Thirteen years of aeolian dust dynamics in a desert region (Negev desert, Israel): analysis of horizontal and vertical dust flux, vertical dust distribution and dust grain size

ARTICLE IN PRESS Journal of Arid Environments Journal of Arid Environments 57 (2004) 117–140 www.elsevier.com/locate/jnlabr/yjare

Thirteen years of aeolian dust dynamics in a desert region (Negev desert, Israel): analysis of horizontal and vertical dust flux, vertical dust distribution and dust grain size Zvi Y. Offera, Dirk Goossensb,c,* a

Desert Meteorology Unit, Center for Environmental Physics, The Jacob Blaustein Institute for Desert Research, Ben-Gurion University of the Negev, Sede Boqer Campus 84990, Israel b Laboratory for Experimental Geomorphology, Katholieke Universiteit Leuven, Redingenstraat 16 bis, B-3000 Leuven, Belgium c Erosion and Soil and Water Conservation Group, Wageningen University and Research Centre, Nieuwe Kanaal 11, NL-6709 PA Wageningen, The Netherlands Received 9 September 2002; accepted 9 May 2003

Abstract At Sede Boqer (northern Negev desert, Israel), aeolian dust dynamics have been measured during the period 1988–2000. This study focuses on temporal records of the vertical and horizontal dust flux, the vertical distribution of the dust particles in the atmosphere, and the grain size of the particles. This study extends results reported earlier on the characteristics of the airborne dust concentration, the accumulation of dust on the desert floor, and the highmagnitude dust events. Vertical dust flux showed a systematic trend over the year, with high values in spring and low values in autumn. Vertical flux was very low from 1989 to 1991 and from 1997 to 1998, and moderately high or high in the other years. Horizontal dust transport, which was quantified by means of the horizontal dust flux, was also highest in spring and lowest in autumn and in winter. During the period 1988–2000, the horizontal dust flux value oscillated systematically with a periodicity of 30 months. At Sede Boqer the amount of dust in the atmosphere always decreased with height. Vertical stratification was strongest during the first months of the year and lowest at the end of the year. Stratification was very low in 1989– 1991, high in 1988 and between 1992 and 1996, and moderate in the other years. The size of the dust (quantified by means of the median grain diameter) was also highest in spring and lowest in autumn. Median grain diameter was around 30 mm in 1988–1989, in 1995–1996, and in 1999–2000; in the other years it was considerably lower, around 25 mm or less. The average *Corresponding author. Tel.: +32-16-32-64-36; fax: +32-16-32-64-00. E-mail address: [email protected] (D. Goossens). 0140-1963/03/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0140-1963(03)00092-2

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annual vertical dust flux curve parallels the average annual wind speed curve. A similar relationship was observed between airborne dust concentration and vertical dust flux, but only for the background concentration. At Sede Boqer, a negative relationship exists between the amount of rainfall and the amount of vertical dust flux. This relationship is not only apparent during the rainy season itself, but also when the amount of rainfall during a rainy season is compared to the amount of vertical dust flux during the next dry season. The dust crust on the desert floor, which is much stronger after rainy seasons with high precipitation than after rainy seasons with low precipitation, is most probably responsible for the latter relationship. r 2003 Elsevier Ltd. All rights reserved. Keywords: Aeolian dust; Vertical dust flux; Dust transportation; Dust grain size; Dust time-series

1. Introduction Although there is a long history of reporting dramatic dust transport and deposition events, it is only during the last two decades that aeolian dust has become a major environmental topic and that a more structured and systematic approach to aeolian dust research has been developed (McTainsh, 1999). As a consequence, many current aeolian dust projects were launched only a few years ago, most since the early 1990s. Hence, much of the data currently available is short-term, restricted to only a few years. Medium-term and especially long-term data on contemporary dust processes remains scarce to very scarce. This is especially true for quantitative data. Qualitative information on dust is more abundant, in both older and more recent literature. Although largely descriptive, the records of dust fall events such as dust rains or dust storms are useful indicators because they indicate the occurrence, frequency and intensity of the dust activity. A detailed overview of older and modern dust records, including a summary of studies on airborne dust concentration measurements and on aeolian dust deposition and accumulation rates was published recently by Offer and Goossens (2001a). Many of these studies contain quantitative information, although the majority only deals with short-term (often event-based) observations and measurements and are not really suitable for long-term statistics. Offer and Goossens (2001a) carried out a medium to long-term (10-year) study on modern dust dynamics, based on quantitative measurements performed at Sede Boqer, in the northern Negev desert of Israel. That study discussed the characteristics and temporal evolution of airborne dust concentration, dust deposition and the high-magnitude dust events (dust storms as well as dust hazes). The present study completes the data set by investigating the vertical and horizontal dust flux, the vertical distribution of the dust in the atmosphere, and dust grain size. Some comments on dust emission are also provided. Direct measurement of the emission of fine-grained dust from a soil is one of the most problematic issues in aeolian dust research. In most studies, emission is estimated indirectly, by (1) using the vertical gradient of airborne dust concentration

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(Stetler and Saxton, 1996; Nickling et al., 1999; Saxton et al., 2000), (2) by measuring the horizontal sediment flux and calculating the sandblasting efficiency, which is the ratio of the vertical to the horizontal sediment flux (Alfaro and Gomes, 2001; Rajot and Valentin, 2001; Gomes et al., 2003), or (3) by trapping the emitted particles downwind of the eroded area and dividing the amount of sediment by the eroded field length (Hagen, 2001). Direct measurements of soil dust emission are very scarce, although Offer and Goossens (1994) and Goossens and Offer (1997) have reported on some experimental work. With the exception of a study by Goossens et al. (2001), who investigated the emission of soil dust in an agricultural area in northern Germany during a period of 15 months, published reports cover short-term periods only (usually event-based), e.g. Nickling and Gillies, 1993; Gillies et al., 1996; Stetler and Saxton, 1996; Niemeyer et al., 1999; Nickling et al., 1999; Saxton et al., 2000; Hagen, 2001. Other reports either describe the emission in a more regional context, without emphasis on the temporal changes in emission (e.g. Golitsyn et al., 1997; Xuan et al., 2000), or largely deal with human-induced dust emissions, for example, on unpaved roads (Moosmuller et al., 1997; Cowherd et al., 1998; Chang et al., 1999), hence they are not really suitable for investigating natural emission. Directly measuring the horizontal transportation of airborne dust (horizontal dust flux) has become a standard procedure in recent wind erosion research, although many studies measure total sediment, i.e., not only the dust fraction (defined in this study as the particle fraction o63 mm). The large majority of the data is for shortterm periods (events) only (e.g. Stout, 1990; Stout and Zobeck, 1996; Stetler and Saxton, 1996; Niemeyer et al., 1999; Saxton et al., 2000; Hagen, 2001). With the exception of the study by Goossens et al. (2001) mentioned above, all medium-term and long-term continuous quantitative data on horizontal dust transport has been obtained indirectly, for example via measurements of the dust content in lakes (Xiao et al., 1997; Kanfoush and Hodell, 1998; Yamada and Fukusawa, 1999), in oceanic sediment cores (Clemens and Prell, 1990; Hovan et al., 1991; Ruddiman, 1997), or in ice cores (Petit et al., 1981; Lunt and Valdes, 2001). In a study by Leys and McTainsh (1996), horizontal dust flux was measured during a period of 22 weeks at a dust station in Australia. However, Table II of their paper provides data for only 8 of these 22 weeks. Data on grain size and grain size distribution of aeolian dust is abundantly available in the literature. Here again, most of the data is for short-term periods (usually dust storms, or other events of high dust activity) (Nickling, 1983; Nickling and Gillies, 1993; Gillies et al., 1996; Goossens, 1996; Chen and Fryrear, 1996; McTainsh et al., 1997; Nickling et al., 1999; Niemeyer et al., 1999). Medium-term data have been published by Goossens and Offer (1995) (Israel, 24 months), Offer and Goossens (2001b) (Israel, 36 months) and Goossens et al. (2001) (Germany, 15 months). For southern Nevada and California, Reheis and co-workers collected data for a period of 17 years (1984–2001), but the data set is still in preparation although some of data was published in a paper by Reheis and Kihl (1995). As for horizontal dust flux, published long-term information for dust grain size is only available via indirect studies, for example, dust in oceanic sediment cores (Arnold, 1996; Clemens,

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1998; Ratmeyer et al., 1999); in ice cores (Steffensen, 1997; Zdanowicz et al., 2000; Unnerstad and Hansson, 2001), or in loess deposits (An and Porter, 1997; Fang, 1999; Zhongli et al., 2000; Chen et al., 2000; and many others). Arimoto et al. (1997) performed grain size measurements of airborne dust collected over the Atlantic Ocean and the Pacific. Zhang et al. (1998) published a general study on (among other things) the size distribution of mineral aerosol over Chinese desert regions. In this paper we investigate temporal changes in the vertical and horizontal flux and in the grain size characteristics of airborne dust at Sede Boqer station from 1988 to 2000. The aim is to see whether there are systematic trends in the evolution of these aspects, whether such trends can be correlated to the evolution of meteorological parameters in the same period, and whether the trends correspond to trends observed previously for airborne dust concentration, dust deposition, and high-magnitude dust events. Our goal is to establish a general picture of the evolution of the airborne dust dynamics in the northern Negev desert over the 13 years investigated.

2. Description of the Sede Boqer region A report by Goossens and Offer (1988) contains a detailed geographical and climatological description of the region near Sede Boqer. A summary of the most important aspects follows. Sede Boqer station is located in the northern Negev desert of Israel, at an altitude of 470 m (Fig. 1). It is located in the middle of a large plain surrounded by hills to the west, north and east, and by a canyon to the south. The major part of the region consists of stony hamadas, developed on limestone and dolomite formations. Topography is determined by both large and small plateaus and by wadi depressions. The topsoil near Sede Boqer is composed of fine loessial sediments, mixed with silex and chert pebbles. On the hills and in the canyon, the loess is thinner and forms a thin surface crust on the limestone bedrock. Vegetation in the region is scanty. It is mainly composed of small desert shrubs. Average annual rainfall at Sede Boqer is about 100 mm, falling mostly between October and May. The climate of the northern Negev is affected by the subtropical high pressure belt mainly during the summer, and by mid-latitude low pressure systems during the winter. The predominant winds blow from the north, northwest and west. Annual average temperature is 18.3
C, with large differences between winter and summer, and between day and night. Summers are hot and dry with daily sea breeze circulation. Winters are wet and cool with considerable cyclonic activity. During the summer, no intense cyclones affect the region, and thus no intense dust storms are observed. Most dust storms are associated with migrating synoptic systems and with wind directions from the westerly (sometimes also from the easterly) quadrant. Wind speeds during these extreme situations are high, and relative humidity low.

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Fig. 1. Location of Sede Boqer experimental station. Distribution of the average annual rainfall is also shown, in the left picture.

3. Data collection and calculation 3.1. General information All measurements were conducted at the meteorological station of the Jacob Blaustein Institute for Desert Research, Sede Boqer. In 1988, the station was still located inside the institute’s campus. In December 1989 it was moved some 300 m to the northwest into the open desert. The area is flat, almost devoid of vegetation, and open to all directions except the south (due to the buildings of the institute). All

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meteorological data obtained before January 1990 were recalculated according to the new location so that any effects of the relocation are eliminated from the data sets. 3.2. Measurements Wind speed and direction were measured using standard cup-anemometers and a wind vane. Average speed and direction were analysed for each hour of the period 1988–2000, at a height of 3.5 m above the desert surface. Rainfall was measured on a continuous basis, using a standard tipping bucket gauge. Atmospheric dust was collected monthly near ground level using plastic trays filled with two layers of glass marbles (marble diameter: 1.6 cm). Rectangular trays 47-cm long, 31-cm wide and 11-cm high were used between January 1988 and June 1990, but in July 1990 all traps were replaced by circular trays 11-cm high and 39-cm diameter. Additional samples were collected each month from trays mounted on a mast at heights of 0.5, 1, 2, 3, 4, and 5 m above ground level. Grain size distribution of the collected dust was determined with a Malvern Mastersizer (type: S). All analyses were performed in an aqueous environment, but care was taken to ensure a minimum level of dispersion of the samples. To exclude all sand, particles >63 mm were eliminated from the analysis. In order to calculate the horizontal dust flux, air dust concentration was measured with a Sierra Ultra High Volume Dust Sampler at a height of 1 m above the surface. The sampler measured total suspended dust (not only PM10 dust). Dust concentration was measured for periods of 12 h (1988–1993), 24 h (1994–1997) or 72 h (1998–1999). No measurements were made between July 1999 and December 2000. 3.3. Calculations Vertical dust flux at ground level was estimated by first calculating the vertical flux, Fv, at various heights ranging from 0.5 to 5.0 m and then extrapolating the flux curve to zero level. For sufficiently small heights, when buoyancy forces remain small, Fv can be calculated as: Fv ¼ kuabzb ;

ð1Þ

where k is von Karman’s constant (0.4), u the friction velocity, z the height, and a and b empirical constants which are derived from the vertical dust concentration profile (Goossens et al., 2001). Strictly speaking, Eq. (1) holds for neutral atmospheres only. For non-neutral atmospheres, the error remains o10% for zo5 m (see Rasmussen et al., 1985 for more information). The vertical dust flux at ground level as calculated above is a measure for dust emission though it is not necessarily equal to emission. It is the net flux, equal to the total (gross) upward flux of the dust minus dust deposition (gravitational as well as turbulent deposition). Horizontal dust flux, Fh, was calculated as the product of dust concentration and wind speed. Fh was calculated at a height of 1 m above the desert floor. The dust

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concentration values at 1 m were directly available from the Sierra instrument; wind speeds at 1 m were calculated from the wind profile measured at Sede Boqer meteorological station.

4. Results 4.1. Meteorology (wind and rain) 4.1.1. Wind The winds at Sede Boqer blow predominantly from the N, NW and W (Fig. 2). Winds from the E, SE and S blow almost exclusively during the night and are generally weak, whereas much stronger winds blow from the opposite quadrant during the day. Fig. 3A shows the variation of wind speed over the year. The data represent averages for the entire test period 1988–2000. In general, wind speed is highest in summer and lowest in winter. The relatively high values in February and March are mainly caused by the high nocturnal winds that occur at Sede Boqer in this part of the year (see Offer and Goossens, 2001a for more information). Fig. 3B shows the variation of monthly average wind speed during the 13-year test period. Apart from the normal variation within a year, wind speed remained fairly

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constant, except for the first half of 1990 and the second half of 2000, which were characterised by significantly lower wind speed. The slight overall decrease in wind speed that can be observed in the curve may not be significant since it is most probably caused by normal wear of the rotating cup-anemometer. 4.1.2. Rain In Sede Boqer the rainy season starts in October-November and ends in March– April (Fig. 4A). The figure shows that there can be very large differences in the amount of rainfall between months, even within a single rainy season. The first

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5 years of the test period (until March 1992) were rather wet, with rainfall values of over 90 mm (Fig. 4B). For the next 9 years the precipitation amounts tended to decrease, although rainfall was >80 mm during several of these years. Average annual rainfall for the period 1988–2000 was 91.7 mm. 4.2. Vertical dust flux (at ground level) To get a general idea of the vertical dust flux at Sede Boqer, we first calculated the frequency distribution (Fig. 5A). The figure shows the average for the period

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January 1988–June 1999, as no dust concentration measurements were made after June 1999. The lowest class (0–0.1 mg m2 s1) accounts for more than one-third of the 135 cases for which data are available, and the proportion of classes decreases rapidly as vertical dust flux increases. The distribution is thus far from normal. Average vertical dust flux for the total period was 0.18 mg m2 s1, which is equivalent to 5.6 g m2 yr1. Vertical dust flux over the year shows a clear and systematic trend, with highest vertical flux at the end of spring and lowest vertical flux at the end of autumn (Fig. 6A). The peak in vertical flux in February is caused by the high frequency of dust events with local emission during this time of the year (see Fig. 21 in Offer and Goossens, 2001a). Monthly average vertical dust flux also changes over the period 1988–1999 (Fig. 7A). After the high flux values in 1988, very low fluxes occurred between 1989 and 1991. Vertical dust flux increased from early 1992 to mid-1994; it stayed high until early 1997 (two local minima occurred in 1995), and then sank to a minimum near the beginning of 1998. From then on it increased until the end of the measurements in June 1999. The occurrence of heavy dust storms, with strong local emission, has only a limited—if any—effect on the temporal evolution of the vertical dust flux curve. Heavy dust events occurred at Sede Boqer in Jan 1988 (not shown in the figure since no flux data were available), May 1988, Nov 1991, Sep 1993, Apr 1994, Nov 1994, Feb 1996 and Mar 1999. Although some coincide with vertical dust flux peaks, these events do not affect the general picture of the flux curve.

4.3. Horizontal dust flux The frequency distribution of the horizontal dust flux at 1 m height is calculated as the average for the period Jan 1988–Jun 1999 (Fig. 5B). The distribution is nearly normal, with 200–300 mg m2 s1 as the modal class. Average horizontal dust flux for the total period was 264.55 mg m2 s1, which corresponds to 8.3 kg m2 yr1. The monthly horizontal dust flux during the year shows that highest dust transport occurred at the end of spring and lowest transport in winter (Fig. 6B). Although the curve in Fig. 6B is more irregular than the one in Fig. 6A, the overall patterns are similar. The monthly average horizontal dust flux shows trends that are similar to those of vertical dust flux (see Fig. 7A), except for the period 1990–1992, when much higher horizontal fluxes were recorded compared to vertical flux. This is mainly because of the very high nocturnal wind speeds that were recorded at Sede Boqer in 1990 and 1991 (monthly average speeds of 3 m s1 and more, whereas the normal nocturnal value is around 2 m s1: see Fig. 5 in Offer and Goossens, 2001a). No increase in wind speed was observed during the daytime hours in 1990 and 1991. Furthermore, the airborne dust concentration showed a maximum in 1991, which directly affects the horizontal flux value.

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Fig. 6. Variation of dust parameters over the year. (A) vertical dust flux at ground level; (B) horizontal dust flux at 1 m height; (C) exponent b; (D) median dust diameter D50.

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Fig. 7. Variation of dust parameters during the period 1988–2000, based on monthly averages. Thick black line is the double 5-year moving average. (A) vertical dust flux at ground level; (B) horizontal dust flux at 1 m height; (C) exponent b; (D) median dust diameter D50.

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4.4. Vertical distribution of the dust in the lower atmosphere Because dust was collected at various heights above the desert surface, it is possible to investigate the vertical distribution of the dust in the atmosphere. In normal circumstances the amount of dust (quantified by, for example, dust concentration) decreases as a function of height according to a power function of the form C ¼ azb ;

ð2Þ

where C stands for the concentration, z for the height, and a and b are regression parameters (Nickling, 1978). The exponent b quantifies the rate of concentration decrease (bo0) or increase (b>0) with height and is thus a good indicator of the vertical distribution of the dust in the atmosphere. The more b differs from zero the more the atmosphere is sedimentologically stratified; when b=0 equal amounts of dust occur at all heights and there is no sedimentological stratification. b-values were calculated for all 156 months of the period 1988–2000. The frequency distribution of b-values is almost normal (Fig. 5C), with an average b-value of 0.45, although the proportion of the class 0.4>b>0.5 is somewhat lower than that of the adjacent classes. All b-values measured at Sede Boqer were negative (at least, their monthly averages); thus, the amount of dust in the atmosphere at Sede Boqer decreased with height. The average monthly b-value shows changes similar to those of vertical and horizontal dust flux (Fig. 6C). b-values are lowest (recall that b is negative) at the end of spring and highest in autumn and in winter. There is, therefore, a significant difference between the warm and the cold season: in the cold season there is substantially less stratification in the lower atmosphere (at least, with respect to the dust) compared to the warm season. Changes in b-values measured over the period 1988–2000 (Fig. 7C) correspond very closely to that of vertical dust flux in Fig. 7A. 4.5. Grain size of the dust The grain size of dust samples (collected at ground level) at Sede Boqer has a skewed frequency distribution, as shown by the median grain diameter D50 (Fig. 5D). The class 30–31 mm is the modal class, and a secondary peak is visible between 22 and 24 mm. All dust samples had a median grain diameter below 34 mm. Recall, however, that only the dust fraction (o63 mm) has been analysed. The average monthly median grain diameter shows a systematic pattern with coarse dust in spring and fine dust in autumn (Fig. 6D), although the differences in the D50-values are small. The temporal evolution of the changes in median grain diameter over the period 1988–2000 (Fig. 7D) show some similarity with the vertical dust flux (and b) curves in the Figs. 7A and C. However, there are also important differences, for example during the first and the last years of the test period. D50-values are around 31 mm in 1988 and 1989, decrease to a minimum of about 23 mm in 1992, systematically

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increase to a maximum of nearly 30 mm in 1995–1996, decrease again to a minimum around 23–24 mm in 1998, and increase to 29 mm at the end of the experimental period.

5. Discussion Systematic trends in patterns of vertical dust flux, horizontal dust flux, the vertical distribution of dust in the atmosphere and the size of the settled dust were observed during the period 1988–2000. We now look for relationships between these parameters and other dust parameters measured at Sede Boqer (Offer and Goossens, 2001a), and for relationships between dust, wind and rain. 5.1. Vertical dust flux at ground level and wind speed As mentioned earlier, the vertical dust flux at ground level can be seen as a measure for dust emission (though it is not necessarily equal to emission). A relationship between dust emission and wind speed seems logical, because the wind lofts the particles from the ground. However, a one-to-one relationship between these parameters should not necessarily be expected for low wind speeds, in particular wind speeds below the deflation threshold. Moreover, other factors such as soil moisture, vegetation, or soil crusting, affect the relationship between dust emission and wind speed and their effects change with time. Thus, emission values probably will not correspond to wind speeds over short intervals. We compare the average annual vertical dust flux (at ground level) curve and the average annual wind speed curve (Fig. 8). Both parameters have similar temporal patterns: in months with high wind speeds the vertical dust flux at ground level is also high, and in months with low wind speeds the vertical dust flux at ground level is also low. 0.60

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5.2. Vertical dust flux and dust concentration A positive relationship between vertical dust flux and concentration is also logical. However, as with vertical dust flux and wind speed, external factors may seriously affect the relationship between vertical dust flux and dust concentration. Probably the most important of these are the dust hazes, i.e., events during which large amounts of dust, which were eroded elsewhere (probably very distant from the measuring site), are travelling over the site. There is no local emission during a dust haze, although airborne dust concentration during a haze may be comparable to that in a locally generated dust storm. About 30% of the dust events in Sede Boqer are dust hazes (Offer and Goossens, 2001a); in some years, however, hazes are more abundant than dust storms. Thus, vertical flux values and concentration values probably will not match over short time intervals. Here, we plot the average annual vertical flux curve and the average annual concentration curve together (Fig. 9). In another complicating factor, Goossens and Offer (1995) and Offer and Goossens (2001a) showed that the monthly average value of dust concentration may be strongly affected by only a few, sometimes even a single, high air dust concentration events (for example, a dust storm). Seasonal trends in dust concentration, which are correlated to natural meteorological variation, may thus become masked by such events. The exact criterion to consider an event as an ‘‘event of abnormally high air dust concentration’’ is somewhat uncertain as the situation may vary from region to region, but the studies by Goossens and Offer cited above have shown that 200 mg m3 is a good criterion for the northern Negev desert. Thus, it is useful to distinguish between the real concentration (that is, the true concentration as measured by the dust concentration meter in operation) and the background dust concentration (the average concentration over a given period, for example one week, or one month, but with exclusion of the events with abnormally high air dust concentration that occurred in that period). The average annual vertical dust flux curve and the average annual dust concentration curve are similar during the first half of the year, but differ during the second half of the year (Fig. 9A). However, if the diagram is replotted for the background concentration (Fig. 9B), the relationship between vertical dust flux and concentration becomes visible. In months with high vertical dust flux the background dust concentration is also high, and in months with low vertical dust flux the background dust concentration is also low. One reason why the relationship is more pronounced for the background concentration than for the real concentration is that all dust hazes (and also all dust storms) are excluded from the analysis when the background concentration is used. As explained earlier, highmagnitude events (dust storms as well as dust hazes) may seriously mask the seasonal trends that exist in dust concentration. The differences between real dust concentration and background dust concentration are smallest in the summer months (June, July, August) due to the absence of dust events during this time of the year. This explains why the deviations between the vertical dust flux curve and the dust concentration curve in Fig. 9A are most pronounced in summer.

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5.3. Dust deposition and grain size Because the mass of a (spherical) dust particle is proportional to the third power of the particle’s diameter, an increase in particle size automatically results in a higher dust deposition value. Fig. 10 shows the average annual curves for deposition (data from Offer and Goossens, 2001a) and median grain diameter (as a representative for grain size). During the first 6 months of the year both dust deposition and grain size of the deposited dust generally increase with time. In the period when dust deposition is highest (spring), the deposited dust is also most coarse. The late summer and early autumn months show little change in either parameter and are thus more or less similar. An important anomaly between dust deposition and dust grain size is observed in November and January however. The high grain sizes in

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November can be explained by the occurrence of two very heavy dust storms, with very intense local erosion, in November 1992 and November 1994. We found no plausible explanation for the low grain sizes in January. 5.4. Vertical dust flux and rainfall A negative relationship between rainfall and vertical dust flux at ground level at Sede Boqer is expectable, because high (and frequent) rainfall results in high moisture content in the topsoil, leading to an effective protection against wind erosion. Rainfall can also affect the emission during the subsequent dry season, because rainfall influences the status of the vegetation during the dry season and, probably more important in a desert environment, also that of the topsoil. Offer and Goossens (2001a) showed that there is a pronounced relationship between the amount of rainfall in a rainy season and dust activity in the subsequent dry season in the Sede Boqer region. High rainfall results in low dust activity (low airborne dust concentrations, low dust accumulation, low number of high-magnitude events), low rainfall in high dust activity. It was assumed that the surface crust that forms at the end of the rainy season is largely responsible for the reduced dust activity in the subsequent dry season. Since vertical dust flux values are available from the present study, it is possible to check whether, and in which way, rainfall affects the vertical dust flux at ground level (as a measure for dust emission). Fig. 11 shows vertical dust flux at ground level as a function of rainfall for all years for which data are available. The upper figure shows the vertical flux during the rainy season itself, the lower figure the vertical flux during the subsequent dry season. Both figures show that, in general, vertical dust flux at ground level decreases with increasing rainfall. However, to analyse the relationship between vertical dust flux and rainfall correctly, it is necessary to distinguish between the dust storm years

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rainfall amount (mm) Fig. 11. Vertical dust flux at ground level as a function of rainfall. Upper figure: vertical dust flux during the rainy season; lower figure: vertical dust flux during the subsequent dry season. ’, dust storm year; &, dust haze year.

(many local dust storms) and the dust haze years. The dust storm years during the 11-year period for which sufficient dust concentration data are available were: 1992, 1994, 1995, 1996, 1997 and 1998; during the remaining years (1988, 1989, 1990, 1991, 1993) many of the high-magnitude events consisted of dust hazes (see Offer and Goossens, 2001a for more details). Fig. 11 shows that the decrease of vertical dust flux (at ground level) with increasing rainfall is very strong (in the lower figure even

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entirely) in the dust storm years. For the dust haze years, where local dust emission was small to very small, the relationship between vertical dust flux and rainfall is much less expressed or even absent. Thus, rainfall is an efficient factor in suppressing the emission during the rainy season, as could be expected. The lower picture in Fig. 11 shows that high rainfall during the rainy season also results in a reduced dust flux during the subsequent dry season. Because the soil near Sede Boqer is largely devoid of vegetation throughout the year, it is nearly certain that the surface crust that forms after the rains have stopped is the principal cause of the reduced emission. Surface crusts are very strong in dry seasons preceded by a very wet rainy season, mainly because of the strong surface sealing that affected the top layer (see Offer and Goossens, 2001a for more detailed information). Apart from physical crusts, also biological crusts (which include a biological film) may help reduce the susceptibility of the soil to wind erosion (Danin et al., 1989; Belnap, 2001). Several other studies (e.g. Stockton and Gillette, 1990; Musick, 1999) have found that a reduction in soil erodibility caused by plant growth in a wet year also extends into following dry years for as long as the dead plant remains. A final remark should be made here. When the predominant soil texture is such that crusting is unlikely, or when crusts are rather weak (on coarse-grained sands, for example), a positive dust-rainfall correlation may eventually be observed. This is because rain showers are often preceded by strong winds, which may carry dust particles to the site or deflate dust from local sources (provided these are available).

6. Conclusions During the 13-year test period, systematic trends were observed for several of the dust parameters examined. The main conclusions of this study can be summarised as follows: (1) Vertical dust flux at ground level showed a systematic trend over the year, with highest values in spring and lowest values in autumn. Average vertical dust flux for the period 1988–1999 was 0.18 mg m2 s1, or 5.6 g m2 yr1. Vertical dust flux was very low from 1989 to 1991 and also in 1997–1998, and moderately high or high in the other years. (2) Horizontal dust transport (quantified by means of the horizontal dust flux) was also highest in spring and lowest in autumn and winter. It showed systematic oscillations, with a periodicity of 30 months. Lowest dust fluxes were observed in 1990, 1992, around 1995, and in 1998, whereas highest dust fluxes occurred in 1988, 1991, 1994, 1996 and 1999. The average flux value (at 1 m above the desert surface) for the period 1988–1999 was 264.55 mg m2 s1, or 8.3 kg m2 yr1. (3) At Sede Boqer, the amount of dust in the air decreases with height. Strongest vertical stratification was observed at the end of spring and also in February, whereas at the end of the year (September–December) stratification was less expressed (but still present). Vertical stratification was very low in 1989–1991, high in 1988 and between 1992 and 1996, and moderate in the other years.

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(4) Median particle diameter of the dust at Sede Boqer was highest in spring and lowest in autumn. In the period 1988–2000 particle size was high in 1988–1989, in 1995–1996, and in 1999–2000, with a median grain diameter around 30 mm. In the other years the dust was considerably finer, with a median grain diameter around 25 mm or less. Average median grain diameter for the period 1988–2000 was 27.2 mm. (5) The average annual vertical dust flux curve paralleled the average annual wind speed curve. In months with high wind speeds the vertical dust flux was also high, and in months with low wind speeds the vertical dust flux was low. A similar relationship was observed between airborne dust concentration and vertical dust flux, but only for the background concentration. The relationship between dust deposition and dust grain size, on the other hand, remained unclear. (6) At Sede Boqer, a negative relationship exists between the amount of rainfall and the amount of vertical dust flux at ground level. This relationship is apparent during the rainy season itself, but also when the amount of rainfall during a rainy season is compared to the amount of vertical dust flux during the next dry season, likely due to the formation of a surface crust.

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