Shape and size constraints on dust optical properties from the Dome C ice core, Antarctica

Abstract

Mineral dust aerosol (dust) is widely recognized as a fundamental component of the climate system and is closely coupled with glacial-interglacial climate oscillations of the Quaternary period. However, the direct impact of dust on the energy balance of the Earth system remains poorly quantified, mainly because of uncertainties in dust radiative properties, which vary greatly over space and time. Here we provide the first direct measurements of the aerosol optical thickness of dust particles windblown to central East Antarctica (Dome C) during the last glacial maximum (LGM) and the Holocene. By applying the Single Particle Extinction and Scattering (SPES) technique and imposing preferential orientation to particles, we derive information on shape from samples of a few thousands of particles. These results highlight that clear shape variations occurring within a few years are hidden to routine measurement techniques. With this novel measurement method the optical properties of airborne dust can be directly measured from ice core samples, and can be used as input into climate model simulations. Based on simulations with an Earth System Model we suggest an effect of particle non-sphericity on dust aerosol optical depth (AOD) of about 30% compared to spheres, and differences in the order of ~10% when considering different combinations of particles shapes.

The PMIP4 contribution to CMIP6 – Part 2: Two interglacials, scientific objective and experimental design for Holocene and Last Interglacial simulations

“…If atmospheric dust loading is not represented in the DECK piControl and historical simulations, it should not be included in the midHolocene and lig127k simulations. The impact of dust on the radiation balance is sensitive to the optical properties prescribed (Perlwitz et al, 2001); it is uncertain how optical properties might change during interglacials (Potenza et al, 2016;Royer et al, 1983). Uncertainties in the protocol and in the interplay between dust and vegetation will be a focus of the analyses.…”

Particle shape accounts for instrumental discrepancy in ice core dust size distributions

“…The aspect ratio of ice core dust particles was measured using the novel single-particle extinction and scattering (SPES) instrument Potenza et al, 2016). The SPES measures both the extinction cross section, which is also measured by the Abakus, and the optical thickness of the particles .…”

Particle shape accounts for instrumental discrepancy in ice core dust size distributions

“…Although both instruments are typically calibrated using standard spheres of known diameter, they produce substantially different size distribution results when ice core samples are measured (Ruth et al, 2003;Lambert et al, 2008Lambert et al, , 2012Koffman et al, 2014). It has been proposed that this difference is because ice core dust is generally non-spherical (Lambert et al, 2012;Potenza et al, 2016). and we show here that the non-spherical shape of the particles does quantitatively account for the main discrepancy between the two instruments ( Figure 1).…”

“…These measurements have revealed that the dust in ice cores come from central Asia during both the Holocene and the last glacial period (Biscaye et al, 1997). The observed 100 times 15 decrease in dust concentration from glacial to Holocene has constrained the aridity, windiness and insolation forcing of glacial climate models (Ruth et al, 2003;Steffensen, 1997).…”

“…The Coulter Counter on the other hand typically integrates a thicker interval, and requires work from the operator all the time during measurements (Delmonte et al, 2004;Lambert et al, 2012). 15 The aspect ratio of ice core dust particles was measured using the novel Single Particle Extinction and Scattering (SPES) instrument Potenza et al, 2016). In addition to the extinction cross section, which is also measured by the Abakus, it measures the optical thickness of the particles .…”