Dynamic evolution of spring sand and dust storms and cross-border response in Mongolian plateau from 2000 to 2021

ABSTRACT
 According to the United Nations Sustainable Development Goals (SDG 15.3), frequent sand and dust storms (SDSs) in the spring are a long-term challenge to the prevention and control of land degradation on the Mongolian plateau. In this study, MODIS remote sensing data are used to monitor and analyse SDS events on the Mongolian Plateau. The annual distribution of spring SDSs (March to May) from 2000 to 2021 are obtained based on the dust storm detection index. The overall classification accuracy is 85.24% and the kappa coefficient is 0.7636. Results show a decrease in the overall frequency of SDS events, where storm events in 2000–2010 are significantly higher than those in the second decade. The cross-border regions between China and Mongolia appear to be SDS intensity centers, particularly those in southern Mongolia. Precipitation exhibits a strong negative correlation with the area affected by SDS, and the correlation coefficient is – 0.72. The increase in barren and sand contributes primarily to the increase in SDS, whereas wind prevention and sand control projects undertaken by the Mongolian and Chinese governments promote regional restoration. Policies pertaining to cross-board sandstorm responses on the Mongolian Plateau are recommended.


Introduction
Sand and dust storms (SDSs) are natural phenomena that occur globally and significantly affect the regional sustainable development on the Mongolian Plateau, which is a vast arid and semi-arid region (Akhlaq, Sheltami, and Mouftah 2012).SDSs, which typically occur in fragile ecological environments, affect regional sustainability by polluting the natural environment, damaging crop growth, and causing human and livestock casualties (Wang et al. 2001).Inadequate vegetation cover and bare surfaces in arid and semi-arid areas are susceptible to SDSs and land degradation.The objective of combating land degradation for SDG 15.3 is similar to that of SDS control in arid and semi-arid regions.SDG 15.3 proposes combating desertification and restoring degraded land and soil, including those affected by desertification, drought, and floods, to achieve zero growth in land degradation by 2030(https://www.undp.org/sustainable-development-goals).
The Mongolian Plateau is an ecological barrier in Northeast Asia; however, it is also a primary source of SDSs in Asia (Jugder et al. 2011;Lee and Sohn 2011).The increasing occurrences of SDSs, land degradation, desertification, and other ecological and environmental problems restrict the sustainable development of this region (Fan, Shi, and Li 2007).In recent years, SDS events on the Mongolian Plateau have presented unstable situations, which consequently exacerbated the challenge of Land Degradation Neutrality processing (Wang et al. 2023) (https://www.unccd.int/land-and-life/land-degradation-neutrality/overview).For example, in the spring of 2021, several strong SDS events occurred on the Mongolian Plateau, which not only directly resulted in livestock and casualties, but also affected the ecological environment of neighbouring countries in Northeast Asia (Wang et al. 2022).Reducing SDSs and preventing desertification on the Mongolian Plateau are challenging but must be considered urgently.
SDSs can be monitored via two approaches: 1) via conventional ground meteorological observations, and 2) remote sensing monitoring (Xiong, Li, and Zhuang 2002;Li 2018).Monitoring via ground observation allows data to be recorded over a large series of observation times; however, monitoring station resources are limited, and the records cannot encompass large regions (Terrigram 2016).As SDSs involve a wide range of areas, remote sensing methods based on earth observation facilities are an effective option.MODIS data obtained from the Terra and Aqua satellites as well as AVHRR data obtained from the NOAA satellite, Fengyun satellite data, and Himawari-8 satellite data are typically used.Among them, MODIS data are advantageous as they feature multiple bands and long monitoring time series; hence, they are most widely used in large-scale studies.
Multiple SDS indices based on MODIS data have been proposed.Steven et al. (1997) used MODIS bands 31 and 32 to establish a brightness temperature difference (BTD) index; when the bright temperature values of bands 31 and 32 are negative, they allow not only the source area of SDS activity to be monitored, but also allows clouds and dust to be separated.Zhang et al. (2017) used the BTD algorithm to perform remote sensing monitoring and analyse 112 dust events from 2000 to 2015 in southern Mongolia; their results indicated that SDS mainly originated from southwestern and southeastern Mongolia.Based on the MODIS visible light band, Han, Li, and Guo (2005) derived different spectral feature discrimination functions and monitored the processes of SDSs using the decision tree method.Xiao, Wang, and Chan (2007) used the best band combination index and the variancecovariance matrix eigenvalue method to derive an SDS monitoring index and graded the intensity of SDSs.Wang et al. (2022) analysed the daily atmospheric circulation and the horizontal and vertical distribution characteristics of SDSs in northern China in 2021 using visual interpretation and the bright temperature difference method.Karimi et al. (2012) analysed the ability of various dust extraction algorithms (BTD, NDDI, deep blue algorithm, and other algorithms) in identifying dust sources in the Middle East and proposed the Middle East dust index.Li et al. (2006) compared and analysed three dust monitoring methods and concluded that the superposition analysis method based on a double-channel domain value was more conducive for accurately extracting SDS information.Li et al. (2022) used the BTD index, a normalised dust index, and two index coupling methods to extract and analyse SDS events in March 2021 in Inner Mongolia and recommended coupling BTD and NDDI algorithms to monitor dust information.
Although the dust indices proposed by different researchers are applicable to their respective study regions, establishing a universal application threshold for various dust events in different regions is challenging.Hence, Jebali et al. (2021) proposed a new SDS detection index known as the dust storm detection index (DSDI), which does not require a specific threshold for each event.The success of the algorithm in detecting dust storms in arid areas was confirmed based on its detection of sandstorm events in Yazd province in central Iran between 2004 and 2009.Owing to the challenge in monitoring SDSs in the vast Mongolian Plateau, monitoring is typically performed on only certain sections of the entire plateau.Long time-series monitoring has not been performed thus far, e.g.monitoring was performed in short periods of 2000-2015 (Zhang et al. 2017) and 2002-2015(Gao et al. 2016).
Hence, the distribution dynamics and driving forces of SDSs on the Mongolian Plateau are not well understood.Therefore, we herein propose the extraction of spring SDS dynamics in the Mongolian Plateau from 2000 to 2021, followed by an analysis of the attribution and countermeasures of SDS control in the cross-border area using big-data processing technologies.

Study area
The Mongolian Plateau is located in the arid and semi-arid region of the hinterland of Northeast Asia, between north latitude 37-53 and east longitude 88-120; its area is approximately 274.95 × 10 4 km 2 .The scope of this study includes the entire territory of Mongolia and the Inner Mongolia region of China (Figure 1).The region has a temperate continental climate with prevailing northwest winds and an average annual rainfall of approximately 200 mm.Its vegetation coverage reduces gradually from north to south, and the region exhibits clear seasonal characteristics.In spring, it is cold and dry, most of the surface is exposed, and the vegetation cover is low before the vegetation growing season.As the temperature begins to increase in the spring, the intersection of cold and warm airflows causes high winds.These coupled weather and ground conditions can easily induce SDS events on the Mongolian Plateau during spring (Lee and Sohn 2011;Wang et al. 2011).

Data source
SDS event data were obtained from meteorological data records as well as through social news mining.The meteorological data refer to the Strong Sandstorm sequence and its supporting dataset from China (https://data.cma.cn).Social news portal data were obtained using search engines such as Google and Baidu.Eighty SDS events were mined in the spring from 2000 to 2021 (Table S1).MODIS L1B images (1 km resolution) during the same period as the SDS events were downloaded from the NASA.

Methods
The MODIS Conversion Toolkit (MCTK) plug-in of the ENVI software was used to correct the MODIS data based on geometry.We constructed a dust extraction algorithm, calculated the parameters required by the model, and extracted the SDS distribution data.
A suitable DSDI was used to extract SDS information.The DSDI is calculated as follows: Here, BT 22 , BT 30 , BT 31 , and BT 34 refer to the bright temperatures of bands 22, 30, 31, and 34, respectively; R 3 and R 4 refer to the reflectivity of bands 3 and 4, respectively; and b31 Emis refers to the firing rate of band 31.
Figure 2 illustrates the data processing flow.MODIS data downloaded from 2000 to 2021 were screened based on the cloud volume (https://ladsweb.modaps.eosdis.nasa.gov).The MODIS data were preprocessed with geometric correction using the MCTK plug-in in the ENVI software.A DSDI was derived to extract the spatial distribution of dust and obtain the spatial distribution of annual SDS events occurring on the Mongolian Plateau.

Accuracy verification
Sample points were randomly generated in ArcGIS and then imported into ENVI; meanwhile, the actual values of the sample points were obtained using relevant station records, text description data, and visual interpretation methods.For visual interpretation, the MODIS images were synthesised into bands 1, 4, and 3.The white area denotes clouds and the yellowbrown area with a feather texture denotes the area with dust storms.In the composite images of 7, 2, and 1, the dark reddishbrown area with a feather texture was the area with dust distribution, and in the composite images of 1, 2, and 20, the bluewhite area with a feather texture was the area with dust distribution.After determining the actual values of the training sample points, an error matrix was used to determine the extraction accuracy of the dust detection index.

Time change of SDSs on Mongolian plateau
Figure 3 shows the frequency statistics of spring SDS events on the Mongolian Plateau from 2000 to 2021.Eighty typical spring sandstorm events were monitored.The results indicated that 31 events (39%) in March, 34 (43%) in April, and 15 (19%) in May.In terms of temporal changes, 52 events (85%) occurred on the Mongolian Plateau between 2000 and 2010.Among them, 2000Among them, -2002 (21 times) (21 times), 2006 (9 times), and 2010 (7 times) were the peak periods of spring SDS on the Mongolian Plateau.From 2011 to 2021, the number was significantly lower, i.e. 28 (35%).Among them, events occurring in 2011-2015 showed a downward trend annually, the frequency of occurrence from 2016 to 2021 was relatively low, and the fluctuation changed negligibly.However, the SDSs began to increase abruptly in 2021, thus indicating an unstable situation.
We extracted the spatial distribution of all single sandstorms in each event, updated the spatial distribution of all sandstorm events to obtain the maximum range of SDSs in the year (spring), and then calculated its annual area.All the SDS events monitored for each spring were integrated to obtain the annual dust area on the Mongolian Plateau (Figure 4).The years most affected by SDS events were 2000, 2001, 2002, 2006, and  value was indicated in 2020, i.e. 6.65 × 10 4 km 2 .However, in 2021, the value increased significantly to 79.59 × 10 4 km 2 (approximately twice the average).

Spatial changes in SDS outbreaks on Mongolian plateau
Based on validation analysis, the maximum overall classification accuracy of the DSDI was 85.24%, and the kappa coefficient was 0.7636.The spatial distribution of the spring SDS over the past 22 years is shown in Figure 5.
In general, SDSs occur annually in the spring on the Mongolian Plateau, and their occurrence trajectory is evident.The overall area with dust in the first decade was significantly greater than that in the next 10 years, and since 2011, the area affected by SDSs decreased.From 2013 to 2017, the SDS characteristics showed a scattered distribution and generally weakened.The concentrated area of spring SDSs on the Mongolia plateau was in the border area between China and Mongolia.The dust transport route suggests that dust originated from the Bayanhongor and Ovorhangay Provinces west of Mongolia (e.g. 2000Mongolia (e.g. , 2002)), which then propagated to the crossboard area of Mongolia andChina (e.g. 2006, 2010) and can reach the far-east area (e.g.2002,2006) and even the south in Alxa Zuoqi and Alxa Youqi in Inner Mongolia, China.
The high-frequency development areas were mainly concentrated in the middle of the Mongolian Plateau, most of which border Mongolia and Inner Mongolia, including the southern Gobi Region of Mongolia and western regions, such as the Alxa Zuoqi, Alxa Youqi, and Ejin Qi regions of Inner Mongolia, as well as the Urad Houqi and Urad Zhongqi regions and their nearby areas.The other high-incidence areas were Dornod Province of Mongolia and the Dong Ujimqin Qi, Xi Ujimqin Qi Sunid Youqi, and Sonid Zuoqi regions, which border Inner Mongolia.However, in these  areas, the overall number of SDS events was less than that in the central and western regions of Inner Mongolia and the southern region of Mongolia.

Spatiotemporal change trend of sandstorms on Mongolian plateau
In terms of location, SDS events typically occurs more in the south and less in the north, and more in the west and less in the east; furthermore, they decrease from west to east and south to north.The main area affected by spring SDSs on the Mongolian Plateau was the border between China and Mongolia, particularly southern Mongolia.These areas include the southern Gobi region of Mongolia as well as the central and western regions of Inner Mongolia, such as the Alxa League, Urad Houqi, Dorbod Qi, Sonid Zuoqi, and Sunid Youqi regions, and others (Zhou and Wang 2002;Qian et al. 2006).In general, the northern forest area in Mongolia and the northeastern region of Inner Mongolia experienced fewer SDSs, and SDSs reached the northeastern region of Inner Mongolia owing to cyclonic action in only a few years (2000, 2001, and 2004).Consistent with the findings of other studies (Lee and Sohn 2011;Wang et al. 2011), the Mongolian Plateau Spring SDS originated from western Mongolia (Govi-Altay and Bayanhongor), propagated to the south of Mongolia (Dundgovi, Dornogovi, and Omnogovi), and then extended to the border area and China in the Inner Mongolia region (Urad Houqi, Dorbod Qi, Sonid Zuoqi, and Sunid Youqi).
As a cross-border event, SDSs in southeastern Mongolia typically propagate to the east and northeast (Sun et al. 2004).The overall dust path from western Mongolia (Govi-Altay, Bayanhongor, and Ovorhangay Provinces), which is the region most affected by dust, constitutes the southern region of the border between China and Mongolia and directly affects Mongolia's Dundgovi, Dornogovi, and Omnogovi Provinces as well as China's Darhan Muminggan Lianheqi, Urad Zhongqi, Urad Houqi, Dorbod Qi, Sonid Zuoqi, Sunid Youqi, Erenhot Shi, and other regions (Liu et al. 2003;Zhao, Sun, and Zhao 2004;Wang et al. 2011;Zong, Zhang, and Ma 2012).

Analysis of relationship between SDS and land cover types
The formation of dust storms is affected by natural and human factors (Chen, Li, and Sareula 2009).The natural factors include strong winds in the spring, limited precipitation in arid regions, and sand sources with low vegetation coverage on the Mongolian Plateau (Zhang et al. 2007).Human factors mainly affect the underlying ground surfaces via social and economic activities, such as overgrazing, excessive reclamation, deforestation, disordered mining, and off-road soil erosion (Li, Yang, and Wang 2002;Gu et al. 2002).
Changes in land use and land cover are due to both natural and human effects combined.Dust from sources present on the ground is scattered into the atmosphere by cyclones and then transmitted over long distances and large ranges under the action of strong winds, thus resulting in the formation of SDSs (Hu, Cui, and Tang 1999).Table 1 shows the land cover changes on the Mongolian Plateau over the past 30 years (Zhang et al. 2022).The main land cover types in the high incidence areas of SDSs on the Mongolian Plateau were bare land, sandy land, and desert grassland, whereas northern Mongolia and the eastern region of Inner Mongolia were mainly covered by forest land and meadow grassland with high vegetation coverage (Wang et al. 2020;Li et al. 2021).
Based on an analysis of the spatial transfer distribution of bare and sandy lands on the Mongolian Plateau from 2000 to 2010 and 2010 to 2020 (Zhang et al. 2022), the sandy area on the Mongolian Plateau increased, reaching 0.61% from 2010 to 2015.The bare land area increased from 2000 to 2010 at a rate of 1.3%.This period was coupled with a high frequency of SDSs between 2000 and 2011.The bare land area decreased from 2010 to 2021, reaching 1.93% from 2010 to 2015 (Zhang et al. 2022).The increase in bare land area accelerated the development of SDSs, whereas its decrease was accompanied by a decrease in the number of SDS.This indicates that changes in bare land are negatively correlated with changes in the frequency of sandstorm occurrences on the Mongolian Plateau.

Analysis of relationship between SDS and meteorological indicators
As mentioned in Section 3.3, high-frequency (> 40) regions for SDSs exist in the Mongolian Plateau.We considered this region as a case study to analyse the relationship between SDSs and meteorological indicators, mainly temperature and precipitation.Owing to the limited amount of meteorological observation data, our discussion is based on time series from 2008 to 2017.
Figure 7 shows the relationship between SDS area and meteorological data (air temperature and precipitation).Based on the calculated correlation coefficient (−0.73), precipitation and sandstorm area were negatively correlated.This implies that the greater the precipitation, the smaller is the area affected by SDSs.Similarly, temperature was negatively correlated with sandstorm area, based on a correlation coefficient of -0.25, which indicates a weak correlation.Other researchers discovered that precipitation directly affect dust emission through changes in soil moisture and vegetation coverage (Lee and Sohn 2011).

Analysis of relationship between SDS and its prevention policies
In addition to the harsh local ecological conditions of the Mongolian Plateau and the increasingly prominent effects of climate change, some policy measures implemented in Mongolia and Inner Mongolia resulted in both positive and negative effects on SDS control.The recognised pillar industries in Mongolia include animal husbandry and mining industries (Melasu 2015;Dong et al. 2019).Based on official data released by the Mongolian government over the past 30 years, the number of livestock has increased rapidly from 25.8 million in 1990 to 28.5 million in 1995 and then to 70.9 million in 2019.In 2020, it decreased to 67.1 million and then reached 67.3 million in 2021, which was 33 million more than the total carrying capacity of the pastures (Wuda et al. 2022).The rapid growth of the livestock population has caused local overgrazing and severe environmental damage in Mongolia (Middleton 2018).Overgrazing and heavy cultivation may decrease the NDVI and induce SDSs (Lee and Sohn 2011).In 2002, the Mongolian Mining Ministry issued approximately 3,000 exploration permits encompassing almost 30% of the country's territory.The coal output of Mongolia has been increasing from 8,000 thousand tons in 2006 to 10,000 thousand tons in 2008, 25,000 thousand tons in 2010, and 30,000 thousand tons in 2011, thus indicating a rapid growth rate of coal output.The flourishing mining industry has resulted in a new source of wealth to the country but has adversely affected surface vegetation and the ecological environment.The development of the two pillar industries mentioned above may affect the spatial and temporal distribution patterns of spring SDS on the Mongolian Plateau (Wuda et al. 2022;Hughejiltu. 2017).
Both the Mongolian and Chinese governments have implemented a series of measures to resist desertification and thus reduce the occurrence of sandstorm disasters.Since 2005, the Mongolian government has implemented a national plan known as the 'Green Wall' to increase vegetation coverage and control desertification in the southern drought and Gobi Desert areas (Huang et al. 2012).In recent years, the Mongolian government launched the 'billion-tree campaign ' scheme.Mongolia plans to plant at least one billion trees between 2021 and 2030.
Compared with the case in the Mongolian region, the effect of SDSs in Inner Mongolia decreased significantly after 2011, which is due to the long-term ecological projects in Northern China.The Chinese government launched the Beijing -Tianjin Sandstorm Source Control Project in 2000, with desertified land management as the core task (Huang et al. 2018;Wang et al. 2012).The 'Three North China' shelterbelt project is another important desertification prevention and control project in China, which was proposed in early 1979 and will continue until 2050.It is a large-scale artificial forestry ecological project conducted in three northern regions of China (Northwest, North, and Northeast China) that encompass the Inner Mongolian region of China.By the end of 2020, the total afforestation area of the three North China projects reached 31.7429 million hectares.
Sandy land areas in China have decreased significantly owing to the abovementioned ecological projects (Jia et al. 2016;Liu, Li, and Feng 2019).However, findings from some assessments were not congruent with the ecological effects of these projects.For example, the decline and death of young trees caused by afforestation reflects ecological adaptability (Wang et al. 2010;Cai et al. 2020;Middleton 2018).Hence, local conditions and the combination of artificial management with natural restoration must be prioritised to effectively control SDSs.

Suggestions for SDS response on Mongolian plateau
Strengthening monitoring and early warning of SDS.The main concentrated areas of spring SDSs in the Mongolian Plateau were the southern area of Mongolia and the central and western regions of Inner Mongolia; therefore, these sandstorm-prone areas should be monitored more closely at more stations to provide early SDS warnings.Currently, 237 meteorological sites are distributed on the Mongolian Plateau (137 sites distributed in Mongolia and 100 sites in Inner Mongolia).Fixed or mobile monitoring stations are recommended in areas susceptible to SDSs, such as Bayanhongor and Ovorhangay Provinces in Mongolia, and Alxa, Mausu, Hunshak, Hulunbuir, and Keerqing Qu in Inner Mongolia.In addition to these fixed regions, researchers have shown that in the spring, most SDSs were originally generated from Mongolia and propagated southeast to northern and northeast China (Qian et al. 2022), which necessitates improvement in the monitoring of long transportation corridors.In terms of early warnings, numerical weather prediction products (Qian et al. 2022), total variableor full-field-based analysis methods (Qian, Du, and Ai 2021), and remote-sensing monitoring should be integrated.
Strengthen ecological projects related to wind and sand control.For areas that encounter sandstorm events more frequently, ecological restoration projects and related policies should be implemented, e.g.afforestation projects in regions rich in water resources.This would prevent overgrazing and mining in fragile areas, allow grazing systems in areas with developed animal husbandry to be optimised, strengthen surface vegetation control, and prevent the movement of sand dunes.Additionally, a few low-frequency sandstorm regions must be protected from climate change.Ecological reserves provide effective measures for regions such as northern Mongolia and the eastern border region between China and Mongolia.In China, experiments for national park construction were performed in ecological reserve regions, such as Sanjiangyuan National Park on the Tibetan Plateau, as well as for water resource protection in the high Asian region (http://sjy.qinghai.gov.cn/).The Man and Biosphere Programme (MAB) led by UNESCO is a prime example of the implementation of ecological and environmental protection on the Mongolian Plateau (https://www.unesco.org/en/biosphere).These reserves are present in the MAB network, such as the Bodhan Ur Biosphere Reserve in Mongolia and Husteunulu Biosphere Reserve in the Selenge River Basin in northern Mongolia.
Strengthen the construction of a coping mechanism related to cross-border dust weather in China, Mongolia, Japan, and Korea.A joint monitoring and cooperative research project for land desertification and dust weather should be established to improve the emergency response capacity for regional SDSs and related ecological issues.Examples include a joint monitoring system, a joint sandstorm simulation and visualisation system, and online land degradation and dust weather monitoring platforms.Since 2007, the Chinese Academy of Sciences has performed cooperative studies pertaining to desertification control and gained experience in desertification control through local studies (Dong 2010).In February 2022, the Government of the People's Republic of China and the Government of Mongolia issued a joint statement, which clearly stated that the two countries should strengthen their cooperation for the ecological environment and desertification control, jointly manage global climate changes, and create a clean and pleasant ecological environment.In November 2022, leaders of the Chinese and Mongolian governments proposed to discuss the establishment of a China -Mongolia Desertification Control Cooperation Center (http://www.forestry.gov.cn/main/586/20221223/081117780672009.html).Recently, the Chinese government issued the National Plan for Desertification Prevention andControl (2021-2030), which prioritises prevention such that the classified protection of desertified land can be realised.(https://www.thepaper.cn/newsDetail_forward_21372372?commTag = actual).Meanwhile, Mongolia and South Korea co-funded the Korea -Mongolia Greenbelt Project in 2005 (Mu¨hlenberg et al. 2006), which is intended to extend 250 km eastwest across the countries, with a width of at least 600 m.

Conclusion
The Mongolian Plateau is one of the locations where SDSs originate in Asia.In recent years, the SDS events on the Mongolian Plateau have shown an unstable and locally aggravating trend, which has rendered it more challenging to realise Land Degradation Nutrition and SDG15.3.In this study, the distribution of continuous SDS events in the spring season  on the Mongolian Plateau was retrieved using the DSDI based on MODIS data.We discovered that SDSs intensified in the first half and weakened in the second half of the year; however, they have become unstable recently due to climate change and human activities.The regional distribution trend of SDSs indicated a gradually increase from east to west and from south to north (with a clear transportation route) and was centred on the cross-board regions of China and Mongolia.SDS events on the Mongolian Plateau were determined by the physical geographical environment and regional governance policies.An increase in bare and sandy lands and a decrease in precipitation were the main physical reasons for the increase in SDSs, whereas ecological measures by the Mongolian and Chinese governments contributed to their regional suppression.Suggestions for cross-board sandstorm responses on the Mongolian Plateau were proposed based on this analysis.This regional practice of using big earth data to support SDS monitoring is expected to be conducted in more arid and semi-arid regions worldwide.This study presents some limitations.To satisfy future demands, we should strengthen the combination of remote sensing and ground meteorological station monitoring, as well as increase the spatial and temporal analyses of dust concentration and dynamic changes in the Mongolian Plateau.
Figure3shows the frequency statistics of spring SDS events on the Mongolian Plateau from 2000 to 2021.Eighty typical spring sandstorm events were monitored.The results indicated that 31 events (39%) in March, 34 (43%) in April, and 15 (19%) in May.In terms of temporal changes, 52 events (85%) occurred on the Mongolian Plateau between 2000 and 2010.Among them, 2000Among them,  -2002 (21  times) (21  times), 2006 (9 times), and 2010 (7 times) were the peak periods of spring SDS on the Mongolian Plateau.From 2011 to 2021, the number was significantly lower, i.e. 28 (35%).Among them, events occurring in 2011-2015 showed a downward trend annually, the frequency of occurrence from 2016 to 2021 was relatively low, and the fluctuation changed negligibly.However, the SDSs began to increase abruptly in 2021, thus indicating an unstable situation.We extracted the spatial distribution of all single sandstorms in each event, updated the spatial distribution of all sandstorm events to obtain the maximum range of SDSs in the year (spring), and then calculated its annual area.All the SDS events monitored for each spring were integrated to obtain the annual dust area on the Mongolian Plateau (Figure4).The years most affected by SDS events were2000, 2001, 2002, 2006, and  2021.Based on frequency comparison, nine and six SDS events occurred in 2006 and 2021, respectively, and the single intensity and impact range of SDSs in spring 2021 were extremely large.During the 22 years, the average area with SDS outbreaks in the spring was 40.87 × 10 4 km 2 .In 2000-2002, such an area measured approximately 77 × 10 4 km 2 , which decreased to 22.17 × 10 4 km 2 in 2003 and then increased rapidly thereafter.In 2006, the area peaked at 120.58 × 10 4 km 2 (approximately three times the average).In 2007, the area decreased and then increased gradually before reaching a new, lower peak in 2010.From 2011 to 2018, a fluctuating trend with minor fluctuations was indicated, where a high value of 46.88 × 10 4 km 2 was recorded in 2018.The lowest

Figure 4 .
Figure 4. Summary of SDS areas on Mongolian plateau.

Figure 5 .
Figure 5. Overall SDS areas on Mongolia plateau for each year (from 2000 to 2021).

Figure 7 .
Figure 7. Relationship between SDS area and meteorological data from 2008-2017 on Mongolian Plateau.

Table 1 .
Land cover area changes on Mongolian plateau.