Particle settling and vertical mixing in the Saharan Air Layer as seen from an integrated model, lidar, and in-situ perspective

Abstract. Long-range transport of aerosol in the Saharan Air Layer (SAL) across the Atlantic plays an important role for weather, climate, and ocean fertilization. However, processes occurring within the SAL and their effects on aerosol properties are still unclear. In this work we study particle settling and vertical mixing within the SAL based on measured and modeled vertical aerosol profiles in the upper 1 km of the transported SAL. We use ground-based lidar measurements and airborne particle counter measurements over the western Atlantic, collected during the SALTRACE campaign, as well as space-based CALIOP lidar measurements from Africa to the western Atlantic in the summer season. In our model we take account of the optical properties and the Stokes gravitational settling of irregularly shaped Saharan dust particles. We test two hypotheses about the occurrence of vertical mixing within the SAL over the Atlantic to explain the aerosol profiles observed by the lidars and the particle counter. Our first hypothesis (H1) assumes that no mixing occurs in the SAL leading to a settling-induced separation of particle sizes. The second hypothesis (H2) assumes that vertical mixing occurs in the SAL allowing large super-micron dust particles to stay airborne longer than without mixing. The uncertainties of the particle linear depolarization ratio (δl) profiles measured by the ground-based lidars are comparable to the modeled differences between H1 and H2 and do not allow us to conclude which hypothesis fits better. The SALTRACE in situ data on size-resolved particle number concentrations show a presence of large particles near the SAL top that is inconsistent with H1. The analysis of the CALIOP measurements also reveals that the average δl profile over the western Atlantic is inconsistent with H1. Furthermore, it was found that the average δl profile in the upper 1 km of the SAL does not change along its transport path over the Atlantic. These findings give evidence that vertical mixing within the SAL is a common phenomenon with significant consequences for the evolution of the size distribution of super-micron dust particles during transport over the Atlantic. Further research is needed to precisely characterize the processes that are relevant for this phenomenon.

[1]  Jeffrey S. Reid,et al.  Mineral dust aerosol size distribution change during atmospheric transport , 2003 .

[2]  V. Freudenthaler,et al.  EARLINET: towards an advanced sustainable European aerosol lidar network , 2014 .

[3]  A. Ansmann,et al.  Optimizing CALIPSO Saharan dust retrievals , 2013 .

[4]  Ilan Koren,et al.  Patterns of North African dust transport over the Atlantic: winter vs. summer, based on CALIPSO first year data , 2009 .

[5]  Jacques Pelon,et al.  The seasonal vertical distribution of the Saharan Air Layer and its modulation by the wind , 2013 .

[6]  V. Freudenthaler,et al.  The May/June 2008 Saharan dust event over Munich: Intensive aerosol parameters from lidar measurements , 2011 .

[7]  T. Nagai,et al.  Backscattering linear depolarization ratio measurements of mineral, sea-salt, and ammonium sulfate particles simulated in a laboratory chamber. , 2010, Applied optics.

[8]  P. Formenti,et al.  Size distribution and optical properties of African mineral dust after intercontinental transport , 2016 .

[9]  Alfons G. Hoekstra,et al.  The discrete-dipole-approximation code ADDA: Capabilities and known limitations , 2011 .

[10]  H. C. Simpson.  Bubbles, drops and particles , 1980 .

[11]  C. Emmel,et al.  Electron microscopy of particles collected at Praia, Cape Verde, during the Saharan Mineral Dust Experiment: particle chemistry, shape, mixing state and complex refractive index , 2011 .

[12]  Rtin W Irth.  Spatial distribution and optical properties of Saharan dust observed by airborne high spectral resolution lidar during SAMUM 2006 , 2009 .

[13]  Josef Gasteiger,et al.  Modelling lidar-relevant optical properties of complex mineral dust aerosols , 2011 .

[14]  Albert Ansmann,et al.  Saharan Mineral Dust Experiments SAMUM–1 and SAMUM–2: what have we learned? , 2011 .

[15]  David M. Winker,et al.  The CALIPSO Lidar Cloud and Aerosol Discrimination: Version 2 Algorithm and Initial Assessment of Performance , 2009 .

[16]  Peter Koepke,et al.  Technical Note: Optical properties of desert aerosol with non-spherical mineral particles: data incorporated to OPAC , 2015 .

[17]  Martin Wirth,et al.  Airborne measurements of dust layer properties, particle size distribution and mixing state of Saharan dust during SAMUM 2006 , 2009 .

[18]  Juan Cuesta,et al.  Dynamical mechanisms controlling the vertical redistribution of dust and the thermodynamic structure of the West Saharan atmospheric boundary layer during summer , 2009 .

[19]  J. Heintzenberg.  The SAMUM-1 experiment over Southern Morocco: overview and introduction , 2009 .

[20]  P. Ginoux.  Effects of nonsphericity on mineral dust modeling , 2003 .

[21]  Shu-Hua Chen,et al.  Modification of Saharan air layer and environmental shear over the eastern Atlantic Ocean by dust-radiation effects , 2010 .

[22]  F. X. Kneizys,et al.  AFGL atmospheric constituent profiles (0-120km) , 1986 .

[23]  T. Carlson,et al.  The Large-Scale Movement of Saharan Air Outbreaks over the Northern Equatorial Atlantic , 1972 .

[24]  David M. Winker,et al.  Fully Automated Detection of Cloud and Aerosol Layers in the CALIPSO Lidar Measurements , 2009 .

[25]  Albert Ansmann,et al.  Scanning 6-Wavelength 11-Channel Aerosol Lidar , 2000 .

[26]  D. Winker,et al.  CALIPSO Lidar Description and Performance Assessment , 2009 .

[27]  Albert Ansmann,et al.  Size matters: Influence of multiple scattering on CALIPSO light‐extinction profiling in desert dust , 2010 .

[28]  R. Draxler,et al.  NOAA’s HYSPLIT Atmospheric Transport and Dispersion Modeling System , 2015 .

[29]  P. Koepke,et al.  Optical Properties of Aerosols and Clouds: The Software Package OPAC , 1998 .

[30]  L. Schütz,et al.  LONG RANGE TRANSPORT OF DESERT DUST WITH SPECIAL EMPHASIS ON THE SAHARA * , 1980 .

[31]  R. Washington,et al.  Optical properties of Saharan dust aerosol and contribution from the coarse mode as measured during the Fennec 2011 aircraft campaign , 2012 .

[32]  D. Winker,et al.  Overview of the CALIPSO Mission and CALIOP Data Processing Algorithms , 2009 .

[33]  Eric Loth,et al.  Drag of non-spherical solid particles of regular and irregular shape , 2008 .

[34]  David M. Winker,et al.  CALIPSO lidar observations of the optical properties of Saharan dust: A case study of long‐range transport , 2008 .

[35]  R. Draxler.  HYSPLIT (HYbrid Single-Particle Lagrangian Integrated Trajectory) Model access via NOAA ARL READY Website , 2010 .

[36]  Dong L. Wu,et al.  Transpacific transport and evolution of the optical properties of Asian dust , 2013 .

[37]  M. Wendisch,et al.  Dust mobilization and transport in the northern Sahara during SAMUM 2006 – a meteorological overview , 2009 .

[38]  Measurement of the Linear Depolarization Ratio of Aged Dust at Three Wavelengths (355, 532 and 1064 nm) Simultaneously over Barbados , 2016 .

[39]  M. L. Laucks,et al.  Aerosol Technology Properties, Behavior, and Measurement of Airborne Particles , 2000 .

[40]  Accuracy of Linear Depolarisation Ratios in Clean Air Ranges Measured with POLIS-6 at 355 and 532 NM , 2016 .

[41]  V. Freudenthaler,et al.  The Saharan Aerosol Long-range TRansport and Aerosol-Cloud-Interaction Experiment 1 ( SALTRACE ) : overview and selected highlights 2 , 2016 .

[42]  Albert Ansmann,et al.  The Saharan Aerosol Long-Range Transport and Aerosol–Cloud-Interaction Experiment: Overview and Selected Highlights , 2017 .

[43]  A. Marshak,et al.  Shape‐induced gravitational sorting of Saharan dust during transatlantic voyage: Evidence from CALIOP lidar depolarization measurements , 2013 .

[44]  Alexander Smirnov,et al.  Analysis of measurements of Saharan dust by airborne and ground-based remote sensing methods during the Puerto Rico Dust Experiment (PRIDE) , 2003 .

[45]  David M. Winker,et al.  CALIPSO Lidar Calibration Algorithms. Part I: Nighttime 532-nm Parallel Channel and 532-nm Perpendicular Channel , 2009 .

[46]  Albert Ansmann,et al.  Optical properties of long-range transported Saharan dust over Barbados as measured by dual-wavelength depolarization Raman lidar measurements , 2015 .

[47]  A. Ansmann,et al.  Dual‐wavelength Raman lidar observations of the extinction‐to‐backscatter ratio of Saharan dust , 2002 .

[48]  N. Mahowald,et al.  The size distribution of desert dust aerosols and its impact on the Earth system , 2014 .

[49]  Andrea Mazzino,et al.  Size distribution and optical properties of mineral dust aerosols transported in the western Mediterranean , 2015 .

[50]  Timo Nousiainen,et al.  Laboratory investigations of mineral dust near-backscattering depolarization ratios , 2016 .

[51]  Martin Wirth,et al.  Microphysical and optical properties of dust and tropical biomass burning aerosol layers in the Cape Verde region—an overview of the airborne in situ and lidar measurements during SAMUM-2 , 2011 .

[52]  Thomas Trautmann,et al.  Solar radiative effects of a Saharan dust plume observed during SAMUM assuming spheroidal model particles , 2009 .

[53]  K. Sassen.  The Polarization Lidar Technique for Cloud Research: A Review and Current Assessment , 1991 .

[54]  L. Mona,et al.  Systematic lidar observations of Saharan dust over Europe in the frame of EARLINET (2000-2002) , 2008 .

[55]  S. Leinert,et al.  State of mixing, shape factor, number size distribution, and hygroscopic growth of the Saharan anthropogenic and mineral dust aerosol at Tinfou, Morocco , 2009 .

[56]  Y. Benayahu,et al.  LIDAR multiple scattering from clouds , 1995 .

[57]  J. Stuut,et al.  Particle size traces modern Saharan dust transport and deposition across the equatorial North Atlantic , 2016 .

[58]  Toby N. Carlson,et al.  Vertical and areal distribution of Saharan dust over the western equatorial north Atlantic Ocean , 1972 .

[59]  Jeremy Bailey,et al.  Alignment of atmospheric mineral dust due to electric field , 2007 .