Articles | Volume 18, issue 13
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-Merino, David Mateos, Carlos Toledano, Joseph M. Prospero, Jack Molinie, Lovely Euphrasie-Clotilde, Ramiro González, Victoria E. Cachorro, Abel Calle, and Angel M. de Frutos

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