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.

Highlights

  • Mineral dust aerosol is an important component of the climate system

  • The AER-D-Saharan Air Layer (SAL) accumulation- and coarse-mode enhancement may occur because AER-D sampled more intense dust events, though this seems unlikely given that the Fennec dust events were often very intense and aerosol optical depth (AOD) were mostly higher than AER-D (Ryder et al, 2015)

  • This enhancement of the accumulation mode is similar to differences between SAMUM1 (Morocco) and SAMUM2 (Cape Verde region), for which enhancements in number concentration between 0.3 and 4 μm during SAMUM2 were attributed to coagulational growth (Weinzierl et al, 2011)

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Summary

Introduction

Mineral dust aerosol is an important component of the climate system. Atmospheric mineral dust is estimated to account for 70 % of the global aerosol mass burden and 25 % of the global aerosol optical depth (AOD) (Kinne et al, 2006). During atmospheric transport and through subsequent deposition, dust exerts an impact the climate system by interacting with both shortwave and longwave radiation (Tegen and Lacis, 1996; Liao and Seinfeld, 1998). These radiative effects can impact on the global energy balance, land and sea surface temperatures, atmospheric heating, and circulation patterns.

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