Coarse mode mineral dust size distributions, composition and optical properties from AER-D aircraft measurements over the Tropical Eastern Atlantic

Claire L. Ryder, F. Marenco, Jennifer K. Brooke, V. Estelles, Richard Cotton, P Formenti, James B. McQuaid, Hannah Price, Dantong Liu, Phil Rosenberg, J. W. Taylor, Thomas Choularton, Keith Bower, Hugh Coe, Martin Gallagher, Jonathan Crosier, Gary Lloyd, Eleanor Highwood

Research output: Contribution to journalArticlepeer-review

Abstract

Mineral dust is an important component of the climate system, affecting the radiation balance, cloud properties, biogeochemical cycles, regional circulation and precipitation, as well as having negative effects on aviation, solar energy generation and human health. Dust size and composition has an impact on all these processes. However, changes in dust size distribution and composition during transport, particularly for coarse particles, are poorly understood and poorly represented in climate models. Here we present new in-situ airborne observations of dust in the Saharan Air Layer (SAL) and the Marine Boundary Layer (MBL) during the beginning of its trans-Atlantic transport pathway, from the AER-D fieldwork in August 2015, within the peak season of North African dust export. This study focuses on coarse mode dust properties, including size distribution, mass loading, shape, composition, refractive indices and optical properties. Size distributions from 0.1 to 100µm diameter (d) are presented, fully incorporating the coarse and giant modes of dust. Within the SAL, particles larger than 20µm diameter were always present up to 5km altitude, in concentrations over 10−5cm−3, constituting up to 40% of dust mass. Mean effective diameter and volume median diameter were 4.0µm and 5.5µm respectively. Larger particles were detected in the SAL than can be explained by sedimentation theory alone. Coarse mode composition was dominated by quartz and alumino-silicates; the accumulation mode showed a strong contribution from sulfate-rich and sea salt particles. In the SAL, measured single scattering albedos (SSAs) at 550nm representing d<2.5µm were 0.93 to 0.98 (mean 0.97). Optical properties calculated for the full size distribution (0.1<d<100µm) resulted in lower SSAs of 0.91–0.98 (mean 0.95) and mass extinction coefficients of 0.27–0.35m2g−1 (mean 0.32m2g−1). Variability in SSA was mainly controlled by variability in dust composition (principally iron), rather than by variations in the size distribution, in contrast to previous observations over the Sahara where size is the dominant influence. It is important that models are able to capture this complex evolution of both dust composition and size distribution with transport in order to accurately represent the impacts of dust on climate. These results provide a new SAL dust dataset, fully representing coarse and giant particles, to aid model validation and development.
Original languageEnglish
Pages (from-to)17225-17257
Number of pages49
JournalAtmospheric Chemistry and Physics
Volume18
Early online date6 Dec 2018
DOIs
Publication statusPublished - 2018

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