جستجوی مقالات مرتبط با کلیدواژه « Wind tunnel » در نشریات گروه « آب و خاک »

Urmia Lake is the largest saltwater in the Middle East, which has been located in the northwest of Iran. Nowadays, various factors have exposed it to dryness and wind erosion, the result of which is the increase in soil salinity, the thinning of solute crystals, and the occurrence of dust storms. Identifying the nature of these dusts specially the mineralogy of them is important in providing solutions to deal with the crisis. Investigating the characteristics and mineralogy of dusts in the region are useful in predicting and controlling ways to reduce their damages, and dust mineralogy is a practical method to determine their origin. In other words, the deposition of dust in different areas can affect the nearby ecosystems by making changes in the texture, composition of elements and even the acidity of the soils of the affected areas. The source of dust is very important in the mineralogy of sedimentary particles and depends on various factors such as geology and soil characteristics as well as climatic conditions. Also, the height of atmosphere which the dusts are there can be useful in studying the affect of mineralogy in this height.

For this purpose, three flat sites without vegetation and prone to fine dust production were selected from the eastern shore of Urmia Lake, and each site was divided into 3 layers based on the height from sea level, but the first layer was omitted from the studying areas because of the high soil moisture due to low distance to lake, which results in decreasing dust production by this layer, and eventually 2 random samples (0-5 cm) were picked up from each layer. This research work carried out based on 12 selected soil samples from 3 sites and their layers. The soil samples from 0 to 5 cm depth as a surface soil of layers were transferred to trays with dimensions of 3 x 40 x 30 cm in the wind tunnel of agriculture faculty of Tabriz University, with 370cm length, 50cm width and 70cm height, and wind erosion was simulated by applying the maximum wind speed of 45 meters per second to it was done for 15 minutes at each height. Then, the dust particles released at 2 heights of the wind tunnel (10 and 30 cm from the floor of the device) and the control soil sample were subjected to XRD analysis and the obtained diffractions were interpreted using High Score software. Finally, statistical analysis was performed using a nested design to find the effect of factors such as the location, layer and height of the wind tunnel on the content of minerals in windblown dust.

The obtained results showed that the dominant minerals in the most dust samples, were quartz, calcite, halite and gypsum with the highest average percent of quartz, because of the widespread occurrences of this mineral in the earth crust. The statistical analysis results revealed the significant effect of layer on quartz at the p<0.01 level, but the effect of site on calcite was significant at the p<0.05 level. But the maximum amount of quartz appeared in layer 2 of site 2, while its minimum amount has observed in layer 2 of site 1. The maximum and minimum amount of calcite was found in site 2 and 3 respectively, but predominance of quartz in the dust samples can be attributed to the wind blow within a short distance. This differences can be related to the nature of the parent materials of sites from which soils were derived. The presence of calcite as the main mineral in samples is due to study area temperature and precipitation increase and decrease respectively, which cause lake dryness, while gypsum and halite are produced in result of chemical equilibrium process and distribute in salty soils. Gypsum also can be produced during the reaction of calcite and sulfates of sea salts. The dryness of the lake also results in rising the gypsum minerals from dried floor of lake. Also, the presence of halite is affected by Urmia Lake salts that remain after water evaporation in dried land as a main mineral in soil and dust samples of affected lands.Key Words: XRD analyze, Wind erosion, Mineralogy, Wind tunnel, Urmia Lake.

Wind erosion is one of the most important natural processes in arid and semi-arid regions. Sistan plain has a hyper-arid climate and is one of the windy regions of the country. Due to the soil characteristics of the Sistan plain, wind erosion is very intense in this region. In this study, the relationships between some soil's physical and chemical properties and wind erosion were investigated in different land cover in a part of the central region of Sistan in 2018. A map of land cover in five classes was prepared using the results of field studies and the classification of satellite images. Fifty soil samples at a depth of 10 cm were collected to investigate the physical and chemical soil properties and the wind erosion threshold was determined at each location using a portable wind tunnel device. Also, the relationship between physical and chemical soil properties including soil texture, soil moisture, specific apparent weight, EC, SAR, ESP, Na+, k+, with the speed of wind erosion threshold was investigated. According to the results, the highest and the lowest threshold speed were 8.2 and 3.8 m s-1 and occurred in agricultural lands and hilly lands, respectively. The results of this study indicated that the velocity of wind erosion threshold in different lands adjacent to sandy areas is less than the average of that cover. Also, the soil texture, EC, and SAR have the most significant effect on soil wind erodibility at P <0.05 in the study area.

The wind erosion occurs when wind speed exceeds the soil erosion threshold and plants or their residues,surface roughness, or other obstacles do not protect the soil surface. Also wind erodibility is oneof the most important determining parameters of wind erosion under certain climatic conditions. The main objective of this research was mapping of soil erodibility through empirical relationship between satellite imagery and physicochemical properties and estimation of soil erosion using a comprehensive assessment model, on the east shore of the Urmia Lake. For this research work soil sampling carried out in 153 points of three elevation classes(1271-1273, 1273-1275 and 1275-1278meters)and4supervised classification methods such as, support vector machine(SVM), Maximum likehood classification (MLC), Minimum distance and Artificial neural network(ANN)were used in classifying and mapping of soil erodibility. Soil samples physicochemical properties measured and 26 samples of them randomly were selected for wind erodibility measurement in wind tunnel laboratory. Wind tunnel experiments at a distance of 20 cm from the tunnel floor, revealed wind erodibility of 2.92 grm-2.min-1/ms-1. Also stepwise regression (SPSSv.25) results showed that among the physicochemical properties of soils, erodible fraction were the most important soil properties, which used in estimating erodibility and have a positive correlation with soil erodibility. The mean weight of aggregate diameter had negative correlation with soil erodibility and no relationship was found between soil chemical properties and erodibility. Of the4supervised classification methods, the artificial neural network has a higher capability in classifying and mapping erodibility and finally the results showed that the overall classification accuracy is57.1%.

Soil surface crusting, especially the physical type, is an important feature of dust source areas in Khuzestan province, which can have a significant impact on soil protection against wind erosion. But in most wind erosion studies, it has not been considered. The aim of this study was to investigate the effect of physical crust disturbance caused by livestock traffic on soil crusted erodibility and dust emission. For this purpose, three dominant soils with the characteristic of surface crust formation were selected from the areas and transferred to the laboratory. The soils were placed into wind tunnel trays and subjected to a simulated rainfall to form a physical crust on their surfaces. After the soils dried and the physical crust formed, the trays were exposed to different trampling by livestock's hoof including 1, 2 and 5 crossings. The trays were then transferred to a wind tunnel laboratory and threshold friction velocity (TFV), erosion rate and PM10 emission flux were measured. The results showed that erosion and dust diffusion in the control treatment (without livestock traffic) was zero in all three soil types and with increasing the number of livestock traffic from one to five times TFV decreased by 25 to 49%, and the amount soil erosion and dust emission increased between 54-317% and 80-545%, respectively. The physical crust of sandy soil was more sensitive to livestock traffic than clay loam and silt loam. The findings of this study show that the formation of physical crust in this area is a desirable and valuable feature for inhibiting erosion and dust, which acts as a natural stabilizer and protecting them can have a huge impact on dust control.

One of the most important environmental hazards that is rising drastically in IRAN in the last decade is wind erosion causing the spread of soil particles. The present study aims to evaluate the effect of soil surface moisture on the wind erosion threshold velocity in three supercritical centers of dust, located in the regions of south and southeast of Khuzestan province. For this purpose, 11 soil samples from the selected regions were collected. The soil samples were prepared in terms of moisture contents (air-dry, 4, 7, and 9%). Then they were tested in a wind tunnel based on a completely randomized design. The results of comparing the mean values of wind erosion threshold velocity using Duncan method at a probability level of 5% confirmed that the effect of moisture on the erosion threshold velocity is significant for all samples. In addition, the two parameters of moisture and erosion threshold velocity was strongly correlated in all soil samples, so that with increasing moisture levels, the wind erosion threshold velocity also increased. The findings also confirmed that the amount of increase in velocity is different depending on the levels of moisture, indicating that increasing the moisture content from air-dry to 4% level yielded the largest change in threshold velocity, which could have a substantial effect in controlling the erosion phenomenon in the study regions.

In recent years, vast areas of Urmia Lake have dried up and that's why, saline sediments appeared on the surface of the earth, which are very susceptible to erosion. This study was conducted to investigation of wind erosion and its relationship with soil physicochemical properties in eastern land of Urmia Lake. In this research, 96 soil samples were collected from an area of 414000 hectares at a depth of 5 cm based on stratified random sampling method. To measure the wind erodibility, the samples were exposed to 5 different wind velocities (9.5, 11, 12.5, 14.1 and 15 m s-1 at the height of 20 cm) in wind tunnel and finally, its relationship with soil physicochemical properties was investigated. Based on results, soil wind erodibility varied within the range of 76.69-9.98, and with a mean of 10.21 and a coefficient of variation of 94.5% have a relatively high variation in studied area. Wind erosion is significantly affected by soil physical properties, including mean weight diameter, the percent of the size classes 2-4.75, 1.7-2 and 0.1-0.25 mm of soil secondary particles (P<0.001). Based on the results, the erodible fraction (secondary particles smaller than 0.85 mm) had the most appropriate relationship with soil wind erodibility (R2= 0.75, P<0.001) and by increasing it, the soil wind erodibility increased exponentially. Considering the less costly and time-consuming in measuring erodible fraction and also less costly and time-consuming compared with measurement of soil wind erodibility in the field and wind tunnel, this index can be used as a part of soil factor in wind erosion prediction models such as WEQ and RWEQ in the studied area.

Wind erosion is one of the most important environmental issues in the world. This phenomenon could have hazardous effect on weather, water quality and human health. In recent years, the use of mulch has been considered for controlling wind erosion and soil stabilization. The aim of this study was to use Montmorillonite Nano clay, Polyvinyl Acetate polymer and wheat straw biochar as a natural mulch for controlling wind erosion and soil stabilization. A factorial experiment was conducted in a completely randomized design with three replications. The factors included (1) mulch (three types), (2) mulch concentration (Nanoclay: 0, 16 and 32, Polyvinyl Acetate polymer: 0, 8, and 16 and biochar: 0, 65 and 200 g/m2) and (3) time duration (21, 42 and 63 days). Two soil samples (sandy and loamy sand) were collected from the dust source of southeast of Ahvaz (center Number.4). Trays with a 500×300×50 mm were filled by the soils. Then, the emulsion of Nanoclay, and Polyvinyl Acetate were sprayed uniformly on the soil surface. The biochar also was uniformly mixed with the soils. The trays were placed on wind tunnel channel and wind erosion test was performed at a speed of 20 m/s for 5 minutes. Then, aggregate stability (MWD), penetration resistance (PR), shear strength (SS) and their changes were measured at all duration times. The amount of soil loss at first duration time in nanoclay, polymer and biochar treatments were decreased in sandy soil 98%, 97% and 43.65%, and in loamy sand soil 97%, 95% and 58%, respectively as compared to the control treatment. Maximum MWD, PR and SS was obtained in the nanoclay treatment. In general, the results showed that the use of nanoclay, polymer and biochar materials significantly reduced wind erosion at different duration times, compared to the control treatment.

Prediction of soil wind erodibility through soil characteristics is an important aspect for modeling soil wind erosion. This study was conducted to compare the efficiency of multiple linear regression (MLR), artificial neural network (MLP), artificial neural network based on genetic algorithm (MLP-GA) and artificial neural network based on whale optimization algorithm (MLP-WOA) for prediction of soil wind erodibility in part of eastern land of Urmia Lake. In this research, 96 soil samples were collected based on a stratified random sampling method and their physicochemical properties were measured. Additionally, the wind erodibility of soil samples was measured using a wind tunnel. Among the 32 measured soil properties, four properties including the percentages of fine sand, size classes of 1.7-2.0, and 0.1-0.25 mm (secondary particles) and organic carbon were selected as the model inputs by stepwise regression. Result showed that the MLP-WOA was the most effective method for predicting soil wind erodibility in the study area regarding to the lowest RMSE (2.9) and ME (-0.11), and the highest R2 (0.87) and NSE (0.87) values; followed by MLP-GA, MLP, and MLR. Considering the high efficiency of MLP-WOA, This method can be used as a promising method for determination of soil wind erodibility in the study area.

Wind erosion is one of the most important environmental challenges in arid and semiarid regions which cause soil loss and dust storm. In recent decades, the potential of soil erosion has been recognized as serious threat against soil sustainability. In addition, accelerated soil erosion has led to harmful environmental effects. Therefore, focus on soil erosion outcomes is necessary in order to mitigate its environmental impacts. Understanding interactions between land use management and topographical properties of landscape are important in order to effectively control soil erosion through implementing best management practices (BMPs). Application of mulch is one of the most prevailing scenarios to prevent the erosive soil against wind as an erosive factor in the hotspots. In this regard the type of much is really important because the environmental aspects and the mulch consistency are important factors for production and selection of mulch between several options. Nowadays, sustainable management is one of the most important scopes in order to achieve the aims of human healthy. In this regards the Bagasse of sugarcane and Conocarpus were selected as feedstocks to produce biochars. Biochar is the by-product of anaerobic process which called pyrolysis. The biogases, energy and so on are other outputs of pyrolysis. Another treatment which evaluated in this study was Zeoplant. Zeoplant is a super absorptive material which is able to hold the water in the soil therefore is capable to enhance the water holding capacity of the soil.      

In this study the effects of biochar of Bagasse from sugarcane, biochar of Conocarpus and Zeoplant in three levels (0, 2 and 4 percentage) and two moisture levels (25 and 50 percentage of FC) and 3 replications in randomized completely design with factorial on physical and mechanical properties of soil as indices of soil erodibility was studied. Soil sampling accomplished from Horalazim marshes and after application of treatment, incubated in tray with the size of 70×30×10 cm for 90 days. After incubations the trays located in wind tunnel in order to simulate wind erosion process under a wind with 15 m/sec speed and 2 m from soil surface. The main measured soil physical and mechanical parameters include mean weight diameter (MWD), penetration resistance (PR), tensile strength (TS), friability index (FI), shear strength, crusting index (CI), soil textural index and organic matter. The statistical analysis was performed using SAS 9.2 software and the mean comparison was accomplished with Duncan test (5 %). In order to draw the graphs Origin 2017 software was used.  

The soil texture was silty loam (SiL) including 62% silt, 26% clay and 12% sand, therefore the soil was sensitive to wind erosion. Soil organic matter before application of biochars and Zeoplant was around 1.93% and after application increased to 3.78%. Application of these treatments and the period of incubation, enhanced the soil porosity. Generally increasing soil organic matter and soil porosity and decreasing of bulk density are the main factors to increase the soil aggregation. Our results showed that all three treatments in two moisture levels significantly increased soil porosity, tensile strength and field capacity and decrease soil crusting index (P<0.01). Biochar of bagasse and Zeoplant (2%) also significantly increased shear strength whereas biochar of Conocarpus has no significant effect on shear strength. Overall the applied treatments with armoring effect (AE) and increase the soil aggregate stability, diminished the wind erosion.

Our study illustrated that application of biochar is able to improve soil physical and mechanical properties. The main aspect of this positive effect is the specific characteristics and the structure of biochar which showed with SEM (Scanning electronic microscope) images. Moreover, Zeoplant is organic-inorganic treatment and including high potential to absorb the water in the soil. Indeed, the mulching is an effective management strategy to maintain and preserve the soil against wind (as erosive agent) however afterwards a vegetation cover must be grow on the surface. Therefore some treatments such as Zeoplant are essential to hold the water in the soils of arid and semiarid regions because in those areas the water scarcity is one of the main challenges. Based on our results and evaluation of these treatments we found two main processes which are effective to mitigate wind erosion. The first is aggregation process because of organic carbon and organic matter in the soil and the binding between organic and inorganic components. The second one is an armoring effect which is originating from amendments especially biochar lumps on the surface. Finally our results confirmed the application of evaluated treatments to preserve the erosive soil against wind.

In this study, a wind tunnel was used to provide a spatial and temporal model of salt emission. Obtainedregression model from wind tunnel test data on the salt of Urmia Lake evaluated with the empirical Chepil equation. In this experiment, four variables were used: wind speed, distance from the salt source, duration of wind blowing and Salinity (EC), where the variation of EC is dependent on the other three variables. The values of coefficient of determination, root mean square error and relative error between the observational and computational data of regression model are 0.96, 79.12 μs/m and 0.1065 respectively. According to the evaluation criteria, it can be concluded that the accuracy of the proposed regression model is very good for evaluating the salt transfer at different times and distances and various wind speeds. The obtained salinity data from the experiment were turned to salt mass using weighing test, which is explained in the text of the paper, and were placed in the Chepil equation. The values of coefficient of determination, root mean square error and relative error between the observational and computational data are 0.70, 0.04 kg and 63.4359 respectively. According to the obtained evaluation criteria, we can use the suggested equation by experimental data, which was evaluated by Chepil equation on a larger scale by accepting some errors.

In this study, temporal and spatial model of salt transfer was investigated using a wind tunnel testing. In this model, four variables namely time, distance from the salty source, salinity and wind speed were used in which salinity was considered as a dependent variable and the others remained variables were considered as the independent variables. By use of this model, the amounts of measured and calculated EC were compared with the regression model output. The evaluation criteria used here were coefficient of determination (R2), root mean square error (RMSE) and mean absolute error (MAE). The values of these criteria were obtained as R2=0.96, RMSE= 79.14 µs/m and MAE= 46.15 µs/m. According to the mentioned criteria, it could be concluded that the precision of the proposed regression model in evaluating the salt dust transfer in different spaces and times as well as in different wind velocities was very good.

Wind erosion is the most important agent of environmental degradation, poverty of soil, air pollution and the dust spread. Wind erosion is causing a lot of damage to crops, buildings, facilities and vehicles. The first step of the wind erosion control is the stabilization of soil particles. Soil stabilizing methods to control wind erosion can be classified into mechanical, biological and chemical stabilization. Mechanical soil stabilization type is relatively time-consuming and costly. Biological stabilization is a traditional way that exhibits a long-term validity but sandy soil cannot provide essential water and nutrition elements needed by plant. Recently, chemical stabilization such as high-molecular-weight anionic polyacrylamide (PAM) has attracted the attention of researchers because of its advantages in easy and quick construction, and the improvement of the growing conditions for plant. However PAM has been mainly used to control water erosion and there is still little available information regarding the effectiveness of PAM on preventing soil loss by wind erosion. The main objective of this study was to investigate the feasibility of using PAM in wind erosion controlling. Also, effects of PAM on some soil physical and chemical properties and their temporal variability were evaluated.
In this study polyacrylamide polymer was used as a restoration of soil and soil structure stabilizer on sandy soil of Azadegan Plain (Khuzestan province, Iran). Consequently, an experiment was conducted as factorial based on completely randomized design with three replicates. The experimental treatments were consist polyacrylamide polymer (PAM) at three levels (0, 0.5, and 1 %), soil moisture at two levels (80% FC and dry) and time duration at three levels (15, 30 and 45 days). The emulsion of PAM was sprayed homogeneously on the soil surface. After passing each time treatment, penetration resistance and some physical and chemical properties of soil was measured. Finally after doing all measurements, the treatment with maximum penetration resistance were selected and the sample was prepared for wind tunnel testing. The wind erosion experiments were conducted in a wind tunnel. Soil samples were located in removable trays. The width and length of the trays was 30 and 100 cm, respectively. The wind erosion experiments were performed under wind velocity of 12 m s−1 according to the actual situation of study area.
The results indicated that in comparison to control, soil acidity decreased at both levels of the polymer with increasing time. The decreasing of soil acidity in wet treatments was more than dry treatment. The lowest amount of pH was obtained in the 30-day wet treatment at 1% polymer level. The results show from the 30th day onwards, soil pH increased, which is probably due to the polymer degradation. With passing time, soil electrical conductivity (EC) at both levels of the polymer (0.5 and 1%) increased and decreased respectively after 30 days. These observations are probably because after 30 days the properties of polymer-hydrophilic units gradually decrease and water adsorption was reduced or that soil soluble salts were adsorbed by polymer particles. The results also showed with passing time, Mean Weight-Diameter of Soil Aggregates (MWD) increased and then after 30 days declined. The largest MWD was observed in 30 days treatment at 1% polymer level. After thirty days, its effect has probably diminished due to polymer degradation. Furthermore, the results showed no significant difference of bulk density among treated soil with different level of polymer, but application of polymer caused to decrease bulk density comparison to control. Polymer application increased soil penetration resistance significantly. Using 1% of polymer increased it to 6 kg/m2. The results also indicated that the soil resistance at first increased with time and then decreased significantly. The amount of soil penetration resistance at 45-day was less than 15-day. The results of wind tunnel with a maximum 12 m/s wind velocity showed that application of the polymer reduced the erosion of sands samples to zero.
The research results indicated that PAM application increased soil penetration resistance and MWD. The polymer could improve the structure of soil aggregates and increase the amount of dry-stable aggregates and therefore decrease soil bulk density. Spraying PAM solution on the surface of soil significantly decreased the wind erosion amounts. Therefore, this inexpensive and easily usable polymer can be considered as a soil stabilizer to control wind erosion in arid and semiarid areas.

Wind erosion is known as one of land degradation aspects in arid and semiarid regions. Soil moisture affects the erosion rate through controlling threshold velocity and soil erodibility. This study was done to investigate the effect of initial soil moisture content on wind erosion rate and threshold velocity as well in two soils with different texture classes using a wind tunnel facility under controlled conditions.
The experiment was conducted in two separate tests on two soils with different texture classes, each as factorial based on completely randomized design. Two factors including wind speed and initial moisture each at three replications were applied for each soil. For this purpose, two soils with different texture classes of sandy loam and sandy were chosen, afterwards various levels of initial moisture were produced for each soil and then were exposed to several wind speeds. Regarding to the different texture and erosion threshold of the soils, three levels of initial moisture contents of 1.5, 6.5 and 11.5% for sandy loam and 1, 2.5 and 4.5% for sandy soil were produced. After preparing the soil samples and placing in the wind tunnel, wind velocities of 5, 7.5 and 10 m s–1 at 10 cm height were generated and finally, sediment yield due to wind erosion was measured. In addition, the threshold wind velocity was determined through the observation method. Furthermore, the critical moisture content for each soil was determined based on the minimum moisture amount in which a significant reduction in erosion rate was observed.
Results of this study showed that with increasing wind speed and initial moisture content, wind erosion rates increased and decreased, respectively. The measured erosion rates for sandy loam and sandy soils ranged from 0.015 to 0.768 and 0.086 to 14.088 g m–2 min–1, this difference was attributed to the soils primary and secondary particle size distribution. With increasing the soils moisture content, the threshold wind velocity increased as a power function. The critical moisture contents of 6.5% and 4.5% were determined for sandy loam and sandy soils, respectively. With increasing moisture content in sandy loam soil from 1.5 to 6.5%, at wind velocities of 5, 7.5 and 10 m s–1, the erosion rate decreased by 64.6, 80.7 and 62.9%, while with increasing moisture content from 1.5 to 11.5%, it was reduced by 82.3, 90.8 and 77.5%, respectively. These reduction values for sandy soil for the increase in moisture from 1 to 1.5% were 27.7, 32.8 and 71.3% and for the increase in moisture content from 1 to 4.5% were 92.2, 86.6 and 93.9%, respectively.
The findings of this research revealed the importance of maintaining and or increasing in soil moisture to combat wind erosion, so that due to soil moisture increase, wind erosion rate can be restricted by 90%. It was concluded that the critical value of moisture content differs in various soils. This critical value for sandy soil (4.5%) was lower than that in sandy loam soil (6.5%). By reducing wind speed toward values less than the threshold velocity, in addition to soil management strategies to maintain and improve soil moisture, wind erosion can be reduced, remarkably.

Quantitative measurement of aeolian dust may help properly monitor and control the wind erosion. The aim of this study was to evaluate the efficiencies of four aeolian dust samplers including the modified Wilson and Cooke sampler (MWAC), cyclone dust sampler with cone (CDSC), cyclone dust sampler (CDS), and marble dust collector (MDCO) in comparison with the big spring number eight sampler (BSNE) in different velocity rates and particles sizes. For this purpose, MWAC, MDCO, BSNE were simulated and CDSC and CDS were designed and constructed. The relative efficiencies of the CDSC, CDS, MWAC, and MDCO were evaluated for the 80, 137, 260 micron diameter particle sizes (D50) in 2-7 ms-1 velocity by wind tunnel. The results showed that relative efficiency of CDSC is higher than CDS, MWAC, and MDCO as a consequence of the wind speed. CDSC and CDS relative efficiencies varied in relation to wind velocity, but MWAC, MDCO relative efficiencies remained constant. Also, CDSC, CDS, MWAC, MDCO relative efficiencies varied from 0.8, 0.48, 2.18, 0.58 times by increasing the particle size diameters from 80 to 260 micrometers, respectively.