Aircraft and ground measurements of dust aerosols over the west African coast in summer 2015 during ICE-D and AER-D

Abstract

Abstract. During the summertime, dust from the Sahara can be efficiently transported westwards within the Saharan air layer (SAL). This can lead to high aerosol loadings being observed above a relatively clean marine boundary layer (MBL) in the tropical Atlantic Ocean. These dust layers can impart significant radiative effects through strong visible and IR light absorption and scattering, and can also have indirect impacts by altering cloud properties. The processing of the dust aerosol can result in changes in both direct and indirect radiative effects, leading to significant uncertainty in climate prediction in this region. During August 2015, measurements of aerosol and cloud properties were conducted off the coast of west Africa as part of the Ice in Cloud ExperimentDust (ICE-D) and AERosol properties -Dust (AER-D) campaigns. Observations were obtained over a 4-week period using the UK Facility for Atmospheric Airborne Measurements (FAAM) BAe 146 aircraft based on Santiago Island, Cabo Verde. Ground-based observations were collected from Praia (14 • 57 N, 23 • 29 W; 100 m a.s.l.), also located on Santiago Island. The dust in the SAL was mostly sampled in situ at altitudes of 2-4 km, and the potential dust age was estimated by backward trajectory analysis. The particle mass concentration (at diameter d = 0.1-20 µm) decreased with transport time. Mean effective diameter (D eff ) for supermicron SAL dust (d = 1-20 µm) was found to be 5-6 µm regardless of dust age, whereas submicron D eff (d = 0.1-1 µm) showed a decreasing trend with longer transport.For the first time, an airborne laser-induced incandescence instrument (the single particle soot photometer -SP2) was deployed to measure the hematite content of dust. For the Sahel-influenced dust in the SAL, the observed hematite mass fraction of dust (F Hm ) was found to be anti-correlated with the single scattering albedo (SSA, λ = 550 nm, for particles d < 2.5 µm); as potential dust age increased from 2 to 7 days, F Hm increased from 2.5 to 4.5 %, SSA decreased from 0.97 to 0.93 and the derived imaginary part (k) of the refractive index at 550 nm increased from 0.0015 to 0.0035. However, the optical properties of Sahara-influenced plumes (not influenced by the Sahel) were independent of dust age and hematite content with SSA ∼ 0.95 and k ∼ 0.0028. This indicates that the absorbing component of dust may be source dependent, or that gravitational settling of larger particles may lead to a higher fraction of more absorbing clay-iron aggregates at smaller sizes. Mie calculation using the measured size distribution and size-resolved refractive indices of the absorbing components (black carbon and hematite) reproduces the measured SSA to within ±0.02 for SAL dust by assuming a goethite / hematite mass ratio of 2. Overall, hematite and goethite constituted 40-80 % of the absorption for particles d < 2.5 µm, and black carbon (BC) contributed 10-37 %. This highlights the importance of size-dependent composition in determining the optical properties of dust and also the ...

Mineralogy and mixing state of North African mineral dust by on-line single-particle mass spectrometry

“…Nevertheless, the temporal evolution of the particle classes is representative and compares well to the the dust fraction of incandescent particles identified in tandem Soot photometer (SP2) measurement (Liu et al, 2018). …”

Classification of iron oxide aerosols by a single particle soot photometer using supervised machine learning

“…Recently, the SP2 has been increasingly used to quantify other light absorbing refractory aerosols (e.g. Moteki et al (2017); Liu et al (2018)). In particular, observations in source regions have shown that iron oxide containing aerosols from anthropogenic origins are present in the 5 atmosphere (Liati et al, 2015;Dall'Osto et al, 2016;Adachi et al, 2016;Li et al, 2017), and these aerosols can be detected via laser-induced incandescence with an SP2 (Yoshida et al, 2016;Moteki et al, 2017).…”

“…Incandescent particles in Icelandic mineral dust and Taklamakan Desert dust also have been detected with low efficiency using the SP2 (Yoshida et al, 2016). An SP2 was used in one study to estimate the hematite content in Saharan dust measured off the West coast of Africa, and demonstrated good closure between 10 the hematite concentration associated with single dust particles and the optical properties observed in the dust plume (Liu et al, 2018). While previous work focusing on anthropogenic FeO x has relied on using the optical size of these aerosols after any volatile coatings have evaporated as an additional criteria to differentiate anthropogenic and non-anthropogenic aerosols with metallic components (Yoshida et al, 2016;Moteki et al, 2017;Yoshida et al, 2018), this method is limited to the range over which the SP2 optically sizes these aerosols (generally limited to ∼170-350 nm volume equivalent diameter 15 for FeO x ).…”

“…We also found that the width of the distribution of color ratios for rBC and FeO x as a function of incandescent peak height strongly 15 depended on the alignment of the red PMT detector. The incandescent peak height (mass) and a low color ratio together provide sufficient contrast to differentiate iron oxide containing aerosols from rBC for larger incandescent peak heights (equivalent to ∼2 fg rBC) (Yoshida et al, 2016;Liu et al, 2018;Moteki et al, 2017). When detecting mixed populations of FeO x and rBC, the incomplete contrast between rBC and FeO x in terms of their incandescent peak heights and color ratio limits detection at smaller sizes.…”

“…Given that natural dust grains typically consist of metallic inclusions surrounded by other materials, even if these portions of the aerosol incandesce in the SP2, the entire particle may not be completely vaporized. Recent measurements using the SP2 to characterize hematite content in dust particles estimated that mineral dust particles detected by the SP2 were generally >500 nm in size (Liu et al, 2018). Post-incandescent scattering, defined as the scattering amplitude of the particle measured after the incandescent signal has returned to the baseline, indicates portions of the aerosol still remain after the refractory portion of the 5 particle has evaporated (Sedlacek III et al, 2012).…”