# Black Carbon in the Air of the Baikal Region, (Russia): Sources and Spatiotemporal Variations
## 3 Results and Discussion
### Black Carbon in the Air Basin of Lake Baikal
Breeze circulations, such as those observed during the 13-h mooring of the RV on 6 August 2023 near the Slyudyanka town, are an important mechanism for the transport of anthropogenic air pollutants onto the water area of Lake Baikal. During the RV moorings near settlements, particularly in the southern part of the lake, the eBC concentrations increased with varying intensity, which is a consequence of the anthropogenic activity: the operation of industrial enterprises and engine exhausts from vehicles and ships, the amount of which has increased to 500 units in recent years. The highest eBC concentrations (6.3 \(\upmu\)g m\({}^{-3}\)) were recorded in the evening and at night during breeze circulation of wind from the coastal zone (Figure 6, Table 2). A similar situation was observed on 14 August 2023 at the Angara River source (west coast of the southern basin). Many hours of research in this area revealed that, due to the transport of air pollutants from local anthropogenic sources (Listvyanka settlement and Baikal port), the bulk of the aerosol was concentrated in the lower 1000 m layer and significantly affected the near-water atmosphere in this area. For example, in the evening, under coastal breeze conditions, the concentrations of eBC (7 \(\upmu\)g m\({}^{-3}\)), nitrogen oxides (30 \(\upmu\)g m\({}^{-3}\)), and submicron and highly dispersed particles (800 cm\({}^{-3}\)) dramatically increased. Additionally, intense navigation in Listvennichny Bay also influenced the eBC measurements. Analysis of backward trajectories calculated from the HYSPLIT model for this day indicated in this area a northwesterly transport of air masses along the Angara  
Figure 6: Spatiotemporal variability of the eBC mass concentration in the coastal water area of Lake Baikal during the expedition in the summer of 2023. River valley from the industrial centers of the region: the cities of Irkutsk and Angarsk, etc. This could also affect the eBC concentration growth in the near-water atmosphere. During stationary measurements near the settlements in other areas of the lake, the highest eBC concentrations were also recorded in the evening and at night with breeze circulation of wind from the coastal zone to the lake. For example, in the evening hours on 9 August 2023, near the Severobaikalsk town, we recorded elevated eBC concentrations of up to 0.59 \(\upmu\)g m\({}^{-3}\). Moreover, its mean concentration during the entire period of this mooring lasting for 17 h was 0.22 \(\upmu\)g m\({}^{-3}\) (Table 2). High eBC concentrations were observed near the Barguzin River mouth in the largest Barguzin Bay. In the evening and night hours, with the formation of breeze from the coastal zone, the eBC concentration increased to 0.95 \(\upmu\)g m\({}^{-3}\) near the Ust-Barguzin settlement. The atmosphere far from settlements showed low eBC concentrations that were regarded as background values. Based on the results of two years of expeditions, the mean eBC concentration at such sites in the southern basin was 0.27 \(\pm\) 0.09 \(\upmu\)g m\({}^{-3}\) (Tables 1 and 2). In the central and northern basins, along the west coast from the Bugulekais settlement to the Severobaikalsk town, the mean eBC concentration varied from \(\sim\)0.28 to \(\pm\) 0.09 \(\upmu\)g m\({}^{-3}\). On the east coast of the northern basin, from Ayaya Bay to the central basin, this was 0.17 \(\pm\) 0.08 \(\upmu\)g m\({}^{-3}\) in 2023. The northeast coast of Lake Baikal, Ayaya Bay, and Khakusy Bay, demonstrated the lowest eBC concentrations (0.15 \(\pm\) 0.07 \(\upmu\)g m\({}^{-3}\)). Figure 7a,b shows the mean daily eBC concentration under background conditions and near populated areas, which was obtained during the 2023 expedition. Under background conditions, the diurnal eBC variability in the air above the lake was poorly manifested. At the same time, under anthropogenic load near large local sources, its concentration, as well as submicron fraction aerosol particles, increased in the evening and at night with breeze circulation.