4.1. SDS Source Mitigation
There are numerous technical means of environmental management designed to mitigate SDS at source. Most of those detailed here are usually applied in places where wind erosion is predominantly a land-use issue. The main exceptions are in desert areas where naturally-occurring mobile sand dunes and blowing sand present challenges to human activities. Preventing wind erosion in some of the very large natural sources of desert dust is not practical, so hazards deriving from such areas are better managed when the dust is in the transport or deposition phase.
SDS source mitigation can employ a wide array of techniques that have been used for wind erosion control, most developed to protect cultivated fields from soil loss [66
]. In any particular location, a range of measures is typically employed and Riksen et al. [26
] distinguish between techniques designed to minimize actual risk (short-term, e.g., cultivation practices such as minimum tillage) from those that minimize potential risk (long-term, e.g., planting windbreaks).
Action taken to mitigate anthropogenic sources of SDS contributes towards the global aspiration to halt and reverse land degradation by 2030 (Sustainable Development Goal target 15.3) and will be in line with the concept of Land Degradation Neutrality, or LDN [68
]. These goals stretch over the longer-term and are synonymous with the aims of sustainable land management or SLM [69
] while in many cases they also embody the ecological restoration of degraded lands. SLM in particular contributes towards the resolution of issues surrounding the need to achieve social, economic and environmental objectives in areas where productive land uses often compete with environmental and biodiversity goals: the concept of integrated landscape management [70
4.1.1. Controlling Wind Erosion on Cropland
When topsoil is entrained from agricultural fields it represents a severe form of accelerated soil erosion with deleterious effects on crop yields [71
] so numerous techniques to control wind erosion on cropland have been developed. They are frequently classified into three categories:
All these measures aim to reduce erosion by decreasing the wind speed at the soil surface. This is achieved by increasing surface roughness and/or by increasing the threshold velocity that is required to initiate soil particle movement by the wind.
Agronomic measures for controlling soil erosion use living vegetation or the residues from harvested crops to protect soil from the wind. Maintaining a sufficient vegetative cover is often referred to as the “cardinal rule” for controlling wind erosion [66
]. One widely-used agronomic measure involves rotating crops grown in rows with cover crops such as grasses or legumes grown on the same field every other year. Mulching—the practice of leaving some residual crop material, such as leaves, stalks and roots, on or near the surface—is another commonly used agronomic technique. It is successful in reducing erosion and in reducing the loss of water from fields by decreasing evaporation. The relationship between soil loss and vegetation cover (live or dead) is generally exponential: the soil loss ratio is at a maximum of 1 on a bare unprotected surface but decreases rapidly to a value of approximately 0.2 with 40% soil cover [72
]. However, work in Sahelian Africa has shown that maintaining a crop residue cover of just 2% on a field reduces the potential wind erosion by at least a factor of three [73
Most soil management techniques are concerned with different methods of soil tillage, an essential management practice that provides a suitable seed bed for plant growth and helps to control weeds. Strip or zone tillage leaves protective strips of untilled land between seed rows, requiring weed control on the protective strips. Other tillage methods leave varying degrees of vegetative matter from the previous crop on the soil surface to provide protection against erosive winds. Minimum tillage incorporates the idea of stubble mulching, while zero-tillage (or no-tillage) leaves most of the soil covered with plant residues. Zero-till technology allows the farmer to lay seed in the ground at the required depth with minimal disturbance to soil structure, the specially designed machinery eliminating the need for plowing and minimizing the tillage required for planting. In addition to being very effective at controlling erosion, these forms of tillage have effectively revolutionized agriculture in many parts of the world by allowing individual farmers to manage greater areas of land with reduced energy, labor and machinery inputs [74
]. Their adoption is widespread in Australia, Canada, the USA and parts of South America. In the pampa region of Argentina, zero-tillage practices are credited with improving soil fertility by reversing decades of degradation [75
Windbreaks and shelterbelts have been used to reduce the erosive force of the wind in many agricultural areas [76
]. Fences or walls placed at right angles to erosive winds serve this purpose, or windbreaks may be created from living plants such as trees or bushes, in which case they are known as shelterbelts. Reductions in wind velocity are achieved both upwind, for a distance of 2–5 times the height of the windbreak, and downwind, extending 10–30 times windbreak height [78
]. Numerous other benefits to crops can also often be associated with the establishment of windbreaks. These include increased soil and air temperatures, reduced pest and disease problems, and an extended growing season in sheltered areas. However, windbreaks occupy space that might otherwise be used to grow crops and require maintenance to preserve their effectiveness, so they are sometimes removed to make way for irrigation developments or modern agricultural machinery.
In practice, many of these techniques to control erosion on cropland are used in combination. In a survey of villages across the southern Sahelian zone of Niger, Bielders et al. [79
] recorded the use of at least 10 different low-cost wind erosion control technologies and noted that all of the techniques used also had advantages in addition to erosion control.
4.1.2. Controlling Wind Erosion on Rangeland
Methods for controlling wind erosion on rangelands are largely comprised of preventive measures designed to reduce the pressure of grazing. Livestock may be excluded from pastures (either permanently, for a few years, or seasonally), a ban often enforced with fencing. Alternatively, authorities may seek to reduce stocking rates, frequently by putting a cap on livestock densities with the introduction of prescribed carrying capacities per hectare in areas where grazing is allowed, although it should be noted that the notion of carrying capacity is a controversial one [80
]. All of these measures have been introduced in various parts of China’s rangelands in recent decades as policy instruments designed to improve the environment by restoring grassland ecology, combatting desertification and reducing dust storms [82
4.1.3. Controlling Blowing Sand and Mobile Desert Dunes
The hazards associated with active sand dunes and blowing sand are addressed using engineering techniques involving fences or surface treatments. The measures typically employed, shown in Table 2
, are designed to reduce impacts by encouraging the upwind deposition of sand, immobilizing susceptible surfaces, enhancing sand transport through the hazardous area, or deflecting sand movement away from the area to be protected [83
]. Specific control measures are chosen depending on the local physical environment, material availability, required life expectancy of the shelter system and its economic feasibility. Reliable climatic information, particularly data on wind and precipitation, is also essential to assessing the local blowing sand hazard [84
Fences are deployed extensively in deserts to reduce sand deposition (e.g., along roads and railways) or to reduce wind erosion in an area behind the fence. These artificial structures are constructed using materials such as concrete, metal, plastic, wood, stone, or vegetation and their efficacy in trapping sand depends critically on fence height and porosity [86
Attempts to stabilize sand dunes have been undertaken for many decades. Initial, temporary reduction in sand movement can be achieved by covering the sand surface with a mulch of some kind. Mulching with sprays of petroleum products has been a common practice in parts of the Middle East, but concerns over the release of heavy metals has spurred interest in alternatives. Primary surface stabilization can also be accomplished by stone mulching, the use of chemical stabilizers, biological crusting, or covering the sand with plastic sheets, nets and various forms of geotextiles. A review of historical and contemporary dune stabilization techniques used in the UAE is provided by Mohsin and Attia [87
]. Longer term stabilization can be achieved by planting dunes with adapted grasses, shrubs or trees, and establishing a vegetation cover is often achieved in combination with a mulch in the early stages. Amiraslani and Dragovich [88
] report that oil mulch is particularly effective in encouraging seed germination in sand stabilization projects in Iran. In Egypt, and many other countries, treated wastewater is used to irrigate new vegetation until established [89
]. Large-scale projects may require specialized aerial seeding from low-flying aircraft, as described by Greipsson and El-Mayas [90
] in Iceland.
Reed checkboards are widely used in Chinese deserts to reduce surface wind speeds over dunes while the sand is planted with xerophytic shrubs. The process is expensive in economic and labor terms so, in the Taklamakan Desert, Dong et al. [84
] explain checkboards are only used along highways that are strategic for the oil industry. The key to establishing vegetation is to select appropriate plant species that will survive the early years of irrigation using often highly saline groundwater. After several decades, soil within the checkerboards typically accumulates elevated levels of silt, clay, carbon and nitrogen that can support a diverse vegetation canopy [91
], meaning the technique can also be used for the reclamation of degraded agricultural land [92
4.1.4. Controlling Wind Erosion at Mining Operations
The range of options available to mining companies to prevent dust generation from tailings dumps includes many of the techniques detailed for controlling blowing sand and mobile dunes [93
]. They include wetting mine tailings with water, and physically covering or capping them with gravel, topsoil or synthetic materials, and the erection of protective barriers to act as windbreaks. In drylands where water is at a premium, chemical suppressants developed from petroleum are widely used to mitigate fugitive dust, and renewable biopolymers may offer a more sustainable alternative [94
]. Stabilizing such dust-emitting surfaces can be achieved over the longer term by promoting the establishment of a vegetative cover, typically with plants that are native to the area as well as drought-, salt-, and metal-tolerant [95
4.1.5. Integrated SDS Control Strategies
Effective SDS control should adopt an integrated multi-scale and multi-functional approach [96
]. Control measures to protect soil and reduce wind speed at the field level must be combined with wider landscape measures to reduce wind speed, reduce sand and dust mobilization and increase deposition of sand and dust from the atmosphere. Such integrated landscape management must simultaneously identify and manage different landscape components, such as cropland, rangeland, dunefields, mines and building sites. An integrated, landscape level approach is especially important given the transboundary nature of SDS and their impacts.
A landscape approach is also critical for the management of water resources, which are relevant to SDS emissions in many ways. Water is vitally important for many of the land uses associated with anthropogenic SDS generation (cropland, rangeland, mine tailings), but there are numerous examples of diversion and/or consumptive use of surface or groundwaters that have resulted in new sources of dust storms, blowing sand and sand dune formation in and around desiccated lake beds all over the world [97
]. The measures required for mitigating SDS at these sources need to be part of sustainable integrated water management strategies. These plans, at national and international levels, should take into account relevant SDS issues.
4.2. SDS Impact Mitigation
Actions designed to mitigate the numerous impacts of SDS associated with the transport and deposition of sand and dust are facilitated by a range of monitoring, prediction and early warning initiatives. The entrainment and transport of small particles is monitored using a combination of data from satellites, networks of lidars and radiometers, air-quality monitoring and meteorological stations [98
]. All of these sources contribute data to modeling efforts, which enhance our understanding of the processes involved and are used to produce predictions and early warnings.
A diverse range of numerical models has been developed, but the prediction of dust events both on the ground and in the atmosphere continues to face a number of significant challenges owing to the complexity of the systems involved [99
]. Foremost among these challenges are the relative scarcity of suitable wind erosion observations and the huge range of relevant scales needed to fully account for all of the physical processes related to dust. One national initiative designed to address some of these issues is the National Wind Erosion Research Network established in the USA in 2014 [100
Nonetheless, numerous methods have been developed to map areas at risk from wind erosion (e.g., [101
]) and operational dust forecasts have been developed at a number of centers around the world in recent years, many of these initiatives evolving as part of the WMO’s Sand and Dust Storm Warning Advisory and Assessment System (SDS-WAS). Established in 2007, SDS-WAS works as an international network of research, operational centers and user groups such as health, aeronautical, and agricultural communities. More than 15 organizations provide daily dust forecasts using 14 numerical models in different geographical regions [105
Communicating warnings of imminent dust hazards, advising on health risks and mitigation options, can be achieved through a variety of means, including media coverage and SMS text alerts. Such early warning systems can reduce the impacts of a dust event. In the transport sector, for example, airlines can activate programs to reschedule or cancel flights before passengers arrive at the airport, thus reducing cancellation costs. In South Korea, warnings of yellow dust events (HwangSa
in Korean) transported across the Korean peninsula from China and Mongolia are issued by the Korea Meteorological Administration using local media and SMS text alerts issued to users who register on their air quality alert website. There are two levels of notice [106
Advisory, issued when the hourly mean PM10 concentration is expected to exceed 400 μg/m3 for over 2 h.
Warning, issued when the hourly mean PM10 concentration is expected to exceed 800 μg/m3 for over 2 h.
Reducing impacts also involves assessing vulnerability (identification and mapping of vulnerable populations and infrastructure/facilities), which, alongside future trend scenarios, provides critical inputs to plans to strengthen socio-economic resilience. Vulnerability can be formulated in a number of ways, but all involve three essential parameters: the stress to which a system is exposed, its sensitivity to the impacts of that exposure, and its capacity to adapt to ongoing and future exposure [107
]. Our understanding of the vulnerability of landscapes to wind erosion is relatively well-developed and is addressed in detail in Section 4.1
, but appreciation of the vulnerability of socio-economic systems to SDS is much less advanced [108
]. That said, certain principles identified in other fields also apply to SDS hazards, particularly the understanding that poor people living in the poorest countries are especially vulnerable because they depend on natural resource-based livelihoods that are disproportionately affected by environmental change and have the weakest ability to adapt to impacts, in large part because of their poverty. Poor people in drylands, where SDS are prevalent, are particularly vulnerable. Globally, about half of all dryland inhabitants are poor, about a billion people in total, dubbed the “forgotten billion” by Middleton et al. [109
] because they have habitually been neglected in development processes.
Many investigations of the health impacts associated with atmospheric soil dust agree that certain sectors of the population in all countries are particularly vulnerable to airborne and respiratory diseases. These include infants, children, pregnant women and the elderly, people with pre-existing heart and lung diseases (e.g., asthma, chronic obstructive pulmonary disease, and ischemic heart disease) and outdoor workers (e.g., laborers, athletes) in high exposure situations. However, assessments of spatial distributions of human vulnerability to SDS health effects are few and far between. Findings from studies that have been conducted, such as the investigation of Asian dust haze effects on children’s respiratory health in Taipei, Taiwan [110
], indicate that geographical patterns are heterogeneous and not straightforward.
Action that can help to mitigate the health impacts of atmospheric dust relies upon an understanding of how dust exposure is linked to various ailments. Although many of the causal links remain unclear, some trustworthy public health warnings can still be disseminated, such as those shown in Table 3
. There is evidence to suggest that such media alerts of poor air quality do result in behavioral changes that lower exposure to air pollutants generally [111
]. A similar finding has been reached in assessments of the health impacts associated with a severe dust storm in Australia. Tozer and Leys [112
] highlighted the importance of Health Alert SMS and emails sent to subscribers to the local health-alert system advising of a high pollution-level event, and Merrifield et al. [113
] concluded that because the dust storm and consequent public health messages had widespread media coverage, the health consequences from this dust event were likely to represent the optimal health outcomes that could be hoped for in similar future events.
However, the lack of understanding of causal links that characterize our appreciation of most dust-related ailments may still have an impact on appropriate mitigation advice. The incidence of epidemics in the Sahelian zone, the so-called Meningitis Belt, certainly appears to be related to Saharan dust intrusions brought by the Harmattan [114
], but several possible explanations have been proposed for how dust may be linked causally to the epidemics (Table 4
). Should the idea that higher meningitis transmission occurs because people gather together in close proximity during dusty periods, as suggested by Remy [116
], prove to be valid, then advice to stay indoors may not be appropriate.
In the case of meningitis in West Africa, the need for more research and a better understanding of precisely how major outbreaks are linked to dust haze from the Sahara is clear. This gap is being addressed by the Meningitis Environmental Risk Information Technologies (MERIT) project, an effort supported by several international organizations including WHO, WMO and the intergovernmental Group on Earth Observations (GEO). MERIT aims to improve current control strategies and provide more timely warnings of the onset of meningitis epidemics [120
One type of dust storm that is particularly hazardous to transport is the dry thunderstorm (or “haboob”) characterized by a moving wall of dust that brings atmospheric conditions of near-zero visibility. Forecasts for such storms at spatial scales of a few km are under development [121
], but systems designed to warn drivers of dusty conditions on susceptible highways have been used on Interstate routes in the US southwest for several decades [122
]. More recently, remotely controlled signs are being replaced by signs linked to in-situ sensors that detect poor-visibility conditions as they form and alert motorists via overhead electronic signs. Information on dust storms and safety, including what to do if caught driving in blowing dust, is also made available on dedicated websites (e.g., pullasidestayalive.org).
In urban landscapes, increasing vegetation cover is a long-term measure that is likely to help reduce the health problems associated with atmospheric PM10
concentrations as well as biological and chemical aspects of pollution [123
]. Green vegetation has the potential to reduce pollutants through filtration [124
], while also regulating microclimatic conditions in a way that offers at least perceived benefits and well-being [125
]. Research into the importance of urban green spaces in pollution reduction has been driven largely by health effects from industrial and vehicle emissions, but is also relevant to soil dust. One study of US cities estimated the monetary value of adverse health effects (i.e., mortality and morbidity) countered by urban trees: Nowak et al. [126
] calculated the value of PM2.5
removal in ten cities ranged from US $
1 million to US $
60 million per year.
In the shorter term, the efficient filtration of air supply into buildings can have health benefits. Another US study found that in areas where sealed and air conditioned buildings are common, the dose-response rate for PM10
induced morbidity was lower than in areas with milder climate where open windows are used more commonly for ventilation, suggesting a safety factor created by the sealed building envelope [127
]. Similar technology can be used to clean air entering sensitive manufacturing plants, such as electronics component manufacturers. Kim [128
] describes how Samsung in South Korea has introduced systems to reduce the number of faults in components manufactured during SDS events.
Outdoor facilities have to use different options. Identification of appropriate cleaning/maintenance operations for photovoltaic (PV) systems is dependent on numerous factors, including site-specific environmental and weather conditions, but Mani and Pillai [129
] present a series of recommendations based on their survey of the literature. Prominent among the options is periodic cleaning and the adoption/application of dust-repelling coatings. The authors also note the balance that needs to be struck in low latitudes between the low tilt angle required for PV systems to maximize solar gain and the fact that such lower tilts accumulate higher dust deposition.