Features of the fluoride behavior in the snow cover under the action of technological and weather conditions

Introduction

It is a common knowledge that snow cover is an effective accumulator of gaseous and aerosol substances falling from the atmospheric air [1, 2]; therefore, this is of interest to study the behavior of fluoride in the snow cover. The companies producing primary aluminum are predominating in terms of emissions of fluorides among industrial sources of emissions of fluoride compounds into the atmosphere [3], [4], [5], [6]. Industrial F pollution are present in the atmospheric air in the form of gases and aerosols [3], [4], [5], [6] in the areas of these companies. In certain cases, concentrations of fluoride compounds are high in the air near industrial sources; therefore, the study of the behavior of fluoride in interfacing media, such as snow cover, is relevant.

According to the terminology of the documents [7], the atmospheric air is tested for the presence of hydrogen fluoride and soluble fluorine compounds that comprise aerosols. The publication [4] and state report [8] note that hydrogen fluoride and poorly soluble fluorides are determined in the atmospheric air. The atmospheric aerosols composition contains fluorine compounds that are water soluble (water-soluble fluorides) and fluorine compounds that are not water soluble (poorly soluble compounds). Probably, such concepts are important for assessing the effect of aerosols on humans, water-soluble aerosols are more dangerous.

Canada and China [5, 6] use the terminology “gaseous and particulate fluorides.” Volcanoes are a natural source of emissions of fluorine compounds in the atmosphere [9, 10].

Meng-Dawn Cheng [3] writes that there is a big amount of monitoring data and computer simulations for the global transfer of HF, but the study of HF in the troposphere is still limited to an industry, which primary interest is focused on evaluating the safety and risk of HF release as it is a toxic gas.

It is estimated [6] that in 2019 the global production of primary aluminum was a little above 64.4 million tons. China was the world’s top producer in 2019, followed, in the order of decrease in the production of primary aluminum, by India, Russia, Canada, UAE, Australia, Bahrain, Norway, USA, Iceland and other countries.

Only few primary aluminum producing countries have stable snow cover, these are Russia, Canada, Norway, and Iceland. In some countries, for example, in Norway, Iceland and the UAE, aluminum smelters are located on the seashore, and emissions are treated and directed towards the sea after the treatment. The environment of the smelters located in the northern countries is marine or forest areas, which in different ways contribute to the post-treatment of air from emissions.

Aluminum smelters in the Russian Federation are located in inland areas, where stable snow cover persists for several months. The smelters are located in Siberia in the following cities and towns: Bratsk, Shelekhov, Sayanogorsk and Krasnoyarsk. Irkutsk aluminum smelter is located in the city of Shelekhov, at a distance of about 80–90 km from Lake Baikal. This work does not discuss whether the emissions from the aluminum smelters could reach Lake Baikal or not. The mineralization of natural surface waters and snow cover in Siberia is low. Ecosystems in the vicinity of Siberian aluminum smelters function in the conditions of low mineralization of natural waters; as a result, Siberian ecosystems are very sensitive to emissions.

The interest in the chemistry of F content in the snow cover is mainly associated with the use that information to assess the risk of polluting industrial facilities. As “the main mechanism of removal of HF in the atmosphere is deposition (including wet deposition processes) on the underlying surfaces, industrial and natural surfaces, including the snow cover” [3].

“The wet deposition involves water such as mist, rain, fog, dew, and cloud droplets and typically resulting in acidic precipitation when HF is absorbed in atmospheric moisture. The dry deposition refers to the process that removes HF from the air without the involvement of atmospheric moisture. Both dry and wet deposition processes decrease the concentration of HF in the air, and again are the two most likely mechanisms to eliminate atmospheric HF. Atmospheric HF removal could also involve uptake by solid phase materials commonly found in the environment such as particulate matter, dust, ice crystals, and so on that are suspended in the air” [3].

As a small number of the world’s aluminum smelters are located on the territory of forest ecosystems in cold regions, there are no publications about the behavior of fluoride in the snow cover when exposed to natural factors.

There is no information in the scientific literature about the mechanisms of fluoride conversion at the boundary between the atmosphere and snow cover, as well as at the boundary between the snow cover and soil under variations of the air temperature and snow cover. As such data is given on the change in the chemical characteristics of the snow cover under conditions of varying physical characteristics of the snow cover.

The work of J. W. Pomeroy et al. [11] discusses whether there are losses of some ions, mainly NO3, during snow sublimation. Beinea H. J., Domine F. and Simpson W. study the assumption of the behavior of nitrate ions in the snow cover during melting and consider the mechanism of active nitrogen release from the snowpack [12]. S. A. Markova and V. N. Makarov report that they were considering the results of simultaneous monitoring of the physical and chemical characteristics of snow cover in the spring of 2016 [13]. Researchers Koichi Watanabe, Yukiko Saito, Syoko Tamura [14] point out that the study of snowpack on a mountain in Japan is associated with anthropogenic emissions in Eastern Asia and industrial emissions in Japan. The authors write that a high-altitude observation of a snow pit is useful for evaluation of air quality in free troposphere during the cold months. It was observed that in layers of snow that fell in spring, pH of snow samples was usually higher as compared to winter snow samples [14]. Yevgen Nazarenko, Uday Kurien et al. [15] report that little is known about how negative ambient temperatures and the presence of snow affect emission gas particles and the behavior of atmospheric organic pollutants.

T. A. Kotenko [9] points out that the impact of physical factors of the environment change causes degassing of the snow cover [9] and this can serve as an additional factor of the atmospheric air pollution. He considers air condition and composition of the snow cover in the city of Severo-Kurilsk after the eruption of Ebeko volcano. He writes that “a faint odor of hydrogen sulfide was frequently present during strong snowstorms, when the wind speed was 13–14 m/s, and blasts reached 30–32 m/s. These facts are apparently associated with the process of deflation of the snow cover. As a result of deflation, gases previously captured and preserved in the thickness of snow were released. Hydrogen sulfide has a specific smelled odor and is organoleptically recognized by people yet when maximum concentration limit is double [9]. Hydrogen fluoride is an ingredient of fumarole gases and fluorides are present in the snow cover in February–March 2005 (0.1; 0.15; 0.08; 0.07 mg/l) in Severo-Kurilsk [9]. Hydrogen fluoride has a specific feature, particularly, hydrogen fluoride is an odorless gas. Instrumental gauging is required only to determine it in the atmosphere.

The purpose of this study is to establish correlations and/or trends between F content in the snow cover and industrial and weather conditions in the area susceptible to aluminum production emissions.

The field and subject of study

One of the world’s largest aluminum smelters with the production rate of approximately 1 million tons of aluminum per year is located in the city of Bratsk (51°31′N 104°08′E). There are other industrial emission sources in the city, which are relatively close to each other, and therefore the study area is the entire zone of emissions from industrial undertakings in Bratsk, however, only local factors (wind direction, etc.) were taken into account. Thus, in 2014, 1848.309 tons of solid fluorides and 1279.25 tons of hydrogen fluoride were released into the atmospheric air [8 p. 174]. In 2019, Bratsk is included into the list of cities with the highest level of air pollution, including HF [16].

The climate in Bratsk is distinctly continental. On average, 369 mm of precipitation falls in Bratsk annually, 25 % of which occur during the cold season and 75 % during the warm season; stable snow cover forms approximately on October 29 and melts approximately on April 14 in Bratsk [17], the duration of stable snow cover is 166 days on average and the height of the snow cover is 30–37 cm. Monitoring of the snow cover and atmospheric air pollution is carried out by Bratsk Center for Hydrometeorology and Environmental Monitoring in the first 10 days of March, at 11 observation points.

The snow cover in Bratsk contains insoluble fluorides, which are contained in solid sediment of snow melt. This sediment is generated on the filter after snow melt filtering. F are also present in filtrated snowmelt, the term “water-soluble fluorides” is used for their reference [18], in this article we will discuss fluoride concentration in snowmelt and will write “F concentration in the snow cover”. It is obvious that there are several sources of accumulation of fluoride in the snow cover, as was said earlier, fluoride is formed by dry and wet deposition. According to [18] “the analysis of the snow cover for a five-year period (2016–2020) in the area of Bratsk, there are significant fluctuations in the density of precipitation of water-soluble fluorine compounds, both in the baseline area (1.2–11.09 kg/km²*month), and at sample sites (3.0–23.4 kg/km²*month) in the area affected by emissions from the aluminum smelter.”

The Bratsk Center for Hydrometeorology and Environmental Monitoring (BCHMEM) of the Federal Service for Hydrometeorology and Environmental Monitoring of Russia is in charge of this monitoring, and it is assumed that the Bratsk reference area is Listvyanka village, on the shore of Lake Baikal, which is 600–700 km away from Bratsk.

The research subjects (weather factors) in the city of Bratsk: air temperature, amount of precipitation, number of hours with temperatures above zero during the period of stable snow cover, concentration of hydrogen fluoride in the air, range of air transport of emissions and F content in filtrated snowmelt.

The article discusses the results of analytical measurement of the fluoride ions in filtrated snowmelt of the snow cover. But the article uses the short terminology “F concentration in the snow cover”.

Research methods

Research methods: interpretation of data from the website https://rp5.ru [19], historical data from the Bratsk Center for Hydrometeorology and Environmental Monitoring (BCHMEM) and the library of the Federal State Budgetary Institution “Irkutsk Department for Hydrometeorology and Environmental Monitoring”. Many results are shown in yearbooks [7, 8, 16, 18, 20].

The results of F content monitoring in the snow cover are shown in various publications, such as: yearbooks “Soil Contamination in the Russian Federation by Toxicants of Industrial Origin in 2020” [18].

“The state of soil pollution in the Irkutsk region by toxicants of industrial origin in 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007” [20]; State reports “On the State and Environmental Protection of the Irkutsk Region” [8]; yearbooks “Air Pollution Status in the cities in Russia for 2019” [16]; yearbooks State reports “On the state and protection of the Environment of the Russian Federation in 2018” [21].

Summary and discussions

It is known that fluorine compounds in the atmospheric air in the zone of impact of emissions from aluminum smelters are in two forms: gaseous fluorine compounds and solid particles of fluorine compounds [4], [5], [6], [7], [8]. Gaseous fluorine compounds and solid particles of fluorine compounds may be soluble or insoluble in atmospheric moisture and in the melt water of the snow cover. In this paper, we discuss only the behavior of fluoride concentration (F content) in the snowmelt of the snow cover.

A correlation has been established between the content of hydrogen fluoride in the air and the density of atmospheric fallout of F content in the snow cover. The interpretation of long-term observations of Bratsk Center for Hydrometeorology and Environmental Monitoring (2000–2014) shows that the snow cover reflects the content of HF in the atmospheric air, and this is evidenced by the coefficient of correlation established by us between the content of HF in the atmospheric air (mg/m³) and the intensity F precipitation in the snow cover (kg/km²•month). The correlation coefficient R = 0.57 was established for the results obtained in the near zone of emissions from the aluminum smelter [22].

The trend of change in the amount of atmospheric precipitation during the period of stable snow cover in the city of Bratsk. The “Assessment Report on Climate Change and Its Consequences in the Russian Federation” [23] states that “in general, over the territory of Russia and in its individual regions, there is a slight increase in the average annual precipitation, which is most noticeable in Western and Central Siberia. It is mentioned that the trend of average annual precipitation per month in 1976–2006 on average in Russia is 0.6 mm per month for 10 years, although it describes only 12.7 % of interannual variability”.

We compared the amount of winter precipitation 1961–2014 (November, December, January, February, March) with precipitation of the climate norm (1961–1990) in Bratsk. The climatic standard normal is usually determined over a 30-year period, in this case from 1961 through 1990.

The dynamics of atmospheric precipitation was assessed using the graphs of anomalies in the amount of precipitation per month (XI-III) in 1961–2014 in reference to the average value obtained for the period taken as the climatic norm (1961–1990). The linear trend equation for the 11-year moving average is obtained. It is found that the average rate of increase in precipitation in Bratsk during the period of stable snow cover is 0.7 mm/10 years [24].

The correlation between the amount of snow precipitation fallen during formation of stable snow cover to the date of sampling and F concentration in the snow cover. For example, in 1965/66 from XI-III (November–March) only 135 mm fell, and in 1981/82 XI-III (November–March) – 49 mm. Of course, there is a reason to assume that such amount of precipitation will affect F concentration in the snow cover, and when comparing concentrations, it is advisable to take into account the amount of atmospheric precipitation or point to this circumstance when comparing the concentrations. In 2000–2014, F concentration in the snow cover varied in the range from 0.5 ml/l to 6 mg/L (for one sampling point). While the amount of atmospheric precipitation was in the range from 60 to 110 mm, but mainly 85–105 mm. Using the data for these 15 years (2000–2014), we established a trend towards decrease of F concentration in the snow cover with the increase in precipitation fallen during the period starting from the formation of a stable snow cover to the date of sampling in March 2000–2014 in Bratsk. In the near industrial emissions zone, there was observed a quantitative relationship of these two parameters with the coefficient of correlation R = −0.5, in the middle zone – R = −0.4 ÷ −0.5, in the far zone – R = R = −0.7 (Table 1).

The trend of ambient air temperature change during the period of stable snow cover in Bratsk in 1961–2014. For example, in 1982/83 from XI-II (November–February), the average temperature for these months was −6.6 °C, and in 1999/2000 for the period from XI-II it was −19.3 °C. Certainly, there is reason to believe that temperature will affect F concentration in the snow cover, and temperature should be taken into account while comparing annual concentrations. The results of studies [25] of the long-term dynamics of ambient air temperature during the period of a stable snow cover (November–March) in Bratsk are shown by the graphs of temperature anomalies vs. average value obtained in the period taken as a climatic norm (1961–1990). It is found that during the period of stable snow cover for the city of Bratsk in the interval from 1961 to 2014, there is a tendency for air temperature increase at a rate of ≈0.50 °C for 10 years [25]. Sharp interannual temperature fluctuations, of course, affect F concentration in the snow cover.

Correlation between air temperature during stable snow cover and F concentration in the snow cover. In this case, the average air temperature is calculated from the date of snow cover formation to the date of snow cover sampling in 2000–2010. It is found that F concentration in the snow cover increases with the decrease in the surface air temperature. Perhaps this is due to the fact with the solubility of gaseous compounds (HF) in atmospheric moisture increases with decreasing temperature [26].

We identify a correlation associated with emission transfer distance. In the near emissions zone of the aluminum smelter, the correlation coefficient between temperature and F concentration is R = −0.64, in the middle zone – R = −0.49, in the far zone – R = 0.84 (Table 1).

Due to the fact that the number of observations may not be enough for a comprehensive statistical processing, we use qualitative and quantitative assessment of correlation coefficients, the results of which are used to identify trends.

The correlation coefficient measures the strength and direction of a linear relationship between two variables. It is found that for the near zone and the middle zone, the force of relationship is estimated as average with a negative linear dependence, for the far zone it is a strong connection with a negative linear dependence (Table 1).

Correlation of thawing weather during stable snow cover and F concentration in the snow cover. Thawing weather is an increase of atmospheric temperature to positive values in winter or early March in Bratsk. Based on information from website https://rp5.ru [19], there was calculated the sum of hours of positive temperatures in the period from the formation of snow cover (late October or early November) to the date of snow cover sampling in Bratsk in 2014–2019.

It is noted that the more the sum of hours with positive temperatures during the period of stable snow cover is, the lower is the height of the snow cover on the day the snow cover sampling for chemical analysis. The sum of hours with temperatures above 0 °C, in the period from the formation of a stable snow cover to the date of snow cover sampling and the height of the snow cover was compared. The observation period is 7 years. It is found that the correlation factor is R = −0.36 [27].

It is also observed that the higher the sum of hours with positive temperatures (duration of thaws) is, the lower is F concentration in the snow cover in the far atmospheric emission zone (Table 1).

The strongest relationship force is observed between F concentration and three weather (climatic) factors at a distance from the source. We see that high correlation coefficients are observed for the far zone. We may assume that these weather factors have a greater effect on gaseous fluorine compounds in the atmosphere than on solid aerosols as there are practically no solid aerosols of F compounds in the far zone.

Conclusion

The results of systematization and interpretation of heterogeneous scientific information, mainly meteorological, are shown in order to identify factors affecting F concentration in the snow cover in the zone of aluminum smelter emissions affect.

The tendency of increasing air temperature and quantity of atmospheric precipitation is observed based on the graphs of anomalies of air temperature and precipitation during the period of stable snow cover for the city of Bratsk in the interval from 1961 to 2014, and the rate of this change is established. It is observed, based on the results of seven-year data, that the duration of thaws during the period of stable snow cover reduces the height of the snow cover at the time of snow sampling.

Qualitative and quantitative relationship characteristics between F concentration in the snow cover and weather factors (amount of atmospheric precipitation, air temperature and duration of thaws) were found. A strong was established between F concentration in the snow cover and the amount of precipitation, air temperature, and the duration of thaws in the period from the date of formation of a stable snow cover to the date of the snow cover sampling. A strong relationship between variables is indicative mainly for the far zone of atmospheric emissions, while the direction of the linear dependence is negative.

In the future, when comparing the annual F concentration in the snow cover, it is advisable to consider these weather (climatic) factors, as this will increase the reliability of the evaluation of the annual dynamics of fluoride emissions and risk assessment.