Size-resolved mixing state of black carbon in the Canadian high Arctic and implications for simulated direct radiative effect

Abstract

Abstract. Transport of anthropogenic aerosol into the Arctic in the spring months has the potential to affect regional climate; however, modeling estimates of the aerosol direct radiative effect (DRE) are sensitive to uncertainties in the mixing state of black carbon (BC). A common approach in previous modeling studies is to assume an entirely external mixture (all primarily scattering species are in separate particles from BC) or internal mixture (all primarily scattering species are mixed in the same particles as BC). To provide constraints on the size-resolved mixing state of BC, we use airborne single-particle soot photometer (SP2) and ultrahigh-sensitivity aerosol spectrometer (UHSAS) measurements from the Alfred Wegener Institute (AWI) Polar 6 flights from the NETCARE/PAMARCMIP2015 campaign to estimate coating thickness as a function of refractory BC (rBC) core diameter and the fraction of particles containing rBC in the springtime Canadian high Arctic. For rBC core diameters in the range of 140 to 220 nm, we find average coating thicknesses of approximately 45 to 40 nm, respectively, resulting in ratios of total particle diameter to rBC core diameters ranging from 1.6 to 1.4. For total particle diameters ranging from 175 to 730 nm, rBC-containing particle number fractions range from 16 % to 3 %, respectively. We combine the observed mixing-state constraints with simulated size-resolved aerosol mass and number distributions from GEOS-Chem-TOMAS to estimate the DRE with observed bounds on mixing state as opposed to assuming an entirely external or internal mixture. We find that the pan-Arctic average springtime DRE ranges from −1.65 to −1.34 W m −2 when assuming entirely externally or internally mixed BC. This range in DRE is reduced by over a factor of 2 (−1.59 to −1.45 W m −2 ) when using the observed mixing-state constraints. The difference in DRE between the two observed mixing-state constraints is due to an underestimation of BC mass fraction in the springtime Arctic in GEOS-Chem-TOMAS compared to Polar 6 observations. Measurements of mixing state provide important constraints for model estimates of DRE.

Radiative Effects of Residential Sector Emissions in China: Sensitivity to Uncertainty in Black Carbon Emissions

“…Realistically resolving the mixing state of BC with other aerosol species is a key computational challenge (Kodros et al, 2018;Matsui et al, 2013). Realistically resolving the mixing state of BC with other aerosol species is a key computational challenge (Kodros et al, 2018;Matsui et al, 2013).…”

“…The biases in AOD and SSA can be explained if there are missing processes in the model or errors in calculation of aerosol optical properties. Realistically resolving the mixing state of BC with other aerosol species is a key computational challenge (Kodros et al, 2018;Matsui et al, 2013). We use a Maxwell-Garnett approximation, which avoids the known biases of a volume-mixing or external-mixing rules (Barnard et al, 2010).…”

Biogenic and anthropogenic sources of aerosols at the High Arctic site Villum Research Station

“…The Arctic haze peaks in early spring (Heidam et al, 1999(Heidam et al, , 2004Law and Stohl, 2007;Stohl, 2006;Abbatt et al, 2019). Arctic haze particles effectively scatter light (Andrews et al, 2011;Schmeisser et al, 2018) and act as cloud-condensation nuclei (CCN) (Earle et al, 2011;Komppula et al, 2005). Due to the expan-sion of the polar dome, a major part of the aerosol mass is long-range transported from source regions outside the Arctic where the primary source region has been identified as the northern part of Eurasia Quinn et al, 2008;Heidam et al, 2004;Stohl et al, 2007;Christensen, 1997;Abbatt et al, 2019).…”

A Review of the Representation of Aerosol Mixing State in Atmospheric Models

“…The mixing state of a population of particles refers to the heterogeneity of the aerosol population. Most commonly, "mixing state" is used to refer to the distribution of component chemical species among a population of aerosol particles (sometimes referred to as the "population mixing state", e.g., [1]). A population is said to be fully externally mixed if each particle is composed of a single species.…”