Tracing sulphur dioxide in volcanic deposits and ash emission during the 2019 Sinabung eruptions

Sulphur dioxide (SO2) emissions from Mt. Sinabung eruption were quantified in time series for 2019. Both pyroclastic materials and gas or aerosol ejected during volcanic eruption contain sulphur as sulphate salt deposits coating volcanic ash grains or gasses. Sulphur dioxide from the eruption will directly impact the surrounding area. Spectral from satellite optical sensors can be used to monitor and measure SO2 gas near real-time after an eruption. The distribution of SO2 column density in the atmosphere was tracked using the Sentinel-5P satellite. Regression kriging (RK) is applied to predict the spatial distribution of sulphur. The area under study is located in a radius of 3 to 7 km from the eruptive center, covering an area of about 4,517 ha. A total of 51 soil samples and volcanic ash were collected from 0- 20 cm soil depth based on a 1x1 km grid interval. All samples were air- dried, sieved, and analyzed for pH, sulphate, and total SO3 using XRF. The Google Earth Engine (GEE) platform was also used to process Sentinel-5P satellite imagery to determine the number and distribution of SO2 column density in the atmosphere during 2019. The pH of the ash is very acidic to neutral (3.56 - 6.55), while soils are considered acidic to neutral (4.67 - 6.52). The available sulphate content in soil ranges from 0 to 303.39 ppm and 0 to 142.47 ppm in volcanic ash samples. SO2 content in ash ranges from 0 to 16.53% and 0 to 3.71% in soils. Sentinel-5P satellite image spectral data shows that SO2 is concentrated mainly in the southern region, with the highest level occurring in August 2019. This study can serve as one of the volcanic mitigation programs and forecast the distribution of SO2 in an active volcanic region of Indonesia.


Introduction
Mt. Sinabung is considered an active volcano in Indonesia after its eruption on August 28, 2010, and ceased in mid-September 2010; then new eruption started on September 15, 2013, and continues to date [1].Gases, liquid, and solid particles are ejected from a volcanic vent during volcanic eruptions.Both pyroclastic materials and gas or aerosol ejected during volcanic eruption contain sulphur as sulphate salt deposits on particle surfaces or gases [2].Sulphur dioxide is considered the third most emitted volcanic gas and accumulates in the atmosphere, is oxidized, and reacts with water vapor to form sulphuric acid vapor (H2SO4) [3].It will lead to acid rain and air pollution in the vicinity of the volcano.
In the last decade, many studies have been conducted to detect volcanic SO 2 emissions.The spectra from Satellite optical sensors are used to monitor and measure SO2 gas near real-time after an eruption [4].The recording data can be processed using the Google Earth Engine (GEE) platform to estimate the mass distribution and total column density near the time after the eruption [5].Remote sensing monitoring methods, direct observation, and on-site sampling provide an overview of the impact of volcanic eruptions on the landscape and the chemistry of soils and spatially analyze the soil chemical properties by using the regression kriging method [6,7].
Previously, research related to monitoring volcanic SO 2 gas emissions using remote sensing methods only focused on the distribution and measurement in the atmosphere.Unfortunately, there is still too little attention to the impact of these emissions on soil chemical properties.The objective of this study is the rapid mapping of sulphur dioxide emissions aftermath of Mt.Sinabung eruption by using satellite spectra and the impact on soil chemical properties.

Study area
The research was conducted at Mt. Sinabung in Karo district, North Sumatra, at an altitude of 2,460 meters above sea level.The peak of the volcano at 3° 10' 16.7" N and 98° 23' 24.66" E. The research area is about 4,517 ha (Figure 1) on the South and Southeast of Mt.Sinabung.One of the biggest eruptions occurred in August 2019, with the eruption columns soaring up to over 2,000 m.Most of the volcanic ash spread to the South and Southeast.The distribution of pyroclastic material is monitored through remote sensing optical sensors Sentinel-5P and Landsat satellite imagery.Distribution of pyroclastic material is carried out by tracing and digitalizing the distribution boundaries that are visible.Then GIS software is used to create several digital maps, such as the distribution of pyroclastic material, and determine the location of the sampling point.

Materials and Methods
Spectral form satellite optical sensors are used to monitor and measure SO 2 column density in near realtime after the eruption of Mt.Sinabung in the atmosphere.Sentinel-5P satellite imagery data is used as it has a special Tropospheric Monitoring Instrument (TROPOMI) sensor to observe the atmosphere.
Another advantage is the temporal resolution of one day to make better observations of the distribution of Mt.Sinabung eruption columns in near real-time.Sentinel-5P satellite data was used with a recording period of 2019 to get the average direction and distribution of the SO2 emissions and density column aftermath Mt.Sinabung eruption.Soil and volcanic ash samples were collected from the eruption area of Mt Sinabung at Karo Regency, North Sumatra, Indonesia.Soil samples (0-20 cm) were collected at 29 locations, with a grid of 1x1km at a radius of 3 to 7 km from the top of the mountain.Volcanic ash samples were collected at 22 locations above the soil surface with a distance of 1x1 km in a 5 km radius from the top of the mountain.
Detection of sulphur dioxide gas distribution resulting from the eruption of Mt.Sinabung was processed using data from the Sentinel-5P satellite sensor recording in 2019.Processing was carried out on the Google Earth Engine (GEE) platform, and the value of the SO2 density column was averaged every month from January 2019 to December 2019.The value of the density column is calculated based on the geometry of the study area so that the average amount of sulphur dioxide gas spreads over the soil surface.Soil chemical analysis was conducted at the soil chemistry laboratory, Department of Soil, Faculty of Agriculture, Andalas University.The samples taken were air-dried and sieved with a 2 mm sieve.Then the samples were analyzed to determine soil pH (H2O), total elemental SO3, and the amount of available sulphur content in the soil.The SO3 element is determined by XRF analysis [8].The available sulphate content was analyzed using the turbidimetric method to determine the amount of element sulphur in the soil that could be utilized by plants.The data obtained were statistically processed using the JMP PRO application.The data for the analysis of each parameter was mapped using the kriging regression method to get an estimate of the distribution of parameters in the studied area.

Soil and volcanic ash pH
After the 2019 eruption, it was found that the volcanic ash pH (H 2O) ranged from very acidic to slightly neutral (3.56 -6.55).The soil on the East side has a lower average pH value, and soils in the Southeast tend to have a higher pH.The soil pH value ranged from acid to slightly neutral (4.67 -6.52).The lowest soil pH values were found in the East, and the highest pH values were in the Southeast.Overall, Soil on the East and Northeast sides have lower pH values than on the Southeast side.The difference in soil pH values in each region is influenced by the direction distribution of the pyroclastic material of Mt Sinabung.In the 2019 eruption, the direct distribution of pyroclastic material was more dominant towards the South and Southeast of Mt.Sinabung [9].The eruption in 2019 covered the soil surface and was estimated about 1,371 ha [6].The pH value of acidity in volcanic ash and soil is influenced by the sulphate and mineral composition [7].In 2019 Mt.Sinabung eruption released sulphur of more than 1680 ± 1070 t/d [9].The pH value of volcanic ash was lower than in the soil due to the higher sulphate content in the volcanic ash [10], and the pyroclastic material released during volcanic eruptions is mostly acidic [11].This acidic ash will also be washed from the soil surface to lower the soil pH value [12] but not as low as the volcanic ash pH value.

Available Sulphate
The available sulphate content in volcanic ash ranges from 0 to 143 ppm (very low to medium).Lower available sulphate is found in volcanic ash deposits on the South side than in the Southeast and East.The soil has higher sulphate content with a range of 0 -303 ppm.The highest soil available sulphate value is found in the South compared to soil in the Southeast side, covering an area of 3,513 Ha (78% of the research area).The amount of available sulphate is influenced by weathering of sulphuric minerals in the soil [13].The volcanic ash contributes to increasing sulphur in the soils.The source of sulphate in the soil does not only originate from volcanic ash but also derives from organic matter [14]; as such, the amount of available sulphate in the soil is higher than in volcanic ash.Plants require nutrients in the form of sulphate molecules, which are absorbed through root hairs, and plants need more sulphur than nitrogen to grow [15].Sulphur absorption can also be carried out through the stomata in the form of SO2 gas molecules, but if the concentration is too high, it can be toxic to plants [13].

SO3
The results showed that the value of the SO3 content in volcanic ash and soil ranged from 0 -16.53% and 0 -3.71%, respectively.The highest SO3 is found in the East > Southeast > South, and not all samples have SO3.The accumulation of dissolved SO3 on the soil surface and in the volcanic ash layer originates from rocks containing sulphate and sulphur dioxide, which react with oxygen [19].The sulphur released during the eruption originates from sulphur-bearing components in magma [9].This SO3 affects the pH value of pyroclastic materials and volcanic soils [20,21], and over time it will become the source of available soil nutrients [22].Pyroclastic materials have various types of mineral content [16], largely controlled by the composition of magma, temperature, pressure, and magma chamber during the crystallization process [17,18].At the beginning of 2019, the average amount of SO2 gas in the study area was very low, and it increased after the volcano erupted and SO2 emissions reached a peak in June and August (Figure 8).The spatial distribution of SO 2 emissions in the research area is increasing towards the East.Even in the lowest month of February, satellite sensors only capture sulphur dioxide gas emissions in the eastern region of the study area.There was a positive relationship exist between the distribution of gas and the chemical properties of the soil described previously.Soils became acidic, increasing available sulphate content and higher emission of SO2 gas are correlated with the Sentinel-5P satellite data.The Tropomi sensor from the Sentinel-5P satellite senses infrared waves from UV rays, short and far waves, as a result of the reflection of sulphur gas particles in the atmosphere [5].Recording data can be freely accessed through the Copernicus data center via the Google Earth Engine platform.

Conclusion
The present study shows that the Sulphur dioxide (SO 2) gas released during the eruption affects the chemical properties of soils, i.e., soil acidity and the amount of total SO3 content and available sulphate.The spatial distribution of Sulphur dioxide and volcanic ash deposition are in good agreement with Sentinel-5P satellite data.Our findings prove the increase in Sulphur dioxide aerosols is directly proportional to the increase in volcanic activity and can be monitored using satellite data.The method used in in this study can be applied for disaster mitigation of volcanic eruptions.

Figure 2 .
Figure 2. (a).The Distribution of pH value Volcanic ash sample (b).Map of Distribution pH value in Mt.Sinabung.

Figure 3 .
Figure 3. (a).The Distribution of pH value Soil sample (b).Map of Distribution pH value in Mt.Sinabung.

5 Figure 4 .
Figure 4. (a).The Distribution of available sulphate content in volcanic ash (b).Map of Distribution of available sulphate content in Mt.Sinabung.

Figure 5 .
Figure 5. (a).The Distribution of available sulphate content in Soil (b).Map of Distribution of available sulphate content in Mt.Sinabung

Figure 6 .
Figure 6.(a).The Distribution of available sulphate content in Soil (b).Map of Distribution of available sulphate content in Mt.Sinabung.

Figure 7 .
Figure 7. (a) The distribution of available sulphate content in Soil (b) Mapping the available sulphate content in Mt.Sinabung.

Figure 8 .
Figure 8.The amount of SO 2 in the studied area in 2019.

Figure 9 .
Figure 9. Distribution of SO 2 gas observed from the Sentinel-5P satellite in 2019.