# Black Carbon in the Air of the Baikal Region, (Russia): Sources and Spatiotemporal Variations
## 2 Methods  
### Equipment and Expedition Route for Analyzing Black Carbon in the Air above Lake Baikal  
Optical methods currently determine the concentrations of eBC particles at the permanent stations and in the water areas of seas and oceans [31; 32]. Most models of aethalometers (AAE31 and AAE33, Magee Scientific, Berkeley, CA, USA) measure light absorption at seven wavelengths, from 370 to 950 nm in the visible range. Detection limit (1 h): \(<\)0.005 \(\upmu\)g m\({}^{-3}\); range: \(<\)0.01 to \(>\)100 \(\upmu\)g m\({}^{-3}\); resolution: 0.001 \(\upmu\)g m\({}^{-3}\) or 1 ng m\({}^{-3}\). Light-absorbing carbonaceous aerosols (LACs) with a wavelength of 880 nm are designated as eBC. An MDA-02 aethalometer, an analogue of that developed by [PERSON] et al. [33], is used at the V.E. Zuev Institute of Atmospheric Optics Siberian Branch of the Russian Academy of Science (IAO SB RAS) in the study of eBC on ships in the Arctic basin, Siberian and Baikal regions. The operation of the device is based on the continuous measurements via aethalometric filter method for diffuse light attenuation by a layer of aerosol particles during their deposition onto the filter from the pumped air. The air intake rate in the device was 2 L min\({}^{-1}\); hose length was 2.5 m with an internal diameter of 8 mm. The concentration limit of the MDA-02 photometer is \(\sim\)10 ng m\({}^{-3}\) when pumping 30 L of air through the device. This measures the mass concentration of an absorbent substance (eBC, MeBC \(\upmu\)g m\({}^{-3}\)), in submicron atmospheric aerosol, which is equivalent to the BC concentration in the air, eBC (\(\upmu\)g/m\({}^{3}\)). Absolute calibration of the device is carried out in the laboratory using a soot generator and comparison of data from synchronous optical and gravimetric measurements [34; 35]. Simultaneous intercalibration measurements of the eBC mass concentration in the atmospheric aerosol are periodically conducted with a MAAP-5012 photometer (Thermoscientific Co., Franklin, TN, USA). In the summer of 2019 and 2023, we carried out comprehensive experiments to study the air composition above Lake Baikal using remote sensing and local monitoring tools that were installed onboard the research vessels (RV) \"Akademik V.A. Koptyug\" (2019) and \"G.Yu. Vereshchagin\" (2023) owned by Limnological Institute Siberian Branch of the Russian Academy of Science (LIN SB RAS) (Figure 1). Expeditions onboard the RV were carried out in the form of online recording of eBC, gas impurities, aerosol particles of different size fractions, and meteorological conditions. Automatic chemluminescent gas analyzers of sulfur dioxide, nitrogen oxides, and ground-level ozone (OPTEC LLC, St. Petersburg, Russia), a DAS2702M diffusion aerosol spectrometer (AeroNanoTech, Moscow, Russia), and a DUSTTAK 8533 spectrometer (TSI Incorporated, Shoreview, MN, USA) to determine number concentration and mass concentration of aerosol particles, as well as an MDA-02 aethalometer to identify eBC, were installed on the upper deck of the RVs. Sampling with an MDA-02 aethalometer was carried out within 24 h in an automatic mode for 10 min: once every half hour in 2019 and once every 15 min in 2023. The values averaged over the sampling period were entered into the computer. Taking into account the weather conditions, measurements influenced by the ship exhaust pipe were excluded from the general dataset. The 2019 measurements were mainly conducted under anticyclonic weather conditions. Aerosol fields were probed with a LOZA-A2 aerosol Raman lidar [36]. The route of the RV began and ended at its home port in the Listvyanka settlement (Listvennichny Bay, southern basin of Lake Baikal) and passed through longitudinal sections along the coastline of the entire perimeter of Lake Baikal, with periodical cross-sections, as well as with some temporary stops at pre-designated observation sites. The total length of the route was \(\sim\)1700 km.