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

Abstract

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 (δ l ) 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 δ l profile over the western Atlantic is inconsistent with H1. Furthermore, it was found that the average δ l 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.

Triple-wavelength depolarization-ratio profiling of Saharan dust over Barbados during SALTRACE in 2013 and 2014

“…An impact of gravitational settling leading to a decrease of coarse-mode dust concentration in the SAL top region after 5-10 days of travel, which would show up in a significant change in the spectral slope of the depolarization ratio (especially at 1064 nm), is not visible. This corroborates the hypothesis proposed by Gasteiger et al (2017) that heating of the dust particles and turbulent mixing of the SAL air masses during daytime hours may widely reduce coarsemode dust removal by gravitational settling of particles. During SALTRACE-3 in 2014, POLIS was not available.…”

“…Many more laboratory and model studies, together with our complex field observations, are required to improve stepwise our knowledge on the complex relationship between fundamental dust properties and related optical effects. Figure 15 provides a comparison of the observations with triple-wavelength polarization lidars (BERTHA, HSRL-2) and a preliminary modeling result of the depolarization ratios described by Gasteiger et al (2017) in their hypothesis with vertical mixing during the day. A mix of irregularly shaped (non-spheroidal) particles and small spherical ammonium sulfate particles is assumed in the simulation.…”

“…3.3 is shown. The observational findings are compared with respective model results, which are based on the dust size-shape characteristics described in Gasteiger et al (2017). 0.20 and 0.28 for dust after long-range transport. The influence of the fine-and coarse-mode dust fractions on the particle linear depolarization ratio is discussed in detail by Mamouri and Ansmann (2017).…”

Profiling of Saharan dust from the Caribbean to western Africa – Part 1: Layering structures and optical properties from shipborne polarization/Raman lidar observations

“…However, our detailed profile observations presented in Sect. 3.3 as well as the SALTRACE observations in Barbados in June and July 2013 and 2014 (Groß et al, 2015;Gasteiger et al, 2017;Haarig et al, 2017a) suggest that further processes in the SAL are active in addition and counteract sedimentation of dust particles. We will continue the discussion of this point in Sect.…”

“…Gasteiger et al (2017) argue that absorption of solar radiation introduces turbulent mixing of dust within the SAL and thus upward and downward transport of dust which weakens the pure sedimentation-based dust removal effect. Colarco et al (2003) and Yang et al (2013) discuss the impact of different shapes of dust particles on falling speed and gravitational settling behavior.…”