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

John K Kodros,Marco Zanatta,Megan D Willis,Andreas B Herber,Allan K Bertram,Jeffrey R Pierce,Julia Burkart,Sarah J Hanna,Jonathan P D Abbatt,Hannes Schulz,W Richard Leaitch

doi:10.5194/acp-18-11345-2018

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.

Highlights

Over the last several decades, the Arctic has warmed at nearly twice the rate of the global mean (Boucher et al, 2013)
For refractory BC (rBC) core diameters ranging from 140–220 nm, the median measured coating thickness decreases slightly from 45 to 40 nm
This results in total particle to rBC core diameter ratios ranging from 1.6 (IQR: 1.4–2.0) for rBC cores at 140 nm to 1.4 (IQR: 1.2–1.6) for rBC cores at 220 nm

Summary

Introduction

Over the last several decades, the Arctic has warmed at nearly twice the rate of the global mean (Boucher et al, 2013). In addition to CO2, short-lived climate forcers (such as black carbon and methane) may contribute to this increased rate of warming (AMAP, 2015). Black carbon (BC), in particular, may be an efficient warming agent in the Arctic as its potential to decrease planetary albedo is magnified when BC is above a white surface, such as snow (e.g., Bond et al, 2013). BC can affect climate through aerosol– radiation interactions (e.g., the direct radiative effect, semidirect radiative effect) or through aerosol–cloud interactions (e.g., the cloud-albedo indirect effect) (Boucher et al, 2013). Kodros et al.: Size-resolved mixing state of black carbon in the Canadian high Arctic

Methods

Results

Conclusion