Description
Annually, the Saharan Air Layer (SAL), formed by the westbound outflow of warm and dry airmasses from North Africa, transports considerable amounts of dust aerosol across the Atlantic
Ocean as far as into the Caribbean Sea and the Americas. Supplying the affected areas with
additional cloud condensation nuclei (CCN), this aerosol influences the formation and properties
of clouds, thereby indirectly impacting on climate.
However, so far there has been a lack of comprehensive investigations of the CCN properties of
the SAL aerosol, complicating the assessment of these impacts. During the Saharan Aerosol
Long-Range Transport and Aerosol-Cloud Interaction Experiment (SALTRACE) in summer
2013, the SAL was therefore thoroughly probed with an extensive airborne in situ
instrumentation at both ends of its transatlantic journey. The measurements covered the full
vertical extent of the layer, on average ranging from about 1 to 6 km a.s.l. at the Northwest
African coast and from about 2 to 4.5 km a.s.l. in the Caribbean. Besides the concentration of
CCN at a water vapor supersaturation of 0.2%, the acquired data set includes information on
particle number size distribution (NSD), size-resolved particle composition and volatility (at
temperatures above 250°C).
Based on these data, we show that the SAL carries large numbers of hygroscopic non-dust
particles that constitute an essential part of its CCN. We find that at 0.2% supersaturation
particles act as CCN down to diameters of about 80 nm, which is close to the position of the
particle NSD maximum. In this size range, that contributes the lion’s share to CCN number, the
particulate matter is dominated by non-refractory material, primarily identified as ammonium
sulfate (AS). This finding of the compositional analysis is corroborated by a high, near-AS
particle hygroscopicity (휅̅ ≈ 0.5) inferred from the combination of CCN and NSD measurements.
The actual mineral dust in the “dust layer” is the dominant component only for particle diameters
larger about 500 nm. Whereas these coarse dust particles control particulate surface and
volume/mass, their contribution to CCN number is subordinate (only around 10%).
We further show that the features of the SAL aerosol, i.e. size-dependent particle composition
and CCN properties, are likewise observed at the Northwest African coast and in the Caribbean,
meaning that they remain approximately preserved during transatlantic transport. Accordingly,
these features are characteristic for the initial dust aerosol in Northern Africa. Finally, we suggest
that a significant fraction of the AS found in this aerosol is ascribable to anthropogenic sources,
shedding new light on the human influence on SAL-induced climate effects.
| Period | 23 May 2018 |
|---|---|
| Event title | 9th International Workshop on Sand/Dust storms |
| Event type | Seminar/Workshop |
| Location | La Laguna, SpainShow on map |