Decades of aerosol measurements on Barbados have yielded a detailed picture of African mineral dust transport to the Caribbean Basin that shows a strong seasonal cycle with a maximum in boreal summer and a minimum in winter. Satellite aerosol products suggest that in spring, there is a comparable transport to northeastern South America. Here we characterize the complete annual cycle of dust transport to the western Atlantic by linking the Barbados record to multiyear records of airborne particulate matter less than 10 μm diameter (PM 10 ) measured in air quality programs at Cayenne (French Guiana) and Guadeloupe. Comparisons of PM 10 at these sites with concurrent dust measurements at Barbados demonstrate that high PM 10 levels are almost entirely due to dust. Cayenne PM 10 peaks in spring in a cycle which is consistent with satellite aerosol optical depth and suggests that the Sahel is the dominant source. The persistent transport of dust during much of the year could impact a wide range of environmental processes over a broad region that extends from the southern United States to the Amazon Basin. Finally, the average 24 h PM 10 concentrations at Cayenne and Guadeloupe frequently exceed the World Health Organization air quality guideline. Thus soil dust PM 10 could be a significant, but generally unrecognized, health factor at western Atlantic sites and also in other relatively remote regions affected by long-range dust from Africa. Because dust emissions and transport are highly sensitive to climate variability, climate change in coming decades could greatly affect a wide range of biogeochemical processes and human health in this region.

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.

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The climatological conditions that are favorable to thunderstorm cloud development in the coastal zone of French Guyana are investigated. The analysis reveals that both the thunderstorm occurrence and the mean monthly lightning frequency are correlated to the Intertropical Convergence Zone annual oscillation, thus increasing from March to August when the South‐East trade‐wind flux drives continental air masses towards French Guyana and decreasing from September to February when French Guyana is under the influence of maritime air masses driven by the North‐East trade‐wind. The study of the lightning frequency values, observed during individual thunderstorm events, as a function of the cloud‐top height shows that these values are intermediate to the lightning frequency values deduced from the parameterizations proposed by Price and Rind (1992) for typical continental and maritime thunderstorm clouds of a similar height. This behavior suggests that the air masses in which thunderstorm clouds develop in the French Guyana coast are mixed air masses with intermediate characteristics. Further, the absence of correlation between the lightning frequency values and both the cloud‐top heights and the corresponding convective available potential energy (CAPE) values indicates that these parameters can not be used to predict lightning activity in these clouds. Most probably, the aerosol concentrations in the subcloud layer and consequently the cloud droplet concentrations play an important role in the development of lightning activity in the considered clouds.