Spectral- and size-resolved mass absorption efficiency of mineral dust aerosols in the shortwave spectrum: a simulation chamber study

Abstract

Abstract. This paper presents new laboratory measurements of the mass absorption efficiency (MAE) between 375 and 850 nm for 12 individual samples of mineral dust from different source areas worldwide and in two size classes: PM 10.6 (mass fraction of particles of aerodynamic diameter lower than 10.6 µm) and PM 2.5 (mass fraction of particles of aerodynamic diameter lower than 2.5 µm). The experiments were performed in the CESAM simulation chamber using mineral dust generated from natural parent soils and included optical and gravimetric analyses.The results show that the MAE values are lower for the PM 10.6 mass fraction (range 37-135 × 10 −3 m 2 g −1 at 375 nm) than for the PM 2.5 (range 95-711 × 10 −3 m 2 g −1 at 375 nm) and decrease with increasing wavelength as λ −AAE , where the Ångström absorption exponent (AAE) averages between 3.3 and 3.5, regardless of size. The size independence of AAE suggests that, for a given size distribution, the dust composition did not vary with size for this set of samples. Because of its high atmospheric concentration, light absorption by mineral dust can be competitive with black and brown carbon even during atmospheric transport over heavy polluted regions, when dust concentrations are significantly lower than at emission. The AAE values of mineral dust are higher than for black carbon (∼ 1) but in the same range as light-absorbing organic (brown) carbon. As a result, depending on the environment, there can be some ambiguity in apportioning the aerosol absorption optical depth (AAOD) based on spectral dependence, which is relevant to the development of remote sensing of light-absorbing aerosols and their assimilation in climate models. We suggest that the sample-to-sample variability in our dataset of MAE values is related to regional differences in the mineralogical composition of the parent soils. Particularly in the PM 2.5 fraction, we found a strong linear correlation between the dust lightPublished by Copernicus Publications on behalf of the European Geosciences Union. 7176 L. Caponi et al.: Spectral-and size-resolved mass absorption efficiency of mineral dust aerosols absorption properties and elemental iron rather than the iron oxide fraction, which could ease the application and the validation of climate models that now start to include the representation of the dust composition, as well as for remote sensing of dust absorption in the UV-vis spectral region.

Influence of light absorbing particles on snow spectral irradiance profiles

“…Coating of LAP by non-absorbing aerosols is, for example, suspected to enhance their absorption efficiency by up to a factor 3 (e.g., Schnaiter et al, 2005;Moffet and Prather, 2009). Caponi et al (2017) highlighted the high variability of the optical properties of dust particles with respect to their size distribution and their origin, leading to one order of magnitude uncertainty in absorption by dust for a given mass. Thirdly, 25 the interactions between LAP and snow are known to impact LAP absorption efficiency but are still poorly understood highlighted that for a given BC concentration in snow, the absorption can be up to twice as much if particles are inside the ice rather than in the air surrounding the ice, but estimating LAP mixing state is challenging.…”

Three years of measurements of light-absorbing aerosols over coastal Namibia: seasonality, origin, and transport

“…Current understanding suggests that atmospheric aerosols increase the global outgoing shortwave radiation, enhancing the atmospheric albedo, thereby counteracting the warming effect of greenhouse gases (Boucher et al, 2013). However, light-absorbing aerosols, such as black carbon (BC) from fossil fuel combustion and biomass burning, can reduce the amount of outgoing radiation at the top of atmosphere (TOA), finally adding to the greenhouse effect (Haywood and Shine, 1995;Jacobson, 2001;Chung and Seinfeld, 2002;Bond and Bergstrom, 2006;Koch and Del Genio, 2010;Bond et al, 2013). The heating radiative effect of BC aerosols is either enhanced or suppressed if they are above or below clouds, respectively Koch and Del Genio, 2010).…”

Black and brown carbon over central Amazonia: Long-term aerosol measurements at the ATTO site

“…It is mobilized from soils and suspended in the atmosphere by windstorms in areas like the Saharan desert in Africa. Dust aerosol particles in the atmosphere efficiently scatter visible radiation and are able to absorb infrared radiation (Andreae, 1996), having a å abs >> 1.0 (Caponi et al, 2017;Denjean et al, 2016). Mineral dust plumes travel over the Atlantic Ocean and are able to reach the American continent.…”