Dust transport and horizontal fluxes measurement with spaceborne lidars ALADIN, CALIOP and model reanalysis data

In this paper, a long-term large-scale Sahara dust transport event occurred during 14 June and 27 June 2020 is 10 tracked with the spaceborne lidars ALADIN and CALIOP observations and the models ECMWF and HYSPLIT analysis. We evaluate the performance of the ALADIN and CALIOP on the observations of dust optical properties and wind fields and explore the capability in tracking the dust events and in calculating the dust horizontal mass fluxes with the combination of measurement data from ALADIN and CALIOP coupled with the products from ECMWF and HYSPLIT. Compared with the traditional assessments based on the data from CALIOP and models, the complement of Aeolus-produced aerosol optical 15 properties and wind data will significantly improve the accuracy of dust horizontal flux estimations. The dust plumes are identified with AIRS/Aqua Dust Score Index and with the Vertical Feature Mask products from CALIPSO. The emission, dispersion, transport and deposition of the dust event are monitored using the data from HYSPLIT, CALIPSO and AIRS/Aqua. With the quasi-synchronization observations by ALADIN and CALIOP, combining the wind vectors and relative humidity, the dust horizontal fluxes are calculated. From this study, it is found that the dust event generated on 14 and 15 June 2020 from 20 Sahara Desert in North Africa, and then dispersed and transported westward over the Atlantic Ocean, and finally deposited in the Atlantic Ocean, the Americas and the Caribbean Sea. During the transport and deposition processes, the dust plumes are trapped in the Northeasterly Trade-wind zone between the latitudes of 5 N  and 30 N  and altitudes of 0 km and 6 km (in this paper we name this space area as “Saharan dust eastward transport tunnel”). From the measurement results on 19 June 2020, influenced by the hygroscopic effect and mixing with other types aerosols, the backscatter coefficients of dust plumes 25 are increasing along the transport routes, with -6 -6 1 1 3.88 10 2.59 10 m sr − −    in “dust portion during emission phase”, -6 -6 1 1 7.09 10 3.34 10 m sr − −    in “dust portion during development phase” and -6 -6 1 1 7.76 10 3.74 10 m sr − −    in “dust portion during deposition phase”. Finally, the horizontal fluxes at different dust parts and heights on 19 June and on entire transport routine during transportation are computed. On 19 June, the dust horizontal fluxes are about 2 1 2.17 1.83 mg m s − −    in dust portion during emission phase, 2 1 2.72 1.89 mg m s − −    in dust portion during development phase and 30 2 1 3.01 2.77 mg m s − −    in dust portion during deposition phase. In the whole life-time of the dust event, the dust horizontal https://doi.org/10.5194/acp-2021-219 Preprint. Discussion started: 8 April 2021 c © Author(s) 2021. CC BY 4.0 License.

the Atlantic Ocean, the Americas and the Caribbean Sea. During the transport and deposition processes, the dust plumes are trapped in the Northeasterly Trade-wind zone between the latitudes of 5 N  and 30 N  and altitudes of 0 km and 6 km (in this paper we name this space area as "Saharan dust eastward transport tunnel"). From the measurement results on 19 June 2020, influenced by the hygroscopic effect and mixing with other types aerosols, the backscatter coefficients of dust plumes 25 are increasing along the transport routes, with The global aerosol distribution and wind profiles have significant impacts on the atmospheric circulation, marine-atmosphere circulation and aerosol activities. As the most abundant aerosol types in the global atmosphere, the mineral dust influences the radiation budget, air quality, climate and weather via direct and various indirect radiative effects. Mineral dust is also considered as a major source of nutrients for ocean and terrestrial ecosystems. By the prevailing wind systems, mineral dust 45 deposited over the ocean and land surface and hence significantly affect the carbon cycle and perturb the ocean and land geochemistry (Velasco-Merino et al., 2018;Banerjee et al., 2019). The atmospheric mineral dust can be transport over tens of thousands of kilometers away from its source regions (Uno et al., 2009, Haarig et al., 2017, Hofer et al., 2017. For instant, the biggest dust source Africa produced over half the global total dust (Huneeus et al., 2011) and African dust transports westward over the Atlantic Ocean and reaches South America (Yu et al., 2015;Prospero et al., 2020), the Caribbean Sea (Prospero and 50 Lamb, 2003) and southern United States (Bozlaker et al., 2013). Hence, the continuous observations of the dust long-range transport are crucial. As one of the best techniques for remotely studying the characteristics and properties of aerosols, lidar contributes much to measure the dust activities. As introduced in previous papers, several comprehensive field campaigns including Aerosol Characterization Experiment ACE-Asia (Huebert et al., 2003;Shimizu et al., 2004), the Puerto Rico Dust Experiment PRIDE Reid et al., 2003), the Saharan Dust Experiment SHADE (Tanré et al., 2003), the 55 Saharan Mineral Dust Experiments SAMUM-1 (Heintzenberg, 2009) andSAMUM-2 (Ansmann et al., 2011), the Dust and Biomass-burning Experiment DABEX (Haywood et al., 2008), the Dust Outflow and Deposition to the Ocean project DODO (McConnell et al., 2008), the Pacific Dust Experiment PACDEX (Huang et al., 2008), the China-US joint dust field experiment (Huang et al., 2010), the Saharan Aerosol Long-Range Transport and Aerosol-Cloud-Interaction Experiment SALTRACE (Weinzierl et al., 2017), the study of Saharan Dust Over West Africa SHADOW, and the Central Asian Dust Experiment 60 CADEX (Hofer et al., 2017(Hofer et al., , 2020a(Hofer et al., , 2020b were conducted. However, the measurement data from these campaigns are still not able to meet the requirements for the investigation of global dust impact on climate, ocean/land geochemistry and ecosystems. Therefore, the spaceborne lidars that are capable of Lidar and Infrared Pathfinder Satellite Observations) provides us the backscatter coefficient and extinction coefficient at the wavelengths of 532 nm and 1064 nm (Winker et al., 2009). Additionally, the CALIOP product Vertical Feature Mask product (VFM) presents the aerosol sub-types classification and the global dust events could be marked. Moreover, large efforts still should be undertaken to monitor the dust emission, transport, dispersion, deposition and to explore dust impact on the Earth's radiation, climate and ecosystem. Hence, the vertical profiling of the global wind field is necessary to calculate the dust fluxes. 70 Thanks to the efforts of the European Space Agency (ESA), a first ever spaceborne direct detection wind lidar, Aeolus, which is capable of providing the globally high spatial and temporal vertical wind profiles is developed under the framework of Atmospheric Dynamics Mission (ADM) (Stoffelen et al., 2005;ESA, 1999;Reitebuch et al., 2012;Kanitz et al., 2019). On 22 August 2018, the Aeolus was successfully launched onto its sun-synchronous orbit at a height of 320 km (Witschas et al., 2020;Lux et al., 2020). A quasi-global coverage is achieved daily (~ 15 orbits per day) and the orbit repeat cycle is 7 days (111 75 orbits). The orbit is sun-synchronous with a local equatorial crossing-time of ~ 6 am/pm. The Atmospheric Laser Doppler Instrument (ALADIN) is a direct detection high spectral resolution wind lidar carried by Aeolus and provides the vertical profiles of the Line-of-Sight (LOS) wind speeds. It is operating at the wavelength of 354.8 nm. In order to retrieve the LOS wind speeds, the Doppler shifts of light caused by the emotion of molecules and aerosol particles need to be identified. Aiming at this, a Fizeau interferometer is applied in the Mie channel to extract the frequency shift of the narrow-band particulate return 80 signal by means of fringe imaging technique (Mckay, 2002). In the Rayleigh channel, two coupled Fabry-Perot interferometers are used to analyze the frequency shift of the broad-band molecular return signal by the double edge technique (Chanin et al., 1989;Flesia and Korb, 1999).
In the simultaneous observations of the dust plume, the aerosol optical properties can be obtained by means of ALADIN and CALIOP. By further using the wind field data from ALADIN, the wind field and relative humidity data from ECMWF 85 and the trajectories from HYSPLIT model, the dust transport route and dust fluxes can be calculated. The paper is organized as follows: in Section 2 the satellite-based instruments, ECMWF and HYSPLIT models are introduced. Section 3 presents the details to the joint dust measurement strategy and methodology. In section 4 we provide the results and discussions on the dust transport measurements on 19 June 2020 and during the whole lifetime of the dust event.

ALADIN/Aeolus
ALADIN, which is the unique payload of Aeolus, is a direct detection high spectral resolution wind lidar. It is a pulsed ultraviolet lidar works at the wavelength of 354.8 nm with a laser pulse energy around 65 mJ and with a repetition of 50.5 Hz.
As the receiver, a 1.5 m diameter telescope is equipped for the collection of the backscatter light. The high spectral resolution design of ALADIN allows for the simultaneous detection of the molecular (Rayleigh) and particle (Mie) backscattered signals 95 in two separate channels, each sampling the wind in 24 vertical height bins with a vertical range resolution between 0.25 km and 2.0 km. This makes it possible to deliver winds both in clear and (partly) cloudy conditions down to optically thick clouds at the same time. The horizontal resolution of the wind observations is about 90 km for the Rayleigh channel and about 10 km for the Mie channel. The detailed descriptions of the instrument design and a demonstration of the measurement concept are introduced in e.g. Reitebuch et al., 2009Reitebuch et al., , 2012Straume et al., 2018;ESA 2008;Marksteiner et al., 2013;Kanitz et al., 2019;100 Witschas et al., 2020 andLux et al., 2020. The data products of Aeolus are processing at different levels mainly including Level 0 (instrument housekeeping data), Level 1B (engineering-corrected HLOS winds), Level 2A (aerosol and cloud layer optical properties), Level 2B (meteorologically-representative HLOS winds) and Level 2C (Aeolus-assisted wind vectors) (Tan et al., 2008(Tan et al., , 2017. Within the Level 2B processor, the Rayleigh-clear and Mie-cloudy winds are classified and the temperature and pressure correction 105 are applied for the Rayleigh wind retrieval (Witschas et al., 2020). In this study, the Level 2A aerosol optical properties and Level 2C wind vectors are used. For the calculation of particle volume concentration distribution and mass concentration, the extinction coefficients and backscatter coefficients at the wavelength of 355 nm are used.
The VFM product describes the vertical and horizontal distribution of cloud and aerosol types along the observation tracks of CALIPSO. In this study, the backscatter coefficients and extinction coefficients at the wavelengths of 532 nm and 1064 nm are used for the calculation of the dust volume concentration distribution and mass concentration. The VFM from CALIPSO are also applied to identify the subtypes of aerosol layers. 115

ECMWF climate reanalysis
Supported by the Copernicus Climate Change Service (C3S), ECMWF is providing the atmospheric reanalysis ERA5 which presents a detailed record of the global atmosphere, land surface and ocean waves from 1950 onwards (Hersbach et al., 2020).
The 4D-Var assimilated ERA5 is producing the hourly vertical profiles (at 37 pressure levels) of global wind fields with a grid resolution of 31 km. After the successful launch of the Aeolus, the ECMWF is starting to simulate the wind products of Aeolus 120 from January of 2020. In this study, the wind field data from ECMWF is applied in filling in the missing data within the region between the tracks Aeolus of and CALIPSO and in connecting the data from these two spaceborne lidars.

HYSPLIT
The Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT) is a modelling system for determine the trajectories, the transport and dispersion of air masses developed by The National Oceanic and Atmospheric Administration 125 (NOAA) Air Resources Laboratory (ARL) (Draxler and Hess 1998;Draxler and Rolph 2012). It is widely used in studying the development of atmosphere. The backward trajectory and forward trajectory are the mostly used model applications to determine the origin of air masses (Stein et al., 2015). In this study, the HYSPLIT is used in describing and re-checking the routes of the dust plumes transport, dispersion, and deposition.

Methodology 130
In the study of dust transport and fluxes measurement, as shown in Figure 1, the dust identification, Aeolus and CALIPSO tracks match, data analysis and the HYSPLIT model analysis are necessary and the schematic flowchart is described briefly.
To identify the dust events and to choose the quasi-synchronization observations with ALADIN and CALIOP, the flowchart is presented in this figure. To preliminarily determine the occurrences of dust events, the "Dust score index" data provided by AIRS/Aqua are used to determine the dust plume coverage and transport route. With this given information, the VFM products 135 from the simultaneously observations with the spaceborne lidar CALIOP are applied to cross-check the identification of dust events. Hence the vertical distributions of dust plumes are presented. To find the original sources and to predict the transport routes of dust plumes, the backward trajectory and forward trajectory is used respectively. When the dust events are determined, the simultaneous observations with ALADIN and CALIOP have to be selected. Starting from the CALIOP observations, the nearest Aeolus footprints could be figured out. Since the orbits of Aeolus and CALIPSO are different, they cannot meet each 140 other at the exactly same time and same location. From our study, the CALIPSO is about 4 hours ahead of Aeolus. Base on the transport directions of dust events modelled with HYSPLIT, the tracks of Aeolus should be always downwind of the tracks of CALIPSO. When the tracks of Aeolus and CALIPSO are selected, the distances between the tracks can be calculated. If the distances are less than 200 km, the following procedures could be continued. In this study, the backscatter coefficient and extinction coefficient at 355 nm from ALADIN, at 532 nm and 1064 nm from CALIOP are collected as the useful dataset. The 145 backscatter coefficients and extinction coefficients at 355 nm correspond to the "Aeolus Level 2A Product" retrieved by SCA (standard correct algorithm). In this study, we choose SCA instead of ICA (iterative correct algorithm) because the extinction coefficients from ICA are noisy and the assumption of "one single particle layer filling the entire range bin" in SCA is reasonable and is met in the situation of the heavy dust events observation. The backscatter coefficients and extinction coefficients at 532 nm and 1064 nm are the "Total_Backscatter_Coefficient_532", "Extinction_Coefficient_532", 150 "Backscatter_Coefficient_1064" and "Extinction_Coefficient_1064". Since the footprints of Aeolus and CALIPSO are not exactly matched, the missing wind data between their tracks have to be filled in using the ERA5 wind field data.
In Figure 2, the flowchart of dust fluxes calculation procedure is provided. Based on the dataset consists of the backscatter coefficients and extinction coefficients at the wavelengths of 1064 nm and 532 nm from CALIOP and those at the wavelength of 355 nm from ALADIN, the aerosol volume concentration distribution can be calculated based on regularization method 155 which was performed by generalized cross-validation (GCV) from Müller et al. (1999). The advantage of this method is that it does not require prior knowledge of the shape of the particle size distribution and the measurement uncertainty is on the order of 20%. After integrating and multiplying an assuming typical dust particle density which is set as 2.65 to previous studies (e.g., Schepanski et al., 2009;Hofer et al., 2017;Mamouri and Ansmann, 2017), the particle mass concentration would be estimated as Engelmann et al. (2008) introduced. Combining with the horizontal wind speed provided 160 by Aeolus and ECMWF, when the relative humidity is lower than 90%, the aerosol mass fluxes can be computed with eddy covariance (EC) method by Eq. (1).
where m is the aerosol mass concentration and v is the horizontal wind speed.   dust events are actually present but are misjudged by the AIRS/Aqua, which may result from the interference from the highaltitude suspended cloud layers.
To cross-check the transport route of the dust events, as shown in Figure 4, the backward trajectories and forward trajectories starting at 0500UTC 20 June 2020 with NOAA HYSPLIT model are conducted. In these trajectories, the heights of 2 km, 3 180 km and 4 km are chosen as the level heights. From Figure 4 (a), it is found that the dust plumes at about 4 km are generated from the central of Sahara Desert while the dust plumes at about 3 km and 2 km are occurred from the west side of the Sahara Desert. In Figure 4 (b), it is clearly indicated 190 that the dust plumes were separated into two directions toward the Caribbean Sea and the Central/South America respectively.
And an obvious deposition process of dust plumes is observed. After 26 June, transported over the whole Atlantic Ocean, most of the dust plumes were settled in the western Atlantic Ocean, the Central America and the Caribbean Sea.

(a) Vertical view of Aeolus and CALIOP tracks and HYSPLIT trajectories; (b) Backscatter coefficient cross-sections measured with Aeolus and (c) Total backscatter coefficient crosssections measured with CALIOP.
In this section, the dust event measurement case occurred on 19 June 2020 is introduced in detail. As shown in Figure  in "cross-section 3". Since the dust plumes in "cross-section 1" are suspended over continental while the dust plumes in "cross-section 2" are suspended over ocean, probably influenced by the hygroscopic effect, the backscatter coefficients in "cross-section 2" are larger than that of "cross-section 1". However, the dust layers in "cross-section 3" are connected to the surface of ocean and mixed well with other aerosol types (e.g., marine aerosol), hence the backscatter coefficients increase significantly. 210 Based on the backscatter coefficients and extinction coefficients at 355nm, 532nm and 1064nm, combining the wind field data from ALADIN and ECMWF, the column dust transport fluxes can be calculated. The L2C wind product provided by Aeolus are result from the background assimilation of the Aeolus HLOS winds in the ECMWF operational prediction model.
The u and v components of the wind vector and supplementary geophysical parameters are contained in L2C data product.
From the literature report (e.g., Lux et al., 2020), the Aeolus L2B Rayleigh LOS winds and the ECMWF model LOS winds 215 are compared and the result shows good agreement with a correlation coefficient of 0.92 and mean bias of 1.62 ms -1 . Hence, in this study, both the ECMWF wind vector data and the "analysis_zonal_wind_velocity" and "analysis_meridional_wind_velocity" from Aeolus could be applied for the calculation of the dust fluxes.
To calculated the dust fluxes during this event, the wind field and relative humidity information are necessary. Since the observations with ALADIN and CALIOP are not exactly simultaneous, the stability of wind field between the scanning tracks 220 of them has to be estimated. Hence, the wind speed, wind direction and relative humidity between the tracks are analysed with the data from ECMWF, as presented in Figure 6. From this figure, the wind speed, wind direction and relative humidity at the height surfaces of 1 km, 2 km, 3 km, 4 km, 5 km and 6 km are shown as examples. The wind fields between the tracks of Aeolus and CALIPSO at all height surfaces are smoothly distributed and the values are stable. Thus, the mean values of speed and directions are applied in the calculation of dust fluxes. It should be emphasized that, during the calculations of the dust 225 fluxes, the results with relative humidity higher than 90% have to be removed.
In Figure 7, the dust horizontal fluxes at different heights of the three cross-sections are presented. It can be found that,   During this dust event, the quasi-synchronization observations with ALADIN and CALIOP are selected to follow the transport and dispersion of dust. As shown in Figure 8, the detailed information about the ALADIN and the CALIOP observations on 15,16,19,24,27 June 2020 along the transport route and the HYSPLIT modelling are shown. In Figure 8(a), the spaceborne lidars ALADIN and CALIOP quasi-synchronization scanning tracks on those days are indicated by dark purple lines and green lines, respectively. Additionally, the forward trajectories starting from 19 June and backward trajectories ending at 19 June are 245 modelled and presented in dark red lines and light purple lines, respectively. In Figure 8(b) and (c), 5 cross-sections of backscatter coefficient at 355nm measured at different times with Aeolus and 5 cross-sections of backscatter coefficient at 532nm measured at different times with CALIOP are plotted, respectively. From these figures, we can find that the dust transport modelled with HYSPLIT match well with the dust masses at different cross-sections of Aeolus and CALIPSO. In In Figure 10, the dust horizontal fluxes at different heights of all the cross-sections during the dust transport, as well as the dust flux distributions are presented. The trend of the dust fluxes is also shown in Figure 10 America) and hence the whole transport route of the dust event are mainly over the oligotrophic regions at the Atlantic Ocean, the Americas and the Caribbean Sea. The mineral dust, probably act as the main nutrient source, delivers micronutrients including soluble Fe and P to the deposition zones and has the potential to fertilize the ocean and increase the primary productivity in the Atlantic Ocean and Caribbean Sea. And thus leading to N2 fixation and CO2 drawdown.
From Figure 12, the L2C wind vectors including u and v components from Aeolus at different times are plotted. In Figure  285 12 (

Summary and conclusions 295
In this study, a long-term large-scale Sahara dust transport event occurred during 14 June and 27 June 2020 is tracked and its horizontal fluxes are calculated with the remote measurement data from ALADIN, CALIOP and the reanalysis data from ECMWF and HYSPLIT. The combination of the spaceborne lidars ALADIN and CALIOP measurement data coupled with the products from ECMWF and HYSPLIT schemes to (1) evaluate the performance of the ALADIN and CALIOP on the observations of dust optical properties and wind fields and (2) explore the capability in tracking the dust events and in 300 calculating the dust horizontal mass fluxes.
We identified the dust plumes with AIRS/Aqua Dust Score Index and with the Vertical Feature Mask products from CALIOP. The emission, dispersion, transport and deposition of the dust event are followed using the data from HYSPLIT, CALIOP and AIRS/Aqua. With the quasi-synchronization observations by ALADIN and CALIOP, combining the wind vectors and relative humidity, the dust horizontal fluxes are calculated. The accuracy improvement of dust horizontal flux 305 estimations with the complement of Aeolus-produced aerosol optical properties and wind data is foreseen when compared with the traditional assessments based on the data from CALIOP and models.
From this study, it is found that the dust event generated on 14 and 15 June 2020 from Sahara Desert in North Africa, and then dispersed and transported westward over the Atlantic Ocean, and finally deposited in the west part of Atlantic Ocean, the Americas and the Caribbean Sea. During the transport and deposition processes, the dust plumes are trapped and transport in 310 the Northeasterly Trade-wind zone between the latitudes of 5 N  and 30 N  and altitudes of 0 km and 6 km (we name this space area as "Saharan dust eastward transport tunnel"). From the measurement results on 19 June 2020, influenced by the hygroscopic effect and mixing with other types aerosols, the backscatter coefficients of dust plumes are increasing along the transport routes, with the Saharan Dust is found transported across the oligotrophic regions Atlantic Ocean towards the Americas and Caribbean Sea, which are also oligotrophic regions. The mineral dust delivers micronutrients including soluble Fe and P to the deposition zones and has the potential to fertilize the ocean and increases the primary productivity in the Atlantic Ocean and Caribbean Sea. And thus leading to N2 fixation and CO2 drawdown. As a further research, the impact of dust transport and fertilization on the biological communities in eutrophic and oligotrophic areas is of great significance. Additionally, under the combined 330 influence of the Western Boundary Current (e.g., Gulf stream), the impact of dust deposition on the Caribbean Sea and the North Atlantic Ocean (such as the occurrence of brown algae blooms) is also worth studying.