Air Mercury Monitoring at the Baikal Area

atmosphere

Article Air Mercury Monitoring at the Baikal Area

Nikolay Mashyanov 1,2,*, Vladimir Obolkin 3 , Sergey Pogarev 1,2, Vladimir Ryzhov 4, Sergey Sholupov 2, Vladimir Potemkin 3, Elena Molozhnikova 3 and Tamara Khodzher 3

1 Institute of Earth Sciences, St. Petersburg State University, 199034 St Petersburg, Russia; [email protected] 2 Lumex-Marketing LLC, 195220 St Petersburg, Russia; [email protected] 3 Limnological Institute, Siberian Branch of RAS, 664033 Irkutsk, Russia; [email protected] (V.O.); [email protected] (V.P.); [email protected] (E.M.); [email protected] (T.K.) 4 Lumex Analytics GmbH, 24558 Wakendorf II, Germany; [email protected] * Correspondence: [email protected]

Abstract: The GMOS (Global Mercury Observation System) project has the overall goal to develop a coordinated observing system to monitor mercury on a global scale. Here we present the long-term (2011–2020) air mercury monitoring data obtained at the Listvyanka station located at a shore of Lake Baikal, Siberia. The long-term monitoring shows obvious seasonal variation of the background mercury concentration in air, which increases in the cold and decreases in the warm season. The short-term anomalies are associated with the wind carrying the air from the industrial areas where

several big coal-ﬁred power plants are located. A positive correlation between the mercury, SO2 and NO2 concentrations is observed both in the short-term variations and in the monthly average concentrations. The analysis of forward and backward trajectories obtained with the HYSPLIT model demonstrates revealing of the mercury emissions sources. During the cruise of 2018, the continuous

air mercury survey over Lake Baikal covered 1800 km. The average mercury concentration over Baikal is notably less in comparison with the average value obtained at the onshore Listvyanka

Citation: Mashyanov, N.; Obolkin, station during the same days of the cruise. That can lead to the conclusion that Baikal is a significant V.; Pogarev, S.; Ryzhov, V.; Sholupov, sink of the atmospheric mercury. S.; Potemkin, V.; Molozhnikova, E.; Khodzher, T. Air Mercury Monitoring Keywords: GMOS; Baikal Lake; air mercury monitoring; acid gases; emission sources at the Baikal Area. Atmosphere 2021, 12, 807. https://doi.org/10.3390/ atmos12070807 1. Introduction Academic Editor: James Cizdziel The GMOS (Global Mercury Observation System) project has the overall goal to develop a coordinated observing system to monitor mercury on a global scale to assess Received: 3 June 2021 its emissions to atmosphere, transport, atmospheric chemistry, and deposition processes. Accepted: 18 June 2021 The GMOS program included developing of the standard operational procedures (SOPs) Published: 23 June 2021 for air mercury monitoring and mercury deposition assessment harmonized with the international standards, data acquisition and data quality management system, creation Publisher’s Note: MDPI stays neutral of a network of ground-based monitoring stations, periodic oceanographic cruises, and with regard to jurisdictional claims in published maps and institutional affil- airborne measurements. More than 40 ground-based stations in the Northern and Southern iations. hemispheres were involved in the monitoring network covering many regions where little to no observational data were available before GMOS [1]. One of such new points has been founded, as a secondary GMOS site, based on the Listvyanka monitoring station located at a shore of Lake Baikal, Siberia, far away from the existing mercury monitoring sites in Asia. Long-term air mercury monitoring started in October 2011. In July 2018, for the first Copyright: © 2021 by the authors. time, the air mercury survey had been carried out throughout all the Baikal area during Licensee MDPI, Basel, Switzerland. the cruise of the research vessel “Akademik Koptyug”. This article is an open access article distributed under the terms and 2. Experimental conditions of the Creative Commons 2.1. Monitoring Site Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ The monitoring station “Listvyanka” is located in a rural area of the southwestern 4.0/). coast of Lake Baikal (51.8467 N, 104.8930 E, 670 m a.s.l.) on the top of a coastal hill

Atmosphere 2021, 12, 807. https://doi.org/10.3390/atmos12070807 https://www.mdpi.com/journal/atmosphere Atmosphere 2021, 12, x FOR PEER REVIEW 2 of 14

2. Experimental Atmosphere 2021, 12, 807 2.1. Monitoring Site 2 of 14 The monitoring station “Listvyanka” is located in a rural area of the southwestern coast of Lake Baikal (51.8467 N, 104.8930 E, 670 m a.s.l.) on the top of a coastal hill approximatelyapproximately 200 m above 200 m the above lake the (Figure lake (Figure 1). The1 ).nearest The nearest major major anthropogenic anthropogenic source source of at- mosphericof atmospheric pollution pollution is the is city the of city Irkutsk, of Irkutsk, located located 70 km 70 away km awayto the to Northwest the Northwest of Lake of Baikal.Lake Baikal. The location The location of the of station the station on ona hill a hill and and outskirts outskirts of of the the same same name name settlement minimizes the the impact impact of of local local sources sources of of poll pollutionution and and contributes contributes to tothe the monitoring monitoring of regionalof regional and and global global transport transport of atmospheric of atmospheric impurities. impurities. The climate The climate of the ofregion the regionwhere thewhere station the stationis located is located is typical is typical for the for south the south of South-Eastern of South-Eastern Siberia: Siberia: cold coldwinters winters and relativelyand relatively hot summers. hot summers. Over Overthe past the 15 past years, 15 years, the air the temperature air temperature in January in January was − was17.6 ◦ ◦ ±− 4.217.6 °C;± the4.2 averageC; the temperature average temperature in July +19.0 in July± 1.0 +19.0°C, the± average1.0 C, theannual average air temperature annual air ◦ variedtemperature over this varied period over in thisthe range period from in the −0.5 range to +3.0 from °C. −The0.5 amount to +3.0 ofC. atmospheric The amount pre- of cipitationatmospheric is approximately precipitation is 400 approximately mm/year (60–100 400 mm/year mm fall (60–100during the mm cold fall duringhalf of the year cold andhalf approximately of the year and 300–350 approximately mm during 300–350 the warm mm during half). Prevailing the warm wind half). directions: Prevailing north- wind westdirections: (especially north-west in the (especially cold season) in theand cold south-east. season) andAverage south-east. wind Averagespeeds are wind relatively speeds low:are relatively 1.5–2.5 m/s low: (stronger 1.5–2.5 m/sin spring, (stronger weaker in spring, in winter weaker and summer). in winter and summer).

(a) (b)

Figure 1. Listvyanka monitoring station: ( a)) location location within the GMOS network; ( b) aerial view.

InIn the Baikal region, the largest contribution to air pollution is made by emissions of sulfur and nitrogen oxides from thermal powerpower plants of the Irkutsk-AngarskIrkutsk-Angarsk industrial complex (Angarsk, Irkutsk and Shelekhov cities). Annual emissions of SOSO22 and NONO22 (in thousandthousand tons)tons) areare estimatedestimated as as 98 98 and and 54 54 for for Angarsk, Angarsk, 49.5 49.5 and and 19 for19 Irkutsk,for Irkutsk, and and 6.5 and 6.5 and1.7 for 1.7 Shelekhov, for Shelekhov, respectively. respectively. The main The fuelmain for fuel the for electrical the electrical power power plants andplants industrial and in- dustrialboilers is boilers coal from is coal the from Irkutsk the and Irkutsk Buryatia and regions,Buryatia which regions, amounts which toamounts more than to more 90%. than Due 90%.to the Due prevalence to the prevalence of the NW of air the mass NW transfers, air mass the transfers, area of thethe most area probableof the most atmospheric probable atmosphericinﬂuence of theinfluence emission of sourcesthe emission extends source in thes extends direction in of the South direction Baikal, of located South atBaikal, only located70–100 kmat only from 70–100 these sources. km from The these station sources. is a part The of station the EANET is a part network of the EANET (Acid Deposition network (AcidMonitoring Deposition Network Monitoring in East Asia) Network whereby in East numerous Asia) whereby parameters numerous of the parameters air pollution, of wetthe airand pollution, dry deposition, wet and as welldry deposition, as the conditions as well of as the the terrestrial conditions and of aquatic the terrestrial environment and aquaticare measured. environment The station are measured. equipment The carries station out equipment continuous carries automatic out continuous registration auto- of maticthe following registration atmospheric of the following gases with atmospheric a time resolution gases with of a 1–2 time min: resolution SO2 and of H1–22S min: (CB- 320), NO and NO2 (P-310A), CO (K-100), CO2 (OPTOGAS-500)—all manufactured by OPTEC, Russia; O3 (Dylec 1006-AHJ, Japan), and some others (Obolkin et al., 2017). The meteorological characteristics are measured with the Meteo-2M ultrasonic meteorological

station (IAO, SB RAS, Russia). Within the GMOS program, the monitoring at the Listvyanka Atmosphere 2021, 12, x FOR PEER REVIEW 3 of 14

SO2 and H2S (CB-320), NO and NO2 (P-310A), CO (K-100), CO2 (OPTOGAS-500)—all manufactured by OPTEC, Russia; O3 (Dylec 1006-AHJ, Japan), and some others (Obolkin et al., 2017). The meteorological characteristics are measured with the Meteo-2M ultrasonic meteorological station (IAO, SB RAS, Russia). Within the GMOS program, the monitoring at Atmosphere 2021, 12, 807 the Listvyanka site includes gaseous elemental mercury (GEM) measurement3 of 14 (RA- 915AM, Lumex, Russia) and precipitation sampling. In addition to this program, particulate bound mercury (PBM) was measured with a portable multifunctional RA-915M analyzer (Lumex,site includes Russia) gaseous that elemental was also mercury used (GEM) for air measurement survey over (RA-915AM, Lake Baikal. Lumex, Russia) and precipitation sampling. In addition to this program, particulate bound mercury (PBM) 2.2. Stationarywas measured Air Mercury with a portable Monitoring multifunctional RA-915M analyzer (Lumex, Russia) that was also used for air survey over Lake Baikal. The ongoing air mercury monitoring within the GMOS project started on 26 October 2011. Stationary2.2. Stationary air Air measurements Mercury Monitoring were made using a Lumex RA-915AM automated mercury monitor,The ongoingwhich provides air mercury direct monitoring continuous within thebackground GMOS project mercury started (GEM) on 26 October determination in2011. compliance Stationary with air measurementsthe EN standard were me madethod using for athe Lumex determination RA-915AM of automated total gaseous mercury monitor, which provides direct continuous background mercury (GEM) determi- mercurynation (EN in 15852:2010 compliance withAmbient the EN air standard quality) method adopted for the as determinationa SOP for the of GMOS total gaseous monitoring network.mercury The (ENmonitoring 15852:2010 data Ambient with 5 air min quality) averaging adopted were as a SOPacquired for the by GMOS the GMOS monitor- Cyber- (e)-Infrastructureing network. and The stored monitoring in the data GMOS with 5central min averaging databases were [2]. acquired by the GMOS TheCyber-(e)-Infrastructure principle of operation and storedof the in RA-915AM the GMOS central mercury databases monitor [2]. (Figure 2a), as well as that of theThe multifunctional principle of operation RA-915M of the analyzer RA-915AM (Figure mercury 2b), monitor is based (Figure on2 a),the as differential well atomicas absorption that of the multifunctional spectroscopy RA-915Mwith the analyzerZeeman (Figure background2b), is based correction on the differential[3]. atomic absorption spectroscopy with the Zeeman background correction [3].

(a) (b)

Figure 2. MeasurementFigure 2. Measurement technique: technique: (a) The (a )RA-915AM The RA-915AM air airmercury mercury monitor; monitor; ( b)) TheThe RA-915M RA-915M portable portable mercury mercury analyzer. analyzer.

The RA-915AMThe RA-915AM mercury mercury monitor monitor is is designed design fored direct,for direct, long-term, long-term, non-attended non-attended mercury measurement; it has a built-in PC for data acquisition and processing, self-diagnostics, mercuryand measurement; data transfer. Ambient it has a air built-in at the flowratePC for data of ca. acquisition 8 L/min is continuouslyand processing, pumped self-diagnostics,through and data the multipath transfer. cell Ambient of the monitor air at having the flowrate the effective of opticalca. 8 lengthL/min of is 9.6 continuously m. The pumpedreadings through are collected the multipath continuously cell of each the 1 smonitor and are averagedhaving the for reportingeffective at optical any chosen length of 9.6 m.time The interval, readings as aare rule collected for 5 min. continuously The combination each of the1 s multipathand are averaged cell with the for Zeeman reporting at any chosenbackground time correctioninterval, makesas a rule continuous for 5 min. real-time The combination measurement ofof background the multipath mercury cell with concentration in ambient air possible. the Zeeman background correction makes continuous real-time measurement of back- The limit of detection (LoD) is defined as a signal that three times exceeds the standard grounddeviation mercury (SD) concentration of the blank when in ambient ambient air air possible. is sucked through an external mercury ab- Thesorbing limit filter of detection placed at the (LoD) monitor is defined inlet. The as SD a valuesignal was that determined three times as 0.1–0.15 exceeds ng/m the 3stand- ard deviationat 5-min averaging.(SD) of the Thus, blank the when instrumental ambient LoD air is of sucked the monitor through is 0.3–0.5 an external ng/m3 that mercury absorbingcorresponds filter placed to the dataat the obtained monitor during inlet. EN The 15852 SD preparatory value was field determined tests [4]. Further as 0.1–0.15 ng/m3reduction at 5-minof averaging. the LoD can Thus, be achieved the instrumental by increasing LoD the averagingof the monitor interval is (e.g., 0.3–0.5 at 30-min ng/m3 that averaging the LoD is 0.1–0.2 ng/m3). Automatic zero drift correction and auto-calibration corresponds to the data obtained during EN 15852 preparatory field tests [4]. Further re- functions provide stable analytical parameters, operational reliability and safety. duction ofEach the measuringLoD can cyclebe achieved consists of by four increasing steps: the actualthe averaging measurement interval when the (e.g., ambient at 30-min averagingair is the directly LoD entered is 0.1–0.2 to the ng/m analytical3). Automatic cell, the measurementzero drift correction of the zero and level auto-calibration when the functionsambient provide air is passedstable throughanalytical the parameters, built-in mercury-absorbing operational filter,reliability and two and intermediate safety. intervals between these two to replace the ambient air by the “zero air” and vice versa.

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Assuming that the drift is linear between two consecutive zero-level tests, a zero-drift corrected signal is calculated. By default, durations of the actual measurement, the zero- level test, and sum of the two intermediate intervals are 4, 0.5 and 0.5 min respectively. The sensitivity of the spectrometer is automatically (RA-915 AM monitor, once per 6 h by default), or manually (portable RA-915M), checked by installing the sealed quartz test cell with the saturated mercury vapour in the optical beam. No mercury containing devices and operator’s contact with liquid mercury or mercury vapour are required. By concentration obtained from the measured temperature using the Dumarey equation and the width of the test cell, the calculated signal produced by the test cell can be obtained. The calibration coefficient is checked by comparing the response produced by the cell and the calculated signal. Experience showed that the sensitivity was very stable, varying within 5% over years. Main atmospheric components have no spectroscopic effect on the mercury measurement with differential ZAAS: N2,O2, CO2, water vapours have no absorption bands in the near-UV region, some trace gases (e.g., ozone, acetone) have absorption bands in the near range, but due to the Zeeman background correction their effect is negligible. To maintain the 0.5 ng/m3 mercury LoD, the following concentrations of trace gases are tolerable: for 3 3 both of SO2 and H2S it is 10 mg/m , for NO2—100 mg/m . Non-specific for atmosphere volatile organic compounds having vibrational-rotational bands near mercury resonance line of 254 nm (such as benzene, toluene, ethylbenzene, xylenes, and others) also do not produce false response at the allowable level of their concentrations in ambient air. The RA-915AM monitor is convenient for long-term, non-attended operation at remote sites as it does not require argon or any other compressed gases, parts replacement, regular maintenance, and has the autorun function in case of power supply failures (this was the most common reason for the records interruptions at the Listvyanka station). Besides stationary measurements, the monitor is used for other applications, such as continuous air surveys when being mounted on an automobile, boat, or aircraft [5], and mercury flux measurements [6,7].

2.3. Air Mercury Survey over Baikal The portable RA-915M analyzer (Figure2b) is applicable for real-time air mercury surveys on board of moving vehicles (car, helicopter, boat). The air mercury survey during the cruise of the research vessel “Akademik Koptyug” over Lake Baikal in July 2018 was made with two portable mercury analyzers RA-915M operated concurrently. Data were collected continuously with the response time of 1 s, averaging over 4 min, and zero control every 5 min (Figure3). Limit of detection was deﬁned, as described above, as 0.5 ng/m 3 at 5-min averaging. Throughout the cruise, the calibration coefﬁcients for both analyzers were stable within 3%. During concurrent measurements, the difference in the averaged readings between the analyzers did not exceed 10%.

2.4. Determination of Particulate Bound Mercury Within the GMOS project, no mercury speciation measurement is required at the secondary stations. Nevertheless, during February 2015–November 2017, 40 air samples were collected for the particulate bound mercury (PBM) determination. Several types of aerosol filters were tested to choose those with the minimum blank value. The PolyFlon PF060 filters, with a pore size of 6 µm (Advantec, Japan), showed the minimal blank value of 0.2 ng on the average as compared with the other filters. The multifunctional RA-915M mercury analyzer with a PYRO-915+ attachment was used for determination of the PBM concentration by direct pyrolysis of the whole filters (Figure4). Mean air volume for PBM sampling was 300 m 3, with the air flow rate of approximately 70 L/min. For such sampling conditions, the LoD value was determined as 1 pg/m3. Atmosphere 2021, 12, x FOR PEER REVIEW 5 of 14

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(a)

(b) Figure 3. An example of concurrent measurements with two RA-915M analyzers during Baikal cruise. Record of 23.07.2018: (a) Analyzer № 1865; (b) Analyzer № 1621. Dots: zero(a control.)

2.4. Determination of Particulate Bound Mercury Within the GMOS project, no mercury speciation measurement is required at the secondary stations. Nevertheless, during February 2015–November 2017, 40 air samples were collected for the particulate bound mercury (PBM) determination. Several types of aerosol filters were tested to choose those with the minimum blank value. The PolyFlon PF060 filters, with a pore size of 6 µm (Advantec, Japan), showed the minimal blank value of 0.2 ng on the average as compared with the other filters. The multifunctional RA-915M mercury analyzer with a PYRO-915+ attachment was used for determination of the PBM(b) concentration by direct pyrolysis of the whole filters (Figure 4). Mean air volume for PBM sampling was 300 m3, with the air flow rate of ap- FigureFigure 3. An 3. Anexample example of of concurrent measurements measurements with with two RA-915M two RA analyzers-915M analyzers during Baikal during cruise. Baikal Record cruise. of 23.07.2018: Record of proximately 70 L/min. For such sampling conditions, the LoD value was determined as 1 23.07.2018:(a) Analyzer (a) Analyzer№ 1865; № (b) 1865; Analyzer (b) №Analyzer1621. Dots: № 1621. zero control. Dots: zero control. pg/m3. 2.4. Determination of Particulate Bound Mercury Within the GMOS project, no mercury speciation measurement is required at the secondary stations. Nevertheless, during February 2015–November 2017, 40 air samples were collected for the particulate bound mercury (PBM) determination. Several types of aerosol filters were tested to choose those with the minimum blank value. The PolyFlon PF060 filters, with a pore size of 6 µm (Advantec, Japan), showed the minimal blank value of 0.2 ng on the average as compared with the other filters.

The multifunctional RA-915M mercury analyzer with a PYRO-915+ attachment was (a) used for determination of the PBM concentratio(b) n by direct pyrolysis of the whole filters FigureFigure 4. Direct 4. Direct analysis analysis of the(Figure of whole the whole aerosol4). Mean aerosol filters: air ﬁlters: (a volume) The (a set) The offor the set PBM RA-915M of the sampling RA-915M analyzer analyzerwas and 300PYRO-915+ and m3 PYRO-915+, with attachment; the attachment;air (flowb) rate of ap- 3 The (analysesb) The analyses of two filters of two exposed ﬁltersproximately exposed in parallel. in 70 parallel. The L/min. measured The For measured suchPBM concen sampling PBMtration concentration co isnditions, (1) 17.1 is (1)and 17.1the (2) and15.9LoD (2)pg/m value 15.9. pg/m was3 .determined as 1 pg/m3. 3. Air Mercury Monitoring at the Listvyanka Station This section covers stationary air mercury measurement at the Listvyanka site from November 2011 to December 2020.

3.1. Seasonal Variations The obtained data show obvious seasonal variation of the background mercury concentration in air, increasing in the cold seasons (November–February) with the monthly average of 1.56–1.95 ng/m3, and decreasing in the warm seasons (June–September) with 3 the monthly average of 1.12–1.63 ng/m (Figure5):

(a) (b) Figure 4. Direct analysis of the whole aerosol filters: (a) The set of the RA-915M analyzer and PYRO-915+ attachment; (b) The analyses of two filters exposed in parallel. The measured PBM concentration is (1) 17.1 and (2) 15.9 pg/m3.

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3. Air Mercury Monitoring at the Listvyanka Station This section covers stationary air mercury measurement at the Listvyanka site from November 2011 to December 2020.

3.1. Seasonal Variations The obtained data show obvious seasonal variation of the background mercury con- Atmosphere 2021, 12, 807 centration in air, increasing in the cold seasons (November–February) with6 of the 14 monthly average of 1.56–1.95 ng/m3, and decreasing in the warm seasons (June–September) with the monthly average of 1.12–1.63 ng/m3 (Figure 5):

Figure 5. Listvyanka:Figure 5. Listvyanka: seasonal seasonal variation variation of the Hg of the (GEM) Hg (GEM) concentration concentration through through November November 2011–December 2011–December 2020, 2020, monthly average. monthly average.

The average mercury background concentration (Cav) for the overall reporting period ofThe November average 2011–December mercury background 2020 is 1.59 concentrat ng/m3. Forion cold (Cav) seasons, for Cav the has overall a value reporting of period 1.75of November ng/m3 that is2011–December higher as compared 2020 with is Cav1.59 obtained ng/m3. forFor warm cold seasons: seasons, 1.44 Cav ng/m has3. a value of 1.75 Theng/m difference3 that is between higher the as mercury compared concentrations with Cav in theobtained cold and for warm warm seasons seasons: amounts 1.44 ng/m3. 3 The todifference 0.31 ng/m between(Table1). the mercury concentrations in the cold and warm seasons 3 Table 1. Monthly averageamounts air mercury to 0.31 concentration ng/m (Table at the 1). Listvyanka station for cold and warm seasons (ng/m3).

2011– 2012– 2013– 2014– 2015– 2016– 2017– 2018– 2019– Seasonal Table 1.Season Monthly average air mercury concentration at the Listvyanka station for cold and warm seasons (ng/m3). 2012 2013 2014 2015 2016 2017 2018 2019 2020 Average Cold 2014– 2015– 2016– 2017– 2018– Seasonal Season(November– 2011–20121.89 2012–2013 1.67 2013–2014 1.70 1.71 1.70 1.56 1.95 1.84 1.72 2019–20201.75 February) 2015 2016 2017 2018 2019 Average Cold (Novem-Warm (June– 1.89 1.421.67 1.37 1.501.70 1.511.71 1.311.70 1.121.56 1.531.95 1.631.84 1.55 1.721.44 1.75 ber–February)September) Warm (June–Sep- 1.42 1.37The variability1.50 of the1.51 mercury 1.31 concentration 1.12 is also1.53 higher in1.63 cold months1.55 due to 1.44 tember) elevated emissions from coal-ﬁred power plants. The behavior similar to the GEM seasonal variation is observed for the particulate boundThe variability mercury (PBM), of the which mercury was beingconcentration measured fromis also January higher 2016 in tocold March months 2017 due to elevated(Figure emissions6): from coal-fired power plants. The behavior similar to the GEM seasonal variation is observed for the particulate bound mercury (PBM), which was being measured from January 2016 to March 2017 (Fig- ure 6):

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FigureFigure 6.6.Listvyanka: Listvyanka: Seasonal Seasonal variations variations of the GEM of and the PBM GEM during and 20 JanuaryPBM during 2016–20 March 20 January 2017, monthly 2016–20 average. March 2017, monthly average. During this period, the GEM concentration averaged at 1.33 ng/m3, whereas the PBM concentration was 11 pg/m3, which is approximately 0.8% of the former one. Seasonal During this period,variations the GEM of both concentration GEM and PBM species averaged are obviously at 1.33 explainedng/m3, whereas by elevating the mercury PBM emissions from coal combustion during the cold season, giving rise to background mercury 3 concentration was 11concentration pg/m , which at aregional is approximately and global level. 0.8% An additionalof the former evidence one. of mercury Seasonal emissions variations of both GEM andfrom PBM coal-ﬁred species power are plants obvi is revealedously explained from an analysis by elevating of the short-term mercury air mercury emissions from coal combustionvariations (seeduring Section the 3.3 ).cold season, giving rise to background mercury concentration at a regional3.2. Diurnal and Variations global level. An additional evidence of mercury emissions from coal-fired power plantsData processing is revealed reveals from a moderate, an analysis statistically of the signiﬁcant, short-term diurnal air cycle mercury of the variations (see Sectionmercury 3.3). concentration both in the warm and cold seasons. Minimum concentration is observed at 1–6 a.m. local time, and then the concentration increases by 0.05 ng/m3 during daytime (Figure7). 3.2. Diurnal Variations Mercury concentration and its variability are higher in the daytime. The increase in the concentration in the daytime may be attributed to the elevation of the ambient temperature Data processingand reveals intensity a moderate, of the surface statistically and anthropogenic significant, emissions. diurnal The corresponding cycle of the increase mer- in cury concentration boththe concentration in the warm variability and isco explainedld seasons. by the airMinimum turbulence enhancement.concentration is observed at 1–6 a.m. local time, and then the concentration increases by 0.05 ng/m3 during daytime (Figure 7).

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FigureFigure 7. Diurnal 7. Diurnal behavior behavior of the mercury of the concentrationmercury concentration (local time, UTC (local +8). ‘Winter’time, UTC (1 December +8). ‘Winter’ 2013–28 (1 February December 2014 and2013–28 1 December February 2014–28 2014 February and 2015);1 December each point 2014–28 is the average February over 1702015); h (2040 each individual point is 5-min the average readings). over ‘Overall 170 data’ (1 April 2013–1 April 2015); each point is the average over 650 h (7800 individual 5-min readings). ‘Summer’ (1 July–30 h (2040 individual 5-min readings). ‘Overall data’ (1 April 2013–1 April 2015); each point is the av- September 2013 and July–30 September 2014); each point is the average over 150 h (1800 individual 5-min readings). erage over 650 h (7800 individual 5-min readings). ‘Summer’ (1 July–30 September 2013 and July– 30 September 2014); each3.3. Short-Term point is the Variations average over 150 h (1800 individual 5-min readings). The short-term (minutes–hours) anomalies of the mercury concentration can reach Mercury concentrationthe value up and to 5–7, its sometimesvariability up are to 15–20higher ng/m in 3the. There daytime. are no significant The increase local sourcesin the concentration inof the mercury daytime emissions may in be the attributed rural surroundings to the elevation of the Listvyanka of the site. ambient These short-termtem- perature and intensityanomalies of the are surface traced to and the mercury anthropogenic air transfer emission from regionals. The sources corresponding of emissions, first increase in the concentrationof all, from coal-firedvariability power is explained plants located by inthe Irkutsk, air turbulence Angarsk, and enhancement. Shelekhov cities 70 and 100 km away from the monitoring points. The long-range transport of acid gases with plumes from regional coal-fired power plants to the South Baikal area was proven by 3.3. Short-Term Variationsthe monitoring data at the Listvyanka site and backward trajectories modelling [8,9]. The The short-termmercury (minutes–hours) observation within anomalies the GMOS of programthe mercury shows positiveconcentration correlation can of thereach mercury the value up to 5–7,peaks sometimes with the localup to anomalies 15–20 ng/m of the acid3. There gases are typical no forsignificant the coal combustion local sources emissions, such as SO2, and NO2 (Figure8). of mercury emissions inThe the initial rural composition surroundings of stack of gas the changes Listvyanka during atmosphericsite. These transport short-term from the anomalies are tracedpoint to sourcethe mercury of emissions air duetransfer to gas from phase regional reactions [10sources,11]. Atmospheric of emissions, ozone first plays an of all, from coal-firedimportant power role plants in NO located oxidation in within Irkutsk, plumes. Angarsk, The dominating and Shelekhov reaction: cities 70 and 100 km away from the monitoring points. The long-range transport of acid gases with NO + O3 → NO2 + O2 (1) plumes from regional coal-fired power plants to the South Baikal area was proven by the monitoring data at isthe faster Listvyanka as compared site to and the ba mixingckward rate thattrajectories leads to themodelling ozone deficit [8,9]. within The themer- plume, cury observation withinrelative the to the GMOS background program air [11 shows]. Such reactionpositive leads correlation to the negative of the correlation mercury of the mercury and ozone concentrations (Figure9). peaks with the local anomalies of the acid gases typical for the coal combustion emissions, such as SO2, and NO2 (Figure 8).

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(a)

(b) (c)

Figure 8. Measurement on 17.10.2020, 5-min average (a). Correlation between: (b) Hg and SO2, (c) Hg and NO2. Figure 8. Measurement on 17.10.2020, 5-min average (a). Correlation between: (b) Hg and SO2,(c) Hg and NO2.

TheAs itinitial was showncomposition earlier of [ 8stack], the gas anomalies changes ofduring acid gasesatmospheric combined transport with the from ozone the pointdepletion source are of well emissions explained due byto thegas meteorologicalphase reactions phenomenon [10,11]. Atmospheric known asozone “low-level plays an jet importantstreams”: role thelong-distance in NO oxidation plume with transferin plumes. from The remote dominating sources reaction: of emissions in Angarsk, Irkutsk, and Shelekhov cities to South Baikal. To assess the relationship between mercury NO + O3 → NO2 + O2 (1) and acid gases anomalies registered at the Listvyanka site and the largest sources of isindustrial faster as emissions,compared weto the used mixing the HYSPLIT rate that (Hybrid leads to Single-Particle the ozone deficit Lagrangian within the Integrated plume, relativeTrajectory) to the model background [12], with air air [11]. mass Such trajectories reaction calculationleads to the at negati the threeve correlation vertical levels of the of mercury50, 150, andand 500ozone m. concentrations (Figure 9). An example of the long-distance air pollutants transfer with low-level jet streams is shown in Figure 10.

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(a) (b) Figure 9. An example of the negative mercury and ozone correlation: (a) Data of 16–20 January 2014, 5-min average; (b) Ozone vs. mercury concentration.

As it was shown earlier [8], the anomalies of acid gases combined with the ozone depletion are well explained by the meteorological phenomenon known as “low-level jet streams”: the long-distance plume transfer from remote sources of emissions in Angarsk, Irkutsk, and Shelekhov cities to South Baikal. To assess the relationship between mercury and acid gases anomalies registered at the Listvyanka site and the largest sources of industrial emissions, we used the HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory)(a) model [12], with air mass trajectories calculation(b at) the three vertical levels of 50, 150, and 500 m. Figure 9. AnAn example example of of the the negative negative mercury mercury and and ozone ozone correlation: correlation: (a) (Dataa) Data of 16–20 of 16–20 January January 2014, 2014, 5-min 5-min average; average; (b) An example of the long-distance air pollutants transfer with low-level jet streams is Ozone(b) Ozone vs. mercury vs. mercury concentration. concentration. shown in Figure 10. As it was shown earlier [8], the anomalies of acid gases combined with the ozone depletion are well explained by the meteorological phenomenon known as “low-level jet streams”: the long-distance plume transfer from remote sources of emissions in Angarsk, Irkutsk, and Shelekhov cities to South Baikal. To assess the relationship between mercury and acid gases anomalies registered at the Listvyanka site and the largest sources of industrial emissions, we used the HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory) model [12], with air mass trajectories calculation at the three vertical levels of 50, 150, and 500 m. An example of the long-distance air pollutants transfer with low-level jet streams is shown in Figure 10.

(a)

Figure 10. Cont.

(a)

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(b) (c)

FigureFigure 10.10. EvidenceEvidence of of the the SO SO2 and2 and Hg Hg long-distance long-distance plumes plumes transf transferer with with low-level low-level jet streams: jet streams: (a) Simultaneous (a) Simultaneous record of the SO2 and Hg concentrations; the forward air trajectories modelling: (b) At UTC 11:00 plumes pass the Listvyanka record of the SO2 and Hg concentrations; the forward air trajectories modelling: (b) At UTC 11:00 plumes pass the Listvyankamonitoring monitoring point; (c) At point; UTC (23:00c) At UTCa superposition 23:00 a superposition of the plumes of thefrom plumes Angarsk, from Irkutsk Angarsk, and Irkutsk Shelekhov and at Shelekhov the monitoring at the point. monitoring point.

The correlated maxima of the SO2 and mercury concentrations at the Listvyanka The correlated maxima of the SO2 and mercury concentrations at the Listvyanka monitoringmonitoring sitesite (Figure(Figure 1010)) areare observedobserved duringduring superpositionsuperposition ofof airair massesmasses carryingcarrying plumesplumes ofof coal-ﬁredcoal-fired powerpower plants plants from from Angarsk, Angarsk, Irkutsk Irkutsk and and Shelekhov Shelekhov cities. cities.

4.4. MercuryMercury inin thethe AirAir overover LakeLake BaikalBaikal BaikalBaikal isis thethe largestlargest freshwaterfreshwater lakelake byby volumevolume containingcontaining approximatelyapproximately 22–23%22–23% ofof thethe world’sworld’s fresh water water reserves. reserves. The The largest largest le lengthngth and and width width of ofthe the lake lake are are636 636and and79.5 79.5km, km,respectively. respectively. Baikal Baikal is the is world’s the world’s deepest deepest lake lake with with a maximum a maximum depth depth of 1642 of 1642 m. The m. Thelake lake is fed is by fed more by more than than300 inflowing 300 inflowing rivers rivers and is and drained is drained through through a single a outlet single located outlet locatednear Listvyanka near Listvyanka village, village,called the called Angara the Angara River. River. ToTo date,date, therethere is is no no datadata on on thethe airair mercurymercury distribution distribution over over the the vast vast Baikal Baikal basin. basin. InIn JulyJuly 2018,2018, forfor thethe firstfirst time,time, thethe airair mercurymercury surveysurvey hadhad beenbeen carriedcarried outout throughoutthroughout allall thethe BaikalBaikal areaarea duringduring thethe cruisecruise ofof the the research research vessel vessel (RV) (RV) “Akademik “Akademik Koptyug”. Koptyug”. TheThe continuouscontinuous airair mercurymercury surveysurvey overover LakeLake BaikalBaikal covered covered 1800 1800 km. km. DuringDuring thethe cruise,cruise, nono significantsignificant anomaliesanomalies ofof thethe mercurymercury concentrationconcentration inin thethe airair aboveabove thethe lake lake were were found. found. The The potential potential sources sources of theof the elevated elevated mercury mercury concentration concentration up 3 toup 3–5 to 3–5 ng/m ng/m(see3 (see Figure Figure3) can3) can be be explained explained by by the the long-distance long-distance mercury mercury transfer transfer from from thethe coal-firedcoal-fired powerpower plantsplants ofof industrial industrial cities cities (Irkutsk, (Irkutsk, Angarsk, Angarsk, Shelekhov), Shelekhov), describeddescribed above,above, andand thethe emissionsemissions sourcessources ofof townstowns locatedlocated along along the the lake lake shore shore (Figure (Figure 11 11).).

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FigureFigure 11.11. LocalLocal mercurymercury anomaliesanomalies inin thethe airair overover LakeLake Baikal.Baikal. CruiseCruise ofof thethe RVRV“Akademik “AkademikKoptyug”, Koptyug”, July July 2018. 2018.

LakeLake BaikalBaikal isis locatedlocated withinwithin thethe seismicallyseismically activeactive BaikalBaikal RiftRift Zone.Zone. NumerousNumerous hothot springssprings andand oiloil andand gasgas seepagesseepages bothboth onon landland andand atat lakelake bottombottom areare present.present. MercuryMercury degassingdegassing throughthrough deep deep fault fault zones zones could could be be potential potential natural natural sources sources of mercury of mercury emissions. emis- However,sions. However, we did we not did ﬁnd not unequivocal find unequivocal evidence evidence of the of connection the connection of the of airthe mercury air mer- anomaliescury anomalies with tectonicwith tectonic structures. structures. The mercury The mercury concentration concentration over the over gas the hydrate gas hydrate pools andpools oil and seepages oil seepages to the laketo the surface, lake surface, up to 1.6up ng/mto 1.6 3ng/m, is slightly3, is slightly elevated elevated as compared as compared with thewith local the backgroundlocal background level oflevel 1.0–1.1 of 1.0–1.1 ng/m ng/m3. 3. TheThe averageaverage mercurymercury concentration concentration of of 1.10 1.10 ng/m ng/m33 recorded overover BaikalBaikal isis notablynotably lessless inin comparisoncomparison withwith thethe averageaverage value value of of 1.60 1.60 ng/m ng/m33 obtainedobtained atat thethe onshoreonshore ListvyankaListvyanka monitoringmonitoring stationstation duringduring thethe samesame daysdays ofof thethe cruise.cruise. TheThe explanationexplanation ofof thisthis phenomenonphenomenon maymay bebe inin thethe speciﬁcspecific airair circulationcirculation overover thethe BaikalBaikal waterwater areaarea inin summersummer time.time. DueDue toto thethe hugehuge massmass ofof coldcold waterwater (with(with thethe averageaverage ◦ ◦ temperaturetemperature ofof aboutabout 44 °CC andand surfacesurface temperaturetemperature inin summersummer ofof 10–1210–12 °C),C), anan invertedinverted layerlayer isis formedformed inin thethe atmosphereatmosphere aboveabove thethe lakelake surface,surface, whichwhich preventsprevents thethe mixingmixing ofof thethe air above above the the surface surface with with the the air air masses masses coming coming from from the thecontinent. continent. In June–July, In June–July, over over most of the lake area, the process of condensation of air moisture over the cold water

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surface occurs over most of the lake area. This leads to the enhanced polluting gaseous and aerosol impurities deposition enhancement. Under such conditions, Baikal can be considered as a sink of atmospheric mercury.

5. Conclusions According to the long-term air mercury monitoring at the Listvyanka station, the following conclusions are drawn: The average total (GEM) concentration is 1.59 ng/m3 throughout the 2011–2020 years of observation. The particulate bound mercury (PBM) makes up approximately 0.8% of GEM. The daily average concentration of GEM varies within a range of 1.44 to 1.75 ng/m3 and that of PBM in a range of 7.8–15 pg/m3 in the warm and cold seasons, respectively, which is evidence of the elevated mercury emissions from coal combustion during the cold season. Statistical data processing shows the moderate diurnal cycling of the mercury concentration at a lower level at night and higher level at daytime. The space-time distribution and short-term variations of the mercury concentration indicate the long-distance (70–100 km) mercury transfer from the industrial sites where the coal-ﬁred power plants are located. The coal combustion plants are the main sources of the elevated mercury concentration at the Listvyanka site, which is conﬁrmed by the evident correlation between the average Hg and SO2, NO2, and O3 concentrations. The average mercury concentration measured during the cruise above Lake Baikal is 1.10 ng/m3, which is notably less as compared with the average value of 1.60 ng/m3 obtained at the onshore Listvyanka GMOS station during the same days of the cruise. Thus, Baikal can be a sink of the atmospheric mercury due to the air temperature inversion in the warm season. The possibility of using the data of air mercury monitoring at stationary points and route surveys demonstrates feasible revealing of mercury emission sources.

Author Contributions: Conceptualization, N.M. and T.K.; methodology, N.M., V.O., V.R. and S.P.; software, S.S. and E.M.; formal analysis, S.S. and V.O.; investigation, V.O., N.M., S.P., V.P., data curation, S.S., V.O. and N.M.; writing—original draft preparation, N.M., V.O., S.S., S.P. and V.R. and; visualization, N.M., V.O., V.R., S.S., V.P. and E.M.; supervision, N.M. and T.K.; funding acquisition, N.M. and T.K. All authors have read and agreed to the published version of the manuscript. Funding: This study was carried out in accordance with project “Global Mercury Observation System” FP7, grant Agreement No 265113. The cruise monitoring of 2018 was supported by the Russian Foundation for Basic Research, grant No 17-29-05044. The results obtained in 2020 were supported by the Ministry of Science and Higher Education of the Russian Federation (No 075-15- 2020-787) for implementation of the large scientific project “Fundamentals, methods and technologies for digital monitoring and forecasting of the environmental situation on the Baikal natural territory”. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data supporting reported results can be found at https://sdi.iia.cnr.it/ gmos/ (accessed on 21 June 2021). Conflicts of Interest: The authors declare no conflict of interest.

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