Coarse and giant particles are ubiquitous in Saharan dust export regions and are radiatively significant over the Sahara

Abstract

Abstract. Mineral dust is an important component of the climate system, interacting with radiation, clouds, and biogeochemical systems and impacting atmospheric circulation, air quality, aviation, and solar energy generation. These impacts are sensitive to dust particle size distribution (PSD), yet models struggle or even fail to represent coarse (diameter (d) >2.5 µm) and giant (d>20 µm) dust particles and the evolution of the PSD with transport. Here we examine three state-of-the-art airborne observational datasets, all of which measured the full size range of dust (d=0.1 to >100 µm) at different stages during transport with consistent instrumentation. We quantify the presence and evolution of coarse and giant particles and their contribution to optical properties using airborne observations over the Sahara (from the Fennec field campaign) and in the Saharan Air Layer (SAL) over the tropical eastern Atlantic (from the AER-D field campaign). Observations show significantly more abundant coarse and giant dust particles over the Sahara compared to the SAL: effective diameters of up to 20 µm were observed over the Sahara compared to 4 µm in the SAL. Excluding giant particles over the Sahara results in significant underestimation of mass concentration (40 %), as well as underestimates of both shortwave and longwave extinction (18 % and 26 %, respectively, from scattering calculations), while the effects in the SAL are smaller but non-negligible. The larger impact on longwave extinction compared to shortwave implies a bias towards a radiative cooling effect in dust models, which typically exclude giant particles and underestimate coarse-mode concentrations. A compilation of the new and published effective diameters against dust age since uplift time suggests that two regimes of dust transport exist. During the initial 1.5 d, both coarse and giant particles are rapidly deposited. During the subsequent 1.5 to 10 d, PSD barely changes with transport, and the coarse mode is retained to a much greater degree than expected from estimates of gravitational sedimentation alone. The reasons for this are unclear and warrant further investigation in order to improve dust transport schemes and the associated radiative effects of coarse and giant particles in models.

Models transport Saharan dust too low in the atmosphere: a comparison of the MetUM and CAMS forecasts with observations

“…The vertical distribution, particle size distribution, and mass concentration are the key properties that are predicted in a dust transport model. On the other hand, the main observable quantity on a global scale is aerosol optical depth from AERONET (Holben et al, 1998), MODIS (Hsu et al, 2004(Hsu et al, , 2006(Hsu et al, , 2013Levy et al, 2013;Sayer et al, 2013Sayer et al, , 2014, and potentially other sources such as the Polar Multi-Sensor Aerosol product (PMAp; Lang et al, 2017), the Visible Infrared Imaging Radiometer Suite (VIIRS; Hsu et al, 2019), and several others. Aerosol optical depth is at the same time an optical property and a vertically integrated quantity, meaning that the same observable AOD can be retrieved e.g.…”

“…MODIS collection 6.1 level 2 atmospheric aerosol products from Aqua (MYD04_L2) and Terra (MOD04_L2) were obtained from the Level-1 and Atmosphere Archive & Distribution System (LAADS, ftp://ladsftp.nascom.nasa.gov/ allData/61/, last access: 21 September 2017). The merged Deep Blue and Dark Target aerosol optical depth at 550 nm from both Aqua and Terra was used to create daily AOD maps (Hsu et al, 2004(Hsu et al, , 2006(Hsu et al, , 2013Levy et al, 2013;Sayer et al, 2013Sayer et al, , 2014. The differences between the collection 5 (used in both models for operational assimilation in August 2015) and the subsequently released collection 6 are treated in detail in the above-referenced papers.…”

Modelling mineral dust emissions and atmospheric dispersion with MADE3 in EMAC v2.54

“…Furthermore, dust particles may have negative implications for human health, e.g. by causing respiratory diseases (Chan et al, 2008;Sajani et al, 2011;Giannadaki et al, 2014). On the other hand, mineral dust provides nutrients such as iron or phosphorus that are essential for the growth of tropical rainforests, as well as oceanic life (Chadwick et al, 1999;Jickells et al, 2005;Nenes et al, 2011;Yu et al, 2015).…”

Spectral Aerosol Optical Depth Retrievals by Ground-Based Fourier Transform Infrared Spectrometry

“…It is well known that most aerosol types show a rather small impact on IR in comparison to their effect in the visible (VIS) region [7], where the strongest aerosol-radiation interaction takes place. However, large aerosols such as mineral dust are able to efficiently interact with both solar and terrestrial radiation [8], with a radiative impact sensitive to particle size distribution and its evolution with transport [9]. A better characterization of larger aerosol particles may enhance, for example, the spectral sensitivity to super-micron (coarse mode) particles in the inversion techniques [10,11].…”

“…This improved characterization could help to estimate more precisely global anthropogenic aerosol forces [4] and to solve the problematic representation of large aerosols like mineral dust in the current global climate models. Traditionally, mineral dust in the atmosphere is considered a particle substantially coarser than the dust represented in climate models, leading to a bias towards a radiative cooling effect in these dust models [9,12]. Actually, as [9,13] have found, the incorporation of coarse particles in climate model simulations is important to account for the substantial radiative effect of dust in terms of the reduction of single-scattering albedo of shortwave radiation, the increase of radiative absorption, and the enhancement of long-wave radiation absorption.…”