2). The table shows that such episodes most often occur in December, January, and February and account for about 8% of all the observations. Based on the average and median estimates (Table 1), the sulfur dioxide (SO2) concentrations at the remote rural Listvyanka station exceed the values recorded at the Irkutsk station. Lower SO2 values at the urban station are associated with its location on an elevated relief in one of the \"clean\" forest zones of the city, away from the main direction of the smoke plume transport from the city's largest thermal power plant, with smoke stacks over 150 m high and a plume rise height exceeding 500 m above ground level [45]. At the 200-500 m altitudes, hot SO2 emissions from this thermal power plant are carried beyond the city and are not recorded in the ground layer of the atmosphere at the Irkutsk station. However, the level of air pollution measured at the Rosgidromet (Federal Service for Hydrometeorology and Environmental Monitoring) stations in the other Irkutsk districts (the stations are located at an altitude of 2 m above ground level) are twice as high [46,47]. Analysis of the nitrogen oxide concentrations (Table 1) in the air above the stations indicated significant differences between them. In the city, the nitrogen oxide concentrations (NO and NO2) were 5-10 times higher than at the Listvyanka station. At the same time, at this station, NO was recorded rather rarely, testifying to the absence or weak influence of the local sources of air pollution on the atmosphere in this area. Because the NO lifetime is ~2-3 h, we can assume that, at the Listvyanka station, we sometimes recorded episodes when this gas managed to reach the shores of Lake Baikal from the remote regional sources of air pollution [48,49], and presented in Section 3. We examine in detail the average monthly variations of nitrogen and sulfur oxides in the air above the stations for the entire observation period (Figures 4-6). The ordinate axis shows the concentration of trace gases (\(\mu\)g m\({}^{3}\)), and the abscissa axis shows the month (time). The diagrams reflect the distribution medians (bold line), the first quartile (lower boundary of the box, Q1), and the third quartile (the upper boundary of the box, Q3). Atypical values beyond the one and a half product of the difference between Q2 and Q1 (so-called outliers) are not given. The figures demonstrate that gaseous pollutants generally have a similar pattern of intra-annual distribution, with the maximum in February and December and a depression from June to August. The highest concentrations of gaseous pollutants in the air were recorded in February. This was due to the combined effect of some of the factors represented below. _Air temperature and precipitation._ The assessment conducted in [50] revealed that February and March are the coldest months in the year with the lowest precipitation. This \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline
**Avg. Time = 1 h** & **I** & **II** & **III** & **IV** & **V** & **VI** & **VII** & **VIII** & **IX** & **X** & **XI** & **XII** \\ \hline SO2 \textgreater{} 85 \(\mu\)g m\({}^{3}\) & 219 & 205 & 68 & 1 & 8 & 0 & 0 & 0 & 2 & 16 & 67 & 238 \\ \hline n & 2924 & 2329 & 2519 & 2097 & 2152 & 2329 & 2030 & 1952 & 2093 & 2089 & 2341 & 2777 \\ \hline NO2 \textgreater{} 35 \(\mu\)g m\({}^{3}\) & 209 & 180 & 33 & 7 & 5 & 2 & 1 & 45 & 6 & 48 & 158 & 260 \\ \hline n & 2787 & 2119 & 2129 & 1959 & 2146 & 1927 & 1781 & 2680 & 2234 & 2564 & 2401 & 2669 \\ \hline \hline \end{tabular}
\end{table}