Size-dependent wet removal of black carbon in Canadian biomass burning plumes

J W Taylor,J B A Muller,H Coe,C J Percival,P I Palmer,G Forster,G Allen,M Le Breton,M J Flynn,P I Williams,D Oram,J D Lee,J D Allan,M Parrington,A R Rickard

doi:10.5194/acp-14-13755-2014

Abstract

Abstract. Wet deposition is the dominant mechanism for removing black carbon (BC) from the atmosphere and is key in determining its atmospheric lifetime, vertical gradient and global transport. Despite the importance of BC in the climate system, especially in terms of its ability to modulate the radiative energy budget, there are few quantitative case studies of wet removal in ambient environments. We present a case study of BC wet removal by examining aerosol size distributions and BC coating properties sampled in three Canadian boreal biomass burning plumes, one of which passed through a precipitating cloud. This depleted the majority of the plume's BC mass, and the largest and most coated BC-containing particles were found to be preferentially removed, suggesting that nucleation scavenging was likely the dominant mechanism. Calculated single-scattering albedo (SSA) showed little variation, as a large number of non-BC particles were also present in the precipitation-affected plume. The remaining BC cores were smaller than those observed in previous studies of BC in post-precipitation outflow over Asia, possibly due to the thick coating by hydrophilic compounds associated with the Canadian biomass burning particles. This study provides measurements of BC size, mixing state and removal efficiency to constrain model parameterisations of BC wet removal in biomass burning regions, which will help to reduce uncertainty in radiative forcing calculations.

Highlights

Black carbon (BC) is the dominant absorbing aerosol in the atmosphere and is an important, ubiquitous climate-warming agent (Ramanathan and Carmichael, 2008; Chung et al, 2012; Bond et al, 2013)
Both BC and organic aerosol (OA) may be affected by wet removal, and OA / CO may increase or decrease due to evaporation or condensation of OA (Donahue et al, 2011)
CO2 data were available for the campaign, which can be used in conjunction with CO to characterise combustion efficiency (Ward and Radke, 1993), we did not consider this calculation robust as the variation in CO2 background was greater than the excess in the plumes, meaning the derived slopes (∂CO/∂CO2) may be misleading (Yokelson et al, 2013)

Summary

Introduction

Black carbon (BC) is the dominant absorbing aerosol in the atmosphere and is an important, ubiquitous climate-warming agent (Ramanathan and Carmichael, 2008; Chung et al, 2012; Bond et al, 2013). The optical properties of BC affect the single-scattering albedo (SSA) of an aerosol layer, which determines the sign of its radiative forcing (Haywood and Shine, 1995). Important uncertainties remain regarding global and local emissions of BC, as well as its chemical processing, lifetime in the atmosphere and optical properties. Observations are required to further constrain and/or validate model parameterisations surrounding BC processes in the atmosphere. Open biomass burning (BB) is the largest source category of BC, responsible for ∼ 40 % of total emissions in the year 2000 (Bond et al, 2013), and the size distribution and mixing state of BC from this source are known to exhibit systematic differences to fossil fuel emissions (Kondo et al, 2011; Sahu et al, 2012)

Results

Discussion

Conclusion