Long‐range transport across the Atlantic in summertime does not enhance the hygroscopicity of African mineral dust

Abstract

We present the first direct evidence that the hygroscopic properties of super micron (>1 μm) African dust particles did not change despite undergoing long-range transport across the Atlantic toward the Caribbean. Concurrent measurements of chemical composition show that most of mineral dust was chemically unprocessed and externally mixed. A minor portion of mineral dust was internally mixed with sulfate and chloride (~13-24% by number) or aggregated with sea-salt particles (~3-6%). Only dust particles aggregated with sea salt showed significant hygroscopic growth above 75% relative humidity (RH), resulting in a decrease in extinction mass efficiency by up to a factor 2.2. All other dust particles did not take up significant amounts of water when exposed to up to 94% RH. These results demonstrate that the direct radiative effect of African dust in this region remained independent of RH and an external mixing state could be considered for evaluating the climate effects of dust.

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

“…Regarding potential changes of the particle optical properties, Denjean et al (2015) observed that internal mixing did not have any effect on the hydrophobic behavior of dust; the optical properties did not change for low and high relative humidities. Groß et al (2016) found a good correlation between in situ observed dust mass concentrations at Ragged Point, Barbados, and lidar-derived dust mass concentrations within the humid MBL with typical relative humidities around 80 %.…”

“…Lieke et al (2011), Kandler et al (2011), Denjean et al (2015, and Weinzierl et al (2017) investigated the potential impact of cloud processing and chemical aging, and analyzed their observations regarding a potential sulfate coating of dust particles or a potential internal mixing with marine (sea salt) aerosol components. According to the airborne in situ measurements performed during SALTRACE over Cabo Verde and in the Barbados region in the summer of 2013 , the abundance of silicate particles (in the diameter range of 500 to 1000 nm) with detectable amounts of sulfate (≈ 1 %) increased from 2.5 % (in the Cabo Verde region) to 4.3 % (in the Caribbean).…”

“…For supermicron particles, no considerable modification (internal mixture) was visible. Denjean et al (2015) performed detailed chemical analysis of long-range transported dust with diameters > 1 µm in Puerto Rico (in situ observations at ground level) in the summer of 2012 and found that about 10 % of the dust particles were internally mixed and showed sulfate and chloride coatings during a strong dust event, and 3-6 % of the dust particles showed an internal mixture with sea salt particles. Lieke et al (2011) (airborne observations) and Kandler et al (2011) (ground-based observations) found during the SAMUM-2 campaign (Cabo Verde, January-February 2008, biomass burning season in central-western Africa) internal silicate-sulfate mixtures in roughly 10-20 % of the analyzed particles in the 500-1000 nm range and about 10 % for particles with diameters > 1 µm.…”

Limited production of sulfate and nitrate on front-associated dust storm particles moving from desert to distant populated areas in northwestern China

“…It was also found that, for a dust plume lofted from the surface by a synoptic mid-latitude cyclone, the plume did not mix significantly with adjacent air parcels polluted by anthropogenic sources; the dust plume and the polluted air were separated as two air parcels by the cold front associated with the cyclone (Bates et al, 2004;Tsai et al, 2014;Wang et al, 2013;Zhang et al, 2005). Some measurements of the chemical composition of longdistance-transported dust particles have also shown that most of the dust particles were not altered chemically and were externally mixed with species produced in the air via gasto-particle reactions, such as sulfate and nitrate (Denjean et al, 2015;Song et al, 2005). These results leave us with a question: why are there so many different rates of sulfate and nitrate production during dust transport to polluted areas?…”

Enhanced ice nucleation efficiency of microcline immersed in dilute NH<sub>3</sub> and NH<sub>4</sub><sup>+</sup>-containing solutions

“…Calcium-rich particles, originating from calcite, have been shown to be more susceptible to processing by SO2 and NOy than silicates and aluminosilicates (Matsuki et al, 2005b). Furthermore, airborne and ground station measurements imply that Saharan dust particles undergo little chemical processing during long-range transport across the Atlantic unless they become incorporated in cloud droplets (Matsuki et al, 410 2010;Denjean et al, 2015;Fitzgerald et al, 2015). An exception are desert dusts originating from the industrial regions at the Atlantic coast of Morocco, Algeria, and Tunisia, which show high concentrations of ammonium, sulfate, and nitrate when sampled at Tenerife, Canary Islands (Rodríguez et al, 2011).…”