Influence of soil texture on the binding energies of fine mineral dust particles potentially released by wind erosion

Models of the two aeolian processes (saltation and sandblasting) that lead to emission of fine dust particles (PM20) by wind erosion in arid and semi-arid areas have been combined to form the so-called ‘Dust Production Model’ (DPM). In this model, the size dependent binding energies of PM20 embedded within the wind-erodible loose soil aggregates or in the soil surface itself are key input parameters. Indeed, their values condition at the same time the intensity of emissions and their initial size distribution. Previous comparisons of vertical mass fluxes measured on-field with the model predictions suggest that these energies might be relatively independent of soil texture and also probably composition. Because this would greatly facilitate application of the DPM at regional or global scale, the objective of this work is to check experimentally the veracity of this result. The strategy that has been designed for this has involved selecting four natural soil samples collected in various source areas of the world and covering a wide range of textures and compositions. Then, these soil samples have been used to perform carefully controlled wind erosion simulations in a wind tunnel. During the experiments, which were carried out at different wind speeds with each soil, the horizontal flux (F_h) of saltating soil aggregates has been monitored. At the same time, number concentrations (C_i) of PM20 released by the sandblasting process were recorded in the 6 size classes of an optical size analyzer. Thus, the efficiency of the sandblasting process (defined as the ratio of C_i to F_h) could be determined for each of these size classes. Analysis of the results obtained in similar saltation conditions shows that for the four tested soils, and within the range of contents in clay and other components favoring aggregation (mostly organic matter and carbonates), the influence of soil composition and texture on binding energies of the PM20 particles within soil aggregates is at best a second order effect that can be neglected in large scale modeling of wind erosion by the DPM.     

若干风蚀粉尘释放模型述评

粉尘气溶胶对大气、陆地和海洋多方面的影响促进了粉尘释放的模拟研究。Gillette和美国FPA的粉尘释放模型相对地比较简单，主要考虑了摩阻速度和土壤质地对粉尘释放的影响，没有考虑粉尘释放过程的微观机制；邵亚平和Alfaro等人的DPM粉尘释放模型都通过土壤粒度分布、粗糙度和起动摩阻速度等参数表达了地表特征对粉尘释放的影响，且都描述了粉尘释放的微观机制。20多年来粉尘释放的模拟研究取得了重要进展，主要表现在通过室内和野外实验证实了粉尘释放的主要机制为跃移颗粒的冲击，并分别从粉尘的结合能与跃移颗粒动能及跃移颗粒产生的弹坑体积角度来描述这个微观机制。然而所有的模拟工作只是从某一方面反映了粉尘释放过程，对粉尘释放机制还没有完全了解清楚，特别的是目前关于粉尘结合能研究还有很大的不确定性，这也是粉尘释放模型需要改进的地方。     

Control of dust emissions by geomorphic conditions, wind environments and land use in northern China: An examination based on dust storm frequency from 1960 to 2003

The degree to which dust emissions are controlled by geomorphic conditions, wind environments and land use was investigated using the dust storm frequency (DSF) and data from more than 300 meteorological stations throughout northern China. Our analysis showed that most dust emissions originated in gobi deserts that developed in piedmont alluvial fans of the Kunlun, Qilian and Helan mountains. Dust emissions are low from other gobi desert regions, such as the northern Gurbantunggut and eastern Taklimakan, where high vegetation coverage restrained dust emissions or where dust-size particles are not abundant after a long period of strong wind erosion. Sandy deserts with relatively high vegetation coverage or an extensive cover by mobile sands are not a major dust source. Although the highest dust emissions did not appear in regions with the highest wind energy, DSF trends in each region from 1960 to 2003 were closely related to local wind activity. DSF was low in regions with high levels of human activity, where the mean DSF from 1960 to 2003 did not exceed 4 days/year; even from the 1960s to the early 1970s, the period with the greatest DSF, frequency did not exceed 8 days/year, which indicates that extensive land use did not contribute to DSF. The low DSF in these areas might result from the fact that although land use could produce abundant fine soil fractions, vegetation coverage and soil moisture remained higher than in the gobi deserts of arid China, thereby decreasing dust-storm occurrence.     

中国西部河谷型城市土壤与灰尘粒级组分分布特征

通过采集兰州市和延安市主城区的土壤灰尘样品,测量了其粒径组成,界定了土壤和灰尘不同粒径的空间分布,分析了环境影响.结果表明:兰州市和延安市城市土壤灰尘粒径组成有明显的分布特征,主要集中在小于100μm的部分,整体粒径分布情况为:粉黏粒(<50μm)>极细砂(50～100μm)>细砂(100～250μm)>中砂(250～...     

风蚀影响因子的敏感性试验

 对风蚀模式的各重要影响因子进行了敏感性试验，结果表明：①随风力的增大，跃动沙粒的粒径范围迅速增大，从而会使更多、更大的尘粒因受到更强烈的撞击作用而释放于空中。但随土壤水分和植被覆盖度的增加，跃动沙粒粒径范围会变窄，较大的粒子很难被激发到空中。②各种土壤沙流通量及尘粒释放率随粒径的变化趋势Q(d)和F(d)与相应的地表土壤有效粒度分布Ps(d)具有相似的特征，说明前人用近地层沙尘粒度分布来代表地表土壤的有效粒度分布是合理的。③若以总沙流通量Q>0.5 g·m^-1·s^-1为风蚀过程开始发生的标准，在干燥、裸露的情况下，沙土、沙壤土、壤土、黏土和粉黏土表面发生风蚀的临界摩擦速度都约为0.3 m·s^-1。相同风力条件下（u*=0.6 m·s^-1），若地表干燥（w=0）并忽略小于0.1 g·m^-1·s^-1的总沙流通量，则抑制5种土壤发生风蚀的最小植被覆盖度分别约为：沙土0.35、沙壤土0.45、壤土0.45、黏土0.55、粉黏土0.55；若地表裸露，抑制风蚀发生的最小水分含量分别为：沙土0.15、沙壤土0.18、壤土0.3、黏土0.36和粉黏土0.33。④通常情况下沙土最不易起尘，它在各个粒径的尘粒释放率比其他土壤均约小3~5个量级。粉黏土最易起尘，且粒径较小，较容易传输到下游很远处。⑤总尘粒释放率F和总沙流通量Q随风力、地表条件的变化一般是同相的，即Q增大，F也会增大。⑥一般情况下F随摩擦速度u*的增大或植被覆盖度cf和土壤水分w的减小而增大；土壤拖曳系数sx和弹性压力垂直分量pye的增加会大大降低尘粒释放率。⑦通常风蚀情况下，5种土壤中粉黏土和沙壤土因聚合粒子破碎产生的尘粒释放率Fc最大，Fc随风力、地表条件变化的敏感度也最强；沙土的Fc最小，其对风力、地表条件的敏感度也最弱。     

Theoretical expressions for soil particle detachment rate due to saltation bombardment in wind erosion

Saltation bombardment is a dominate dust emission mechanism in wind erosion. For loose surfaces, splash entrainment has been well understood theoretically. However, the mass loss predictions of cohesive soils are generally empirical in most wind erosion models. In this study, the soil particle detachment of a bare, smooth, dry, and uncrusted soil surface caused by saltation bombardment is modeled by means of classical mechanics. It is shown that detachment rate can be analytically expressed in terms of the kinetic energy or mass flux of saltating grains and several common mechanical parameters of soils, including Poisson's ratio, Young's modulus, cohesion and friction angle. The novel expressions can describe dust emission rate from cohesive surfaces and are helpful to quantify the anti-erodibility of soil. It is proposed that the mechanical properties of soils should be appropriately included in physically-based wind erosion models.     

1980—2015年风蚀影响下中国北方土壤有机质与养分流失时空特征

中国北方干旱/半干旱区是全球主要的沙尘源区之一,风蚀造成的沙尘排放可导致大量的土壤有机质（SOM）与养分流失,并通过传输与沉降过程对其进行空间再分配,对空气质量、气候变化、植被生长及生物地球化学过程等具有重要影响。本文利用WRF/Chem（Weather Research Forecasting with Chemistry）v3.7.1大气化学传输模型,对1980—2015年间中国北方沙尘排放及其引起的SOM、全氮（TN）与全磷（TP）的时空变化过程进行了精细化模拟,探究了中国北方风蚀引起的SOM、TN与TP养分流失的时空变化特征。结果表明：① 1980—2015年来平均每年约有66.59 Tg的沙尘颗粒排放至大气;② 沙尘排放具有较大的时空差异,沙尘排放源区主要集中在新疆东部、内蒙古西部的巴丹吉林沙漠和腾格里沙漠等地区;③ 每年因沙尘排放引起的SOM、TN和TP流失量分别约为0.07 Tg、0.004 Tg和0.005 Tg;④ 1980—2015年间中国北方沙尘排放及其引起的SOM、TN与TP的流失量具有较强的年际波动,未呈现显著的趋势性变化特征。本研究对于认知风蚀在碳循环与养分循环过程中的作用,以及对深入了解中国北方的土地退化机理具有重要意义。     

柴达木盆地表土与大气降尘粒度Weibull组成及其环境意义

柴达木盆地北部现代地表不同沉积环境沉积物粒度分析结果显示,多数盆地地表沉积物质呈Weibull三峰分布,具有超细粒、细粒、粗粒3个组分,个别样品包含沙粒组分,绝大多数的样品匮乏细粒-超细粒组分,以高含量分选性较好的粗粒组分和低含量分选性差的细粒-超细粒组分为总体特征。现代沙尘暴天气降尘粒度分布模式较为单一,以高含量窄峰态的粗粒组分为显著特征。依据含量最高主峰所在Weibull粒度组分,本研究将全部粒度分布分为3种类型:以高含量细粒组分主导的F型、以高含量粗粒组分主导的C型和以高含量沙粒组分主导的S型。比较沙尘暴降尘、季节降尘和年降尘粒度分布模式的结果表明,研究区全年和季节大气降尘主要由沙尘暴天气降尘贡献,在细粒和超细粒组分上,同时混合有尘暴释放和正常天气条件下大气本底粉尘沉降两种来源的风尘成分。分析冬季研究区湖泊（苏干湖）冰层粉尘粒度分布模式发现,细粒组分含量显著偏高,可达70.6%,明显有别于其他沉积环境粒度组成,对比年降尘细粒-超细粒组分成因我们初步认为,细粒组分可用于指示冬季风盛行季节近地面大范围风场搬运风尘物质的状况。     

CHEMICAL WEATHERING PROCESSES ON THE VANTAGE PEAK NUNATAK,JUNEAU ICEFIELD,SOUTHERN ALASKA

On the Vantage Peak nunatak in the Juneau Icefield of southeastern Alaska, grus and soils display evidence of extensive chemical alteration in a self-evidently periglacial environment. Accompanying the alteration of bedrock to grus and soil is a decrease in grain size. Grus is dominated by very coarse sand while the soils are predominantly fine sand. Grain-size reduction is attributed primarily to mineral grain dissolution. Total chemical analyses show that alkali earths (calcium and magnesium) and alkalis (sodium and potassium) are lost as weathering progresses, while silicon and iron increase relative to resistant elements. Secondary clay minerals present in the grus and soils appear to have been derived from clay-size primary minerals. Vermiculite is the principal secondary clay mineral and appears to have formed by the alteration of biotite. Scanning electron microscopy shows that quartz and feldspars are primarily weathered by dissolution with no evidence of feldspar transformation to secondary clays. Superimposed on these weathering transformations is evidence of eolian processes. Chemical weathering processes, notably dissolution and clay mineral transformation, do occur in the periglacial environment of the Vantage Peak nunatak and are clearly an important component of the periglacial geomorphic process suite.     

Characteristics of the Gobi desert and their significance for dust emissions in the Ala Shan Plateau (Central Asia): An experimental study

Using intact samples of Gobi surfaces, we conducted wind tunnel experiments and related analyses to describe the surface characteristics of Gobi deserts and their significance for dust emissions in the Ala Shan Plateau, an area of Central Asia with high dust emissions. Under relatively high wind velocities (22 m s⁻¹), the total sediment transport approached 700 g m⁻², with the fine fractions (<50 μm in diameter) accounting for up to 26 g m⁻². In consecutive experiments, the emission rates decreased greatly due to depletion of erodible particles. In the Ala Shan Plateau, coverage by gravels varied, and aeolian sediment transport increased with increasing gravel cover (to about 30%); above that level, transport decreased. Because gravel cover was less than 30% in most areas, the gravel may not play important role in dust emissions in this region. Although the Gobi surfaces are covered by gravel, high clay contents that may restrain sediment transport. In the Ala Shan Plateau, dust emissions therefore appear to be controlled mainly by the availability of fine particles, which is in turn controlled by their deposition by ephemeral streams, by their creation via physical, salt, and chemical weathering, and by other processes such as aeolian abrasion.     

Wind tunnel test of the influence of moisture on the erodibility of loessial sandy loam soils by wind

Loessial sandy loam soils are the major soil categories in the northern Loess Plateau, China. Owing to a dry, windy climate and sparse surface cover, wind erosion is a serious problem and dust (sand) storms occur frequently. Soil moisture is one of the most important factors influencing resistance to wind erosion. The influence of moisture content on the erodibility of sandy loam soils was investigated through wind tunnel simulations. Results showed that the threshold velocity for soil particle movement by wind increases with increasing soil moisture by a power function. The intrinsic factor in the increase in soil resistance due to moisture content is the cohesive force of soil water. Cohesive forces of the film and capillary water are different; the influence of soil moisture on threshold velocity was shown to follow a step-like pattern. The wind erosion modulus of sandy loam was directly proportional to the cube of the wind velocity or the square of the effective wind velocity (V−V_t). There existed a negative exponential relationship between the wind erosion rate and soil moisture content. Initially, as soil moisture increased the decrease in the wind erosion rate was rather rapid. When the moisture content reached more than 4%, the rate of decrease in erosion slowed and became almost constant with successive increments of moisture. This suggests that different soil moisture contents can prevent wind erosion at different levels. Four percent soil moisture could only reduce the erodibility of the sandy loam soil by a small degree.     

Wind erodibility of major soils in the farming-pastoral ecotone of China

Wind erosion and desertification are severe problems in China's farming-pastoral ecotone. In this study, wind erodibility of five major soils in both uncultivated and simulated cultivated conditions, were determined through wind tunnel tests at nine wind speeds ranging from 10 to 26 m s⁻¹. The average wind erosion rate (g m⁻² min⁻¹) under the uncultivated condition (q₀) for the five soils could be set in the order: chestnut soil (28.5)>brown soil (24.8)>sierozem (21.8)>chernozem (19.9)>fixed sandy soil (11.4). The highest natural wind erosion might take place in the semi-arid steppe zone where the Chestnut soils predominate. Cultivation can significantly accelerate wind erosion, the mean wind erosion rate under the cultivated condition (q_c) for all five soils was 743.7 g m⁻² min⁻¹ in the following order: sandy soil (3313.2)>brown soil (227.2)>chernozem (221.8)>sierozem (85.1)>chestnut soil (81.2). For both the uncultivated and cultivated soil samples, the relationship between wind erosion rate (q) and wind speed (U) could be expressed in general as q=A e^BU (A and B are constant coefficients). There was a critical wind speed for each soil type except for the sandy soil. Below the critical wind speed, cultivation reduced wind erosion rate possibly due to soil clodiness and roughness effects. Above the critical speed, cultivation greatly intensified wind erosion rates due to the break down of the original soil structure. The critical wind speed measured at 20 cm above the soil surface was 20 m s⁻¹ for the brown soil, 14 m s⁻¹ for chernozem and the chestnut soils, and 10 m s⁻¹ for the sierozem. Among the five tested soils, the high wind erosion rate of the cultivated sandy soil showed its extreme sensitivity to cultivation, possibly because of the structureless nature of the loose sand. The “effect of cultivation on wind erosion” index, η (=q_c/q₀), increased exponentially with the increase of wind speed, indicating that under higher wind speed conditions, cultivation could result in more severe wind erosion.     

Eolian Dust Erosion from an Agricultural Field on the North Carolina Coastal Plain

Eolian erosion typically has not been considered a significant process on the humid southeastern coastal plain of the United States. A preliminary study of eolian erosion from an agricultural field was undertaken during the late winter of 2002 and early spring of 1999. During those times local agricultural practices leave fields bare while frontal systems produce frequent high wind events. Dust emissions were measured with two samplers; modified Wilson and Cooke passive dust traps and high-volume air samplers. Results of the study indicate that wind erosion is a significant process on agricultural fields of the North Carolina Coastal plain. Dust flux off of the field during the largest of five measured events was estimated as high as 126 kg/m with total losses of 3070 kg/ha. Atmospheric concentrations of suspended material were measured at 58,815 μgm^-3. Sediment erosion was not evenly distributed across the field. Erosion was focused over soils that are better drained. Low levels of soil moisture did not eliminate erosion but instead produced pulses of sediment emission as sustained wind continually dried then activated sequential layers of the field surface. Soil moisture and topography appear to be the primary controls on spatial erosion differences and soil characteristics likely play a secondary role.     

Organic C, N and P in size fractions of virgin and cultivated soils of the semi-arid pampa of Argentina

Nutrient and organic carbon (OC) losses are important components of the soil degradation processes produced by continuous agriculture. It was hypothesized that coarse textured soils will be more affected by cultivation than fine textured ones. The purpose of this study was to evaluate the status of OC, total nitrogen (Nt), and three phosphorous fractions (total inorganic, Pi; organic, Po; and available, Pa) in different textured virgin (under Prosopis caldenia forest) and cultivated soils of the semi-arid pampa of Argentina. Three virgin and non-fertilized cultivated soil pairs, representative from soils of the region, were selected for this study: two fine sandy loam Entic Haplustolls and a sandy Typic Ustipsamment. The element content of each soil was analysed from the <0·1-mm to 0·1–2-mm sized aggregates. Results indicated that cultivation decreased the OC, Po and Pi contents in fine aggregates of the fine textured soils, and in coarse aggregates of the coarse textured soil. Plant absorption and wind erosion were probably the main processes which decreased element contents in both textural soil types. It was speculated that in the sandy soil the elements lost from fine sized aggregates were restored by the break down of the weak coarse aggregates produced by tillage. The C/N ratios showed mostly small changes due to cultivation, indicating that no changes of organic matter quality occurred. Only the sandy soil showed C/N increases in the fine sized aggregates and decreases in the coarse sized aggregates. The C/Po quotients were not changed by cultivation, indicating that the qualitative composition of P organic compounds remained unchanged. Large OC decreases and Pa increases after cultivation detected in one of the fine textured soil were apparently linked to the occurrence of natural fires.     

呼伦湖西南部咸水湖干涸湖滨带沉积物粒度特征

沙粒是区域地表沙化的重要物质基础,以呼伦湖西南部典型间歇性干涸盐湖的湖滨带为研究对象,通过野外采样和室内测定,分析沉积物粒度组成及平均粒径、标准偏差、偏度和峰态值等粒度参数,探讨粒度与可蚀性特征。结果表明:区内干涸盐湖湖滨带沉积物属沙壤土,干涸湖滨带是潜在的风蚀粉尘释放源;粉砂和细砂是湖滨带沉积物呈现空间变化的主要粒级,从湖心到湖滨带,沉积物粒径频率分布曲线呈三峰态向双峰态渐变,地表粗化现象明显;风力搬运侵蚀是表层沉积物粒度发生变化的主要原因。受主风向影响,呼伦湖西南部咸水湖干涸湖滨带存在沿主风向扩大趋势,如进一步连通成片可能发展为呼伦贝尔沙地新的风蚀区,应该引起重视。     

陕西北部榆林第四纪地层剖面的粒度分析与讨论

本文根据榆林剖面及其粒度分析结果,讨论了剖面中地层的沉积相、古气候和环境演化.     

Effects of High-Intensity Forest Fires on Soil Clay Mineralogy

High-intensity forest fires can degrade, collapse, or completely destroy clay minerals in soils, with signatures of these changes remaining for years after the burns. To ascertain immediate impacts of high-intensity fire on soil clay minerals and mineral recovery over time, soil from the 2002 Hayman, Colorado, fire was analyzed by X-ray diffraction. Sample locations included burned soil from within the perimeter of the fire, unburned soil near the origin, and soil from adjacent historic burns. The unburned soils contain mixtures of illite, mixed-layer illite/smectite and illite/vermiculite, kaolin, and mixed-layer chlorite. Surface soils (surface-7.7 cm) contain illite, mixed-layer illite/smectite, and kaolin. Sub-surface soils (7.7-13.0 cm) contain mixed-layer illite/vermiculite, in addition to the same minerals found at the surface. Deep soils (13.0-27.0 cm) show disappearance of mixed-layer illite/smectite and illite/vermiculite and show evidence of the presence of mixed-layer chlorite. Comparisons between recently and historically burned soils and unburned soils showed slight trends in alterations of clay mineral structures in the surface soil, including alteration of the 001 illite peak, the 001 kaolin peak, and a decrease in the swelling component of mixed-layer illite/smectite. These trends indicate fire impacts the structure of soil clay minerals.     

Dust sources in arid and semiarid China and southern Mongolia: Impacts of geomorphological setting and surface materials

Analysis of the geomorphological settings and surface materials in arid and semiarid China and southern Mongolia revealed that the dominant dust sources are ‘Gobi’ deserts and alluvial fans adjacent to Gobi deserts in the piedmonts of the Kunlun, Qilian, Helan, and Gobi Altai mountains. These areas have persistent supplies of dust-sized particles, deposited by intermittent floods from the mountains, and fine materials are also generated by weathering and aeolian processes acting on the coarse fractions of Gobi surfaces. Clear spatial differences in mineral types and contents and in the salt contents of surface materials also strongly affected the intensity of dust emissions. Inconsistencies in monitoring, simulation, and calculation of dust emissions in previous studies of Northern China and Mongolia may have been caused by insufficiently detailed considerations of variations in the geomorphological setting. Where the geomorphological settings in these areas are not considered in sufficient detail, deviations arise in the estimation and prediction of dust using current models. Although the present study did not quantify the persistence and replenishment of dust sources in the study area, our conclusions about the importance of geomorphology can improve future studies of dust sources and controlling factors, improve assessments of the effects of climate change on dust emissions, and facilitate the interpretation of paleoclimate records.     

Contribution of resuspension to particulate matter inmission levels in SE Spain

Ambient air particulate matter concentrations were measured at three locations in semi-arid SE Spain during 2005-2007. Sites representative of urban and rural background levels, as well as one representative of a rural area influenced by local mineral industry, were selected.The contribution of coarse particle resuspension (mainly crustal) in the area was assessed by studying the influence of wind speed, human activity and African dust outbreaks on the daily mass concentration and the aerosol number size distribution. Wind and soil characteristics in the area, typical of many semi-arid environments, are not conducive to major dust entrainment events.Twenty-four hour PM₁₀ mass concentrations, subjected to air quality regulation, present a net decrease as wind speeds increase at the three study sites. Size-resolved measurements in the diameter range 0.25-32 μm with higher temporal resolution, however, show a net increase in the coarse particle concentrations with increasing wind speed, while the smallest particles are diluted. Although suspension is found to occur at all wind speeds, threshold values for an increase in particulate concentration can be identified and show some dependence on the particle size.African dust outbreaks, human activity and wind speed are (in this order) the main contributors for increasing particle sizes.     

西南地区水耕人为土诊断层土壤特性及SEM特征

对我国西南地区代表性水耕人为土诊断层土壤特性进行研究,并采用扫描电镜(SEM)进行土壤微形态研究,结果表明:不同母质和发育程度的水耕人为土,在土壤特性及在剖面上的变化特征都有明显差别.通过SEM观察研究表明:粗、细颗粒物质形态与矿物组成密切相关,如以薄片状、针条状等细粒物质为主的土壤,在水耕条件和外力作用下,易于定向、粘结聚合而引起土壤压实和板结.水耕人为土的微团聚体含量少而分散,对肥力的贡献有限.长期培肥的水耕人为土能够形成丰富的各类微孔隙和微结构类型,是调节土壤水、热、气和养分状况的主要机制.