Articles | Volume 18, issue 13
Atmos. Chem. Phys., 18, 9411–9424, 2018

Special issue: The Saharan Aerosol Long-range Transport and Aerosol-Cloud-interaction...

Atmos. Chem. Phys., 18, 9411–9424, 2018

Research article 05 Jul 2018

Research article | 05 Jul 2018

Impact of long-range transport over the Atlantic Ocean on Saharan dust optical and microphysical properties based on AERONET data

Cristian Velasco-Merino1, David Mateos1, Carlos Toledano1, Joseph M. Prospero2, Jack Molinie3, Lovely Euphrasie-Clotilde3, Ramiro González1, Victoria E. Cachorro1, Abel Calle1, and Angel M. de Frutos1 Cristian Velasco-Merino et al.
  • 1Grupo de Óptica Atmosférica, Dpto. de Física Teórica Atómica y Óptica, Universidad de Valladolid, Valladolid, Spain
  • 2Cooperative Institute for Marine and Atmospheric Studies, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida, USA
  • 3Laboratory of Geosciences and Energy, Université des Antilles, Pointe-à-Pitre, Guadeloupe, France

Abstract. Arid regions are a major source of mineral dust aerosol. Transport from these sources can have a great impact on aerosol climatology in distant regions. In order to assess the impact of dust on climate we must understand how dust properties change after long distance transport from sources. This study addresses the changes in columnar aerosol properties when mineral dust outbreaks from western Africa arrive over the eastern Caribbean after transport across the Atlantic Ocean, a transit of 5–7 days. We use data from the NASA Aerosol Robotic Network (AERONET) located at five Caribbean and two western Africa sites to characterize changes in columnar aerosol properties: aerosol optical depth (AOD), size distribution, single scattering albedo, and refractive indexes. We first characterized the local aerosol climatology at each site and then using air mass back trajectories we identified those days when trajectories over Caribbean sites back-tracked to western Africa. Over the period 1996–2014 we identify 3174 days, an average of 167 days per year, when the air mass over the Caribbean sites could be linked to at least one of the two western Africa sites. For 1162 of these days, AOD data are available for the Caribbean sites as well as for the corresponding western Africa sites about 5–7 days earlier, when the air mass passed over these sites. We identified dust outbreaks as those air masses yielding AOD  ≥  0.2 and an Ångström exponent below 0.6. On this basis of the total 1162 days, 484 meet the criteria for mineral dust outbreaks. We observe that the AOD at 440 nm decreases by about 0.16 or 30 % during transport. The volume particle size distribution shows a similar decrease in the volume concentration, mainly in the coarse mode. The single scattering albedo, refractive indexes, and asymmetry factor remain unchanged. The difference in the effective radius over western Africa sites with respect to Caribbean sites ranges between 0 and −0.3 µm. Finally we conclude that in about half of the cases only non-spherical dust particles are present in the atmosphere over the western Africa and Caribbean sites, while in the other cases dust particles were mixed with other types of aerosol particles.

Short summary
We present the first comparison of columnar aerosol properties recorded by sun photometry of Saharan dust between western Africa and Caribbean Basin. A comprehensive climatology of 20 years of data is presented in the two areas. To our knowledge, we present the first global climatology of columnar aerosols in the Caribbean Basin. Changes after transport in aerosol load, size distribution, shape, and absorbing and scattering variables are quantified using long-term records between 1996 and 2014.
Final-revised paper