The first FRAGMENT field campaign took place in September 2019 in a small ephemeral lake, locally named “L'Bour”, located in the Lower Drâa Valley of Morocco. L'Bour (29∘49′30′′ N, 5∘52′25′′ W) lies at the edge of the Saharan Desert, ∼15 km west of M'Hamid El Ghizlane, ∼70 km east of Lake Iriki, ∼50 km east of the Erg Chigaga dune field, ∼1.5 km north of the dry Drâa river, ∼30 km north of the Moroccan–Algerian border, and ∼25 km south of the Jbel Hassan Brahim mountain range (840 ma.s.l.) (Fig. a, b and c). We chose the location and time period of the campaign based on the analysis of remote sensing data , in situ inspection, and local advice, considering both scientific criteria and logistic aspects such as accessibility.

L'Bour is approximately flat and devoid of vegetation or other obstacles within a radius of ∼1 km around our measurement location. Small sand dune fields surround the lake, and dunes south of the site were accompanied by some vegetation and shrubs during the campaign. The surface of L'Bour consists of a smooth hard crust (hereafter referred to as paved sediment) mostly resulting from drying and aeolian erosion of paleo-sediments . In Fig. S1 in the Supplement, we include a close-up of a small dune and the lake's paved sediment surface, along with their respective PSDs analysed using dry dispersion (minimally dispersed) and wet dispersion (fully dispersed) techniques . The paved sediment PSDs exhibit two prominent modes peaking at ∼100 µm and ∼10 µm. The fully dispersed PSD of the paved sediment shows disaggregation of silt aggregates observed at sand sizes in the minimally dispersed PSD. The sand dune PSDs display a dominant mode ranging between ∼50 and ∼400 µm, peaking at ∼150 µm, and contain only a small fraction of particles smaller than 50 µm. The fully dispersed PSD of the sand dune shows disaggregation of clay aggregates observed at silt sizes in the minimally dispersed PSD. The volume median diameter of sand dune particles (and therefore of the saltators) for minimally and fully dispersed techniques are 132.2 and 137.6 µm, respectively. According to the fully dispersed PSD, the texture of the surface paved sediment is loam . During the campaign, we did not observe any substantial change in the paved sediment. We observed some growth of vegetation in nearby areas, particularly to the south, after a flooding event that took place during the night of 6 September. The flooding, which did not affect our site, was caused by a convective storm that produced heavy rain upstream of the Drâa river and whose cold pool outflow generated a strong “haboob” dust storm that passed our site (see Sect. ).

L'Bour is surrounded by other dust sources in all directions, including dunes concentrated in small flat areas and other ephemeral lakes such as Iriki and Erg Smar (Fig. c). Therefore, the fetch length (i.e. the distance between the measurement location and the upwind border of the source area ) is not limited to the dimensions of L'Bour. We estimate long fetches of about 60 and 10 km in the western and eastern predominant wind directions, respectively, which are approximately parallel to the Drâa river bed and perpendicular to the alignment of our instruments (Fig. d), as described in Sect. .

The site layout is shown in Fig. d and e. The alignment of the instruments was informed by prior analyses of nearby automated weather stations maintained by the IMPETUS and FENNEC projects , the enerMENA initiative , and ERA5 and ERA-Interim wind reanalysis, which suggested a southwesterly predominant wind direction. To avoid shadowing between instruments as much as possible, instruments were aligned roughly perpendicular to this predominant wind direction. Below we describe only the instruments and measurements used in this paper. Measurements performed during the campaign with other instruments displayed in Fig. d are discussed in companion papers e.g..

Most studies have traditionally assumed that the diffusive flux PSD obtained a few metres above the surface is equivalent to the emitted dust PSD at the surface, neglecting the gravitational settling and the turbulent dry deposition flux. Considering the schematic shown in Fig. , the emitted flux (Femi) can be estimated as the diffusive flux (F) plus the gravitational settling (Fg) at the intermediate level between the two Fidas minus the dry deposition flux at the surface (Fdep): 12Femi(Di)=F(Di)+vdep(Di)cint(Di)-vg(Di)cint(Di)=F(Di)+(vdep(Di)-vg(Di))cint(Di), where vdep is the dry deposition velocity, vg is the gravitational settling velocity, cint is the concentration at the intermediate height between the two Fidas, and Di is the mean logarithmic diameter of each bin i. The gravitational settling velocity is calculated as vg(Di)=CcσpagDi2/(18ν) where Cc is the Cunningham slip correction factor, ν=1.45×10-5m2s-1 is the air kinematic viscosity and σpa=(ρd-ρair)/ρair is the particle-to-air density ratio. Note that this expression assumes a Stokes regime, which is applicable to particles with Di∼10 µm or less.

The dry deposition velocity vdep(Di) can be calculated as the sum of the diffusive dry deposition velocity, vdiff(Di), and vg(Di). We obtain vdiff(Di) for each 15 min period as vdiff(Di)=-F(Di)/cint(Di) . vdiff(Di) is positive when downward, so the diffusive flux in integrated size bin resolution F(Di) must be negative. Due to the presence of dust emission, these observation-based estimates of vdep must be restricted to periods when dust emission is negligible, i.e. for u*<u*th.

In the absence of observation-based vdep(Di) during wind erosion conditions (u*>u*th), we use resistance-based dry deposition velocity parameterizations, which are typically used in dust transport models, to estimate vdep(Di) for all u* values. We first evaluate two different parameterizations , described in Appendix , with our observation-based estimates. Given that the parameterizations severely underestimate vdep(Di), we update the parameterization of based on (see Appendix ) and tune key parameters and processes within the parameterization to fit the observation-based estimates for u*<u*th. This tuned parameterization is used to estimate the dry deposition flux, which is then used to estimate the emitted dust flux for all u* conditions using Eq. ().

Times series of measured atmospheric conditions and near-surface dust concentrations are displayed in Fig. ; u* and atmospheric stability, along with saltation and diffusive fluxes, are displayed in Fig. . As expected, the diurnal cycles of temperature and relative humidity are anti-correlated (Fig. b), and temperature inversions (Fig. a), along with atmospheric stability (Fig. b), are prevalent during nighttime. Temperature at 2 m ranges from slightly less than 20 ∘C during the night to up to ∼40 ∘C during the day, and surface relative humidity ranges from as low as 6 % during the day to up to ∼65 % during the night. There is a shift after 14 September, with substantial increases in temperature and decreases in relative humidity, with the exception of 17–18 September, when relative humidity appears to be temporarily high.

The diurnal cycles of surface wind (Fig. d) and u* (Fig. a), along with the associated cycles of saltation and diffusive fluxes (Fig. c, d and e) and dust concentration (Fig. f and g), are generally associated to the diurnal cycle of solar heating. In the early morning, as the surface starts to warm and releases turbulent sensible heat, the lower atmosphere becomes unstable. As the day evolves, momentum is mixed downward from the stronger winds aloft increasing wind speed and u*, while stability progressively tends towards neutrality (Fig. b). Winds are generally channelled through the valley, broadly parallel to the Drâa river, alternating between two opposite and preferential wind directions, centred around 80 and 240 ∘ (Fig. e). The distribution of wind direction and u* during the campaign is shown in Fig. S4. We refer to the dust events associated to these recurring diurnal cycles as “regular” events, for which maximum winds at 10 m can reach 15 min average values up to ∼11 ms-1 (Fig. d). From 22 to 25 September winds remain relatively calm, and after 25 September diurnal cycles are less marked and dust events are more intermittent and short-lived.

In addition to these regular events, we also captured two strong cold pool outflows (hereafter referred to as “haboob” events) in the evening of 4 September and in the afternoon of 6 September, both marked with a red “H” in Figs.  and . Cold pool outflows result from density currents created by latent heat exchange of evaporating rain in deep convective downdrafts. The arrival of sharply defined dust walls, caused by the gust fronts at the leading edge of the outflow winds, were not only directly witnessed by the field campaign team but can be also clearly detected in the measurements. As a video supplement we provide a 1 min frequency time-lapse video recorded from the Fidas location during 6 September, which clearly shows the arrival of the haboob in the afternoon. Both haboob events are characterized by the highest 10 m winds recorded during the campaign (15 min averages of ∼11.5 and ∼14 ms-1, respectively) and unusually fast changes in atmospheric conditions with values consistent with previous haboob studies : sudden increases in wind speed, decreases in 2 m temperature of ∼8–9 ∘C, increases in relative humidity of ∼24 %–32 %, and a rise of ∼2 hPa in surface pressure (Fig. c). During these events, precipitation was not detected by our rain gauge, but during the night of 6 September there was water flowing downriver, which caused flooding of large areas in the vicinity of our lake on the next day (not affecting the lake itself), suggesting that heavy showers occurred over the mountain range to the north of our location (Fig. c).

Dust concentration (Fig. f and g) exhibits peaks of varying intensity about every ∼1–2 d, consistent with the wind speed and u* patterns. Number and mass concentrations were 5×107 m-3 and 1243 µgm-3 on average, respectively, and there were 10 d when the 15 min dust mass concentration exceeded 104 µgm-3. As expected for dust, the number concentration was dominated by fine particles, and the mass concentration was dominated by coarse and super-coarse dust. Dust concentration is generally correlated with saltation (Fig. c) and diffusive fluxes (Fig. d and e), with the notable exception of an event that extends over the evening of 17 September and the morning of 18 September. During this event, concentrations reached values that are among the highest recorded during the campaign (Fig. f and g), although winds were low (Fig. d), saltation was absent (Fig. c), and diffusive fluxes were negative (note that negative fluxes are not represented in Fig. d and e). The latter implies that dust was transported from elsewhere and deposited, but it was not emitted from our site. Given that convective storms were spotted from a distance during that evening and the event was characterized by high relative humidity values (Fig. b), we hypothesize that those highly dust-loaded air masses that slowly and persistently reached our site were generated by precedent haboob activity upwind.

During the campaign, we also detected the presence of anthropogenic aerosols with diameters below ∼0.4 µm, whose influence is most visible when winds are weak and mass concentrations low (see Fig. S5), consistent with measured optical properties analysed in a companion contribution . This is particularly evident between 8 and 10 September, when low wind comes from the east (i.e. from M'Hamid). Such anthropogenic aerosol influence at the lower end of the measured PSD range is further evidenced and discussed in Sect. .

Saltation and diffusive fluxes are highly correlated and occur regularly throughout the campaign, peaking typically between 12:00 and 18:00 UTC in accordance with maximum surface winds and u*. In our case, the threshold friction velocity u*th is 0.16 ms-1 (see Sect. S3), which is reached nearly every day. The u* value shows peaks up to ∼0.4 ms-1 during regular events and reaches up to ∼0.6 ms-1 during the haboob event that occurred on the afternoon of 6 September (Fig. a). Wind erosion occurs mostly under unstable or close to neutral atmospheric conditions (Fig.  b). For u*>u*th, the 15 min average of total vertical diffusive flux in terms of number and mass are on average 3.7×106 m-2s-1 and 191 µgm-2s-1, respectively, reaching maximum values of 8.4×107 m-2s-1 and 5116 µgm-2s-1 on 6 September.

Figure a, b, and c display the diffusive flux, saltation flux, and sandblasting efficiency against u*. We use coincident 15 min data between saltation and diffusive flux, and only when the diffusive flux is positive in all dust size bins with Di>0.4 µm, i.e. we consider the bulk diffusive flux between 0.37 and 19.11 µm (see Sect.  for more details). The points corresponding to the haboobs on 4 and 6 September are depicted with squares and triangles, respectively. Regression curves of the form a⋅u*b are also represented for u*>u*th. The 95 % confidence intervals of the parameters of each regression curve are shown in Table S1 in the Supplement. The diffusive flux ranges mostly between ∼101 and ∼103 µgm-2s-1 and the power law exponent b is 3.88 (Fig. a). The obtained exponent is within the range shown in (their Fig. 5), where b varies between approximately 3 and 6 across different datasets gathered from the literature ; this is likely due to differences in soil type and soil surface conditions.

The saltation flux ranges between about 10-1 and 102 gm-1s-1. The power law exponent b is slightly higher than that obtained for the diffusive flux, i.e. b=4.31 (Fig. b). This value is larger than that reported in for most soils (b≈3). In comparison with (their Fig. 4), where data of two major dust field campaigns (JADE and WIND-O-V) are re-analysed, we obtain larger saltation fluxes for similar ranges of u*. For u*≈0.25–0.45 ms-1, our 15 min saltation fluxes vary between 100 and 102 gm-1s-1, while the 1 and 16 min measurements from the JADE and WIND-O-V campaigns, respectively, vary between 10-1 and 101 gm-1s-1. Using the same instrument (SANTRI) as in our study, reported a maximum 1 min saltation flux of almost 101 gm-1s-1 for u*>0.8 ms-1, approximately an order of magnitude smaller than our 15 min maximum values occurring during the haboobs for smaller u*. The large saltation fluxes suggest, despite the hard surface crusting, that the sand supply was such that our site did not experience considerable supply limitation, i.e. that saltation transport was mainly driven by atmospheric momentum and not by particle availability. Comparison of the height-dependent saltation flux obtained with SANTRI4 with that from the co-located MWAC sampler (not shown) confirmed that both are largely consistent, with SANTRI4 tending to record slightly higher fluxes. This is in qualitative agreement with the comparison of saltation measurement devices from .

The intensity of saltation impacts the aerodynamic roughness length z0 due to momentum absorption by the saltating particles . In our experimental site z0 ranged mostly between 10-5 and 10-4 m and increased with u*. This increase was also observed in and , although we obtained roughness lengths about one order of magnitude smaller, consistent with values obtained in other playas . Further details about z0 at our site and its relationship with u* under saltation conditions are shown in Sect. S7.

The sandblasting efficiency ranges between about 10-6 and 10-3 m-1, although most values are concentrated between 10-5 and 10-4 m-1 (Fig. c). These results are similar to those obtained in (corresponding to a soil nominally of silt loam texture in Spain), (for a sandy soil with a very low clay and silt content in Niger), and the results of the soils 4 (sandy), 5 (sandy), and 9 (clay) reported in . However, our values are on the lower end of the range reported in and , where most sandblasting efficiencies are above 10-4 m-1. The sandblasting efficiency tends to decrease slightly with increasing u* when considering all wind directions, i.e. the exponent of the power law is negative (b=-0.43), but R2 (in logarithmic space) is very small. There is some dependency of the sandblasting efficiency upon wind direction. For example, sandblasting efficiencies are higher under southeasterly winds (135–180 ∘) than under the dominant wind directions (45–90 and 225–270 ∘). The exponent of the power law also changes between predominant wind directions (See Figs. S6 and S7), but the amount of data is rather small and shows significant scatter, and R2 (in logarithmic space) is small. Interestingly, some of the lowest sandblasting efficiency values (∼10-5 m) are obtained during the haboob events, at least in part due to an enhanced reduction of coarse and super-coarse particles in the diffusive fluxes during the haboob events as discussed in Sect. .

There is a more robust decrease in sandblasting efficiency with increasing saltation fluxes (Fig. d), which is also evident in each of the two dominant wind directions (See Figs. S6 and S7). Such decreases in the sandblasting efficiency with increasing u* and saltation flux are also found in using data from the JADE and WIND-O-V field campaigns. To explain this result, suggests that the proportion of emitted fine particles produced by sandblasting should increase with Q due to enhanced aggregate disintegration, which leads to lower sandblasting efficiencies. We discuss in Sect.  a variety of potential mechanisms to explain the variations in the diffusive flux PSD with u* that contribute to the decrease in sandblasting efficiency with increasing u*.

All in all, our results highlight the prominence of saltation in our site, which produces strong diffusive fluxes despite the relatively low sandblasting efficiencies. These features are consistent with the measured surface sediment properties. On the one side, L'Bour is surrounded by small dunes with a minimally dispersed volume median diameter of 132.2 µm and a considerable amount of saltators below 100 µm (see Fig. S1), which translates into rather optimal saltation conditions. For instance, saltation can be detected even when u*<u*th based on 15 min averages (Fig. b). During such situations, saltation is typically intermittent during the 15 min period; hence, instantaneous momentum fluxes can be large enough to enable particle transport. On the other side, the low sandblasting efficiencies are attributed to the paved sediment that constitutes the surface of the ephemeral lake.

In this section, we analyse variations in the dust PSD after identifying and removing any potential anthropogenic aerosol influence. To provide a comprehensive view, we study the number and mass-normalized and non-normalized PSDs of concentration (Figs.  and ) and diffusive flux (Figs.  and ). For dust concentrations, we refer to concentrations from FidasL. The results from FidasU are analogous and provided in Sect. S8. We consider all available measurements covering the full range of u* for concentration PSDs, but we only consider diffusive flux PSDs when u*>0.15 ms-1, i.e. well-developed erosion conditions, and when the diffusive flux is positive in all size bins with Di>0.4 µm (this minimum size is taken to avoid any anthropogenic aerosol contamination as discussed in Sect. ). Figures – group the PSDs into u* intervals, types of events (regular versus haboob events), and wind directions (for the sake of simplicity we only show two 180 ∘ wind direction sectors to the east and west of the alignment between the Fidas and the 10 m tower, as shown in Fig. d). Our preliminary analysis did not show any effect of atmospheric stability independent of u* upon the PSD, in agreement with and in contrast with some recent studies , likely due to the small range of stability conditions during our campaign (Sect. ). However, this aspect was not analysed in detail.

In the previous section we have seen how and to what extent the concentration and diffusive flux PSDs depend upon u*, wind direction, and type of event (regular versus haboob). Here, we discuss the potential mechanisms that may explain these PSD variations, which include the effect of dry deposition modulated by the fetch length, aggregate disintegration during wind erosion, and the impact of the haboob gust front.

The proportion of sub-micrometre particles decreases in the concentration PSD between calm (purplish and blueish lines) and well-developed erosion conditions (yellow, orange and red lines) (Fig. c and d). When u* is low, i.e. in the absence of local emission, the PSDs represent background conditions and therefore present a smaller fraction of super-micrometre particles due to their shorter lifetime. As u* increases, the concentration becomes increasingly dominated by freshly emitted dust, reducing the influence of the background dust and hence enhancing the proportion of super-micrometre dust. However, during regular dust events, the proportion of sub-micrometre particles increases and that of super-micrometre particles decreases in the diffusive flux PSD as u* increases (Fig. c and d). This is also observed, although to a lesser extent, in the concentration PSDs for well-developed erosion conditions when u*>0.25 ms-1 (Fig. c and d). This could be compatible with two different mechanisms or the combination thereof. On the one side, it could be due to a reduction in super-micrometre particles by dry deposition, which increases with u* . On the other side, the relative enhancement of sub-micrometre particles may be the result of more aggregate disintegration with increasing u* . We examine these two hypotheses more thoroughly below.

The potentially large effect of dry deposition upon the diffusive flux PSDs has been recently suggested based on numerical experiments . More specifically, these studies clearly illustrated the key roles of the dust fetch length and u* in this process. The dust fetch is defined as the uninterrupted upwind area generating dust emissions. This differs from the flux footprint, which is the upwind area that contributes substantially to the concentration at the measurement location , and which is here much smaller than the dust fetch, a couple of hundred metres versus several kilometres, respectively. For a given surface and uniform u* along the fetch, the deposition of dust particles, which is size dependent, slowly increases with the fetch as the concentration of dust is enhanced. This way, a longer fetch results in a higher enrichment of the diffusive dust flux in small particles . Additionally, for a given fetch, an increasing u* can substantially modify the diffusive flux PSD by enhancing the deposition of super-micrometre particles through impaction, i.e. the direct collision of particles to a surface resulting from their inertia, and hence reducing the fraction of these particles. Our observations suggest a major role of dry deposition in shaping the variations in the concentration and diffusive flux PSDs. On the one side, for equivalent u* intervals during regular events, there are in general higher total number and mass concentrations for the western sector (Figs. a and a versus b and b, respectively), consistent with the longer fetch in that direction (60 km versus 10 km in the western and eastern sectors, respectively, as described in Sect. ). Furthermore, in the normalized number concentration and diffusive flux PSDs we observe a higher proportion of sub-micrometre particles in the western sector compared to the eastern sector (Figs. c and c versus d and d). On the other side, during regular events when u* increases, the mass fraction of super-coarse particles (Di>10 µm) decreases and that of fine and coarse particles (Di<10 µm) increases, both in the concentration and the diffusive flux PSDs (Figs. c, d, c and d). This effect is more visible when winds come from the western sector, which has a longer fetch. Our hypothesis is further confirmed when applying the tuned resistance-based dry deposition velocity parameterization described in Sect.  and Appendix , whose results are discussed in detail in Sect. .

Parallel to the effect of deposition, at least part of the enhancement in sub-micrometre particles with u* could be attributed to an increased aggregate disintegration. However, while this explanation can hold for regular events, there is no detectable increase in the proportion of sub-micrometre particles with increasing u* in the haboob events in either direction. In addition, the proportion of sub-micrometre particles during the haboob events is lower than during regular events although the former are associated with equivalent or higher u* values (Fig. c and d). This further favours the prevalence of the fetch-deposition mechanism over any potential enhanced aggregate disintegration with u*.

It is indeed quite remarkable that haboob events tend to show a much higher proportion of super-micrometre particles, especially for Di∼(1–5) µm, and a lower proportion of sub-micrometre particles than the regular events for equivalent or higher u* intervals in the normalized number concentration PSDs (Fig. c and d). In terms of normalized number diffusive flux PSDs (Fig. c and d), haboob events are similar to the regular events for the u* interval (0.15–0.2] ms-1, although coarse and super-coarse dust mass fractions with Di>3 µm during the haboob events show much more variability than during the regular events (Fig. d). To try to explain these features we revisit the formation process of a haboob. A convective storm or thunderstorm is formed when there is vertical transport of heat and moisture in the atmosphere (convection) that produces updrafts. As the convective storm matures, besides updrafts there are also downdrafts caused by evaporative cooling. When these downdrafts are very strong and hit the ground in a dust source area, large amounts of sand and dust are lifted into the air and can spread several kilometres wide horizontally, producing a wall of dust and strong wind gusts, a phenomenon known as a “haboob”. Therefore, a haboob is formed from the outflow of a convective storm. We hypothesize that the location where the downdraft of the thunderstorm hits the surface represents a new beginning of the dust fetch, which would be closer to our experimental site than the original start. Following the argument given to explain the differences in PSDs between western and eastern sectors, this shorter “effective” fetch could at least partially explain the relative reduction in sub-micrometre particles and the increase in super-micrometre particles. At the same time, despite the overall increase in the fraction of super-micrometre particles, dry deposition visibly more strongly affects the fractions of coarse particles (Di>3 µm) and super-coarse particles (Di>10 µm) in the diffusive flux PSDs during the haboob events than during the regular events (Fig. b and b). This is because the dry deposition flux scales with the concentration, and during the haboobs the concentration of the super-micrometre particles is substantially higher (Fig. f and g). In addition, a haboob is not a static phenomenon and its gust front, where u* and dust emission are maximized, moves towards and away from our measurement site. Therefore, there is non-uniformity of u* and dust emission across the fetch, which may explain the higher variability in the haboob PSDs. Finally, higher air humidity along the haboob outflow and its potential effect upon the soil bonding forces cannot be discarded. During the haboob events, the relative humidity at our site increased substantially, from 15 %–25 % to ∼50% (Fig. b). Although our near-surface soil moisture measurements (2–3 cm deep) (not shown) did not register any associated increase, it has been argued that wet bonding forces in the soil surface, which are dominated by adsorption in arid regions, increase with relative humidity within approximately the observed variation range . This mechanism would be consistent with the smaller proportion of sub-micrometre particles due to an increased resistance of soil aggregates to disintegration with increasing relative humidity as suspected in .

If the deposition process causes the variability observed in the diffusive flux PSD, the emitted dust PSD should have a higher coarse and super-coarse fraction while showing less variability than the diffusive flux PSD. To test this hypothesis, we calculate the emitted dust flux, which requires estimating the dry deposition flux (see Eq. ) for the same 15 min samples used in Figs.  and . Figure a and b display for different u* intervals the median dry deposition velocities vdep (solid lines) obtained by applying the parameterizations described in Appendix  of (referred to as F19) and (referred to as Z01), respectively, for which field measurements have been used. In both cases vdep increases strongly with particle size from Di∼1.5 µm due to gravitational settling. At the same time, vdep scales with u*, which is more noticeable in F19 for coarse particles in the size range 2.5<Di<10 µm (Fig. a). In Z01 the scaling of coarse particles with u* is much more subtle than for particles with Di<2.5 µm (Fig. b). The stars in purple, blue, and cyan represent the observation-based vdep for the first three intervals of u*. The two parameterizations predict vdep reasonably well for u*<0.05 ms-1 and Di>∼1 µm but strongly underestimate it for the u* intervals (0.05–0.10] and (0.10–0.15] ms-1. For instance, for the u* interval (0.10–0.15] ms-1 F19 and Z01 underestimate the observed vdep (cyan star) by a factor of ∼3 for particles with Di=17.15 µm. Note that our observation-based estimates are broadly consistent with measurements reported by , corresponding to an intense dust deposition event occurred in June 2006 in Niger (see Fig. S17).

Given the systematic underestimation of the parameterized vdep applying F19 and Z01, we updated and tuned Z01 vdep parameterization to best fit the observation-based estimates as described in Appendix . The more suitable configuration was achieved for B1=0.02, dc=0.0009 m and Ain=15 (Fig. c). We note the low value required for the scaling factor of the aerodynamic resistance B1 (see more details in Appendix  and Sect. S10). The resulting size-resolved number and mass dry deposition fluxes obtained using Z01, F19 and the tuned parameterization are provided in Sect. S11.

Figure  shows the estimated size-resolved emitted dust mass flux calculated from Eq. () applying the vdep estimated with the tuned parameterization (results in number and from the other two schemes are shown in Sect. S12). The normalized emitted flux PSDs clearly show less variability as a function of u*, along with a lower shift towards finer dust and a lower reduction of super-coarse particles with increasing u* (Figs. c, d, S25c and d), in comparison to the normalized diffusive flux PSDs (Figs. c, d, c and d). These features can be better appreciated by integrating the fractions over four size ranges in Fig. S31, which is analogous to Fig. S30 but for the estimated emitted flux. The increase in the number fraction for ∼0.37<Di<1 µm with increasing u* (comparison between the u* intervals (0.15–0.20] and (0.30–0.35] ms-1) during regular events is reduced by ∼41 % and ∼28 % for the western and eastern sectors, respectively, in the estimated emitted dust flux in comparison with the diffusive flux (Figs. S31a and b versus S30a and b). However, the remaining difference between u* intervals is still statistically significant (p value <0.05) when individually considering the two smallest size bins (0.37–0.49 and 0.49–0.65 µm) for both wind sectors (Fig. c and d). The increase in the mass fraction for ∼2.5<Di<10 µm and the decrease for Di>10 µm with increasing u* (comparison between the u* intervals (0.15–0.20] and (0.30–0.35] ms-1) during regular events are also both reduced up to ∼13 % and ∼18 %, respectively, in the estimated emitted flux (Figs. S31c and d versus S30c and d). Despite the much lower decrease in super-coarse particles with increasing u* (Figs. c and d versus c and d), the differences between the u* intervals (0.15–0.20] and (0.30–0.35] ms-1 are still statistically significant (p value <0.05) for both wind sectors considering both the whole mass fraction Di>10 µm (Fig. S31c and d) and only the last integrated size bin (Fig. c and d). Similar trends are observed for the haboob on 4 September, while those for the haboob on 6 September seem to be the opposite, consistent with the higher variability in the haboob PSDs reported in Sect. .

Table  shows the mean and standard deviation of the number and mass percentages for the four size ranges in the diffusive and emitted fluxes during regular events for each wind sector, calculated from the average values of each u* interval shown in Figs. S30 and S31. For both wind sectors, the mean number percentage in the particle size range ∼0.37<Di<1 µm is reduced by ∼9 % in the estimated emitted flux compared to the diffusive flux, at the expense of both an increase of ∼23 % and >100 % for the size ranges ∼2.5<Di<10 and Di>10 µm, respectively. Mean mass percentages are reduced in the emitted flux compared to the diffusive flux for all size ranges except for Di>10 µm, where it increases by ∼29 %, for both wind sectors.

Our results show the potential importance of dry deposition as clearly depicted in Fig. S32, which displays the size-resolved ratio of the estimated dry deposition flux to the emitted flux determined using the tuned vdep parameterization. During regular events, we estimate dry deposition to represent up to ∼80% of the emission for super-coarse particles between 55 % and 60 % for particles with Di∼10 µm and between 30 % and 45 % for particles with Di∼5 µm. During the haboob events these fractions are generally higher and more variable under similar u* intervals, reaching up to ∼90% for super-coarse particles, up to 80 % for particles with Di∼10 µm, and between 50 % and 65 % for particles with Di∼5 µm.

In this section we sidestep wind direction differences and compare the obtained normalized concentration, diffusive flux, and estimated emitted flux PSDs with the emitted PSDs formulated in (Fig. ) and (Fig. ), both based on BFT. The former depends on the fully dispersed PSD and the latter on both the fully dispersed and aggregated soil PSDs. Here, our comparison focuses on the simplified parameterization proposed for modelling, which assumes a constant soil PSD and thus an invariant emitted PSD given the lack of spatially resolved soil PSDs.

For the sake of clarity, in Figs.  and  only two haboob PSDs are represented, corresponding to the two highest values of u* reached during the haboob events. While our number concentration PSD is close to the PSD derived from the parameterization (dashed pink line), particularly during regular events, our measurements show a substantially higher proportion of super-micrometre particles in the diffusive flux and the estimated emitted flux PSDs (Fig. a, c and e). In terms of mass, the super-coarse fraction is much higher in our PSDs (Fig. b, d and f), especially in the estimated emitted flux. Consequently, the fine and coarse mass fractions are smaller in our measurements.

While the measured PSDs shown in Fig.  assume that dust particles are PSL latex spheres with a refractive index of 1.59+0i, results shown in Fig.  consider a more realistic representation of the shape and composition of the measured dust particles, i.e. it assumes tri-axial ellipsoids and a refractive index of 1.49+0.0015i. Furthermore, these transformed PSDs are compared with the updated BFT parameterization (dashed blue line), which accounts for super-coarse dust and is constrained with measured PSDs harmonized to geometric diameters assuming dust is a tri-axial ellipsoid . The proportion of particles with Di∼(0.5-2) and Di>∼14 µm is higher and that of particles with Di<∼0.5 and with Di∼(2-14) µm is lower in the updated parameterization than in the original one (dashed blue versus dashed pink lines in Fig. a, c, and e). In terms of mass, the proportion of particles with Di<∼3 and Di>∼12.5 µm is higher and that of particles with Di∼(3-12.5) µm is lower in the updated parameterization than in the original one (dashed blue versus dashed pink lines in Fig. b, d and f).

Our converted PSDs show substantial differences with respect to the parameterization. Firstly, our number concentration (Fig. a), diffusive flux (Fig. c) and estimated emitted PSDs (Fig. e) have a higher proportion of particles with Di<∼0.8 µm. Conversely, our PSDs show a lower proportion of particles with Di∼(0.8–2) µm and a higher proportion of particles with Di>∼2 µm. The latter is more pronounced in the case of the diffusive flux PSD, and especially in the estimated emitted PSD. Secondly, the mass concentration PSD (Fig. b) show from relatively similar to lower fractions of particles with Di<∼2.5 µm. The lower fractions are particularly noticeable in the range of particles with Di∼(0.8–2.5) µm. Conversely, the fractions are relatively similar to higher for particles with ∼2.5<Di<12 µm. Furthermore, depending on the type of event and u*, there is a higher or lower fraction of dust particles with Di>∼12 µm. Thirdly, the mass diffusive and estimated emitted flux PSDs (Fig. d and f, respectively) display a similar pattern to the mass concentration PSD (Fig. b). However, they exhibit higher fractions of coarse dust (with Di>∼(6–8) µm) and generally super-coarse dust. Additionally, they show lower fractions of dust with Di<∼(6–8) µm and a substantial reduction in the range Di∼(0.8–2.5) µm, especially in the estimated emitted flux PSD.

Table  is analogous to Table  but considering tri-axial ellipsoids. The trends in the mean number and mass fractions of the diffusive and estimated emitted fluxes are similar to those described when using the original diameters in Sect. . However, the mean number fractions for ∼0.37<Di<1 µm are ∼22 %–24 % and ∼25 %–27 % higher for the diffusive and the estimated emitted flux, respectively, than when assuming PSL latex spheres. At the same time, the mean number fractions ∼2.5<Di<10 µm are ∼41 %–43 % and ∼38 %–40 % lower for the diffusive and the estimated emitted flux, respectively. In terms of mass, the most remarkable when considering tri-axial ellipsoids is the increase of ∼31 %–33 % and ∼28 %–29 % in the fraction Di>10 µm of the diffusive and estimated emitted flux, respectively.