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

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

Properties and evolution of biomass burning organic aerosol from Canadian boreal forest fires

“…CO concentrations of up to 800 ppb were observed between 5000-8000 m, whereas OA did not exceed 30 µg m −3 . This disparity is attributed to the removal of OA from plumes encountered during flight B622 (20 July) by precipitation prior to sampling following advection through clouds, as corroborated by meteorological observations and back trajectory models (Griffin et al, 2013;Taylor et al, 2014). Wet deposition of aerosol reduced OA to background levels, while CO concentrations remained elevated to similar levels as observed at lower altitudes.…”

“…The AMS provides highly time-resolved mass concentrations of sub-micron, non-refractory aerosol, and a broad chemical characterisation across a complete range of constituent ion mass-to-charge ratios (m/z). Operation of the AMS, including calibration and necessary correction factors, during aircraft deployment (Bahreini et al, 2003) and specifically onboard the BAe-146 (Crosier et al, 2007;Morgan et al, 2009;Taylor et al, 2014) have been described in detail. Refractory black carbon (BC) was measured using a Droplet Measurement Technologies single particle soot photometer (SP-2; Schwarz et al, 2006;Taylor et al, 2014).…”

“…Operation of the AMS, including calibration and necessary correction factors, during aircraft deployment (Bahreini et al, 2003) and specifically onboard the BAe-146 (Crosier et al, 2007;Morgan et al, 2009;Taylor et al, 2014) have been described in detail. Refractory black carbon (BC) was measured using a Droplet Measurement Technologies single particle soot photometer (SP-2; Schwarz et al, 2006;Taylor et al, 2014). Although analysis of the chemical and optical properties of single BC particles was not performed as part of this study, mass concentrations in smoke plumes, particularly in relation to OA concentrations, were used as a means of evaluating the proportional contributions of different combustion phases.…”

“…1. Back trajectories for air masses encountered throughout the durations of flights B621-624 are presented by O'Shea et al (2013a), detailing the transport pathways of plumes and agreement with active fire locations, while Taylor et al (2014) provide air mass histories for individual plumes sampled during flight B626. Comparison of emissions of different ages is subject to potential contrasts in fire behaviour, given that each set of measurements were obtained at different stages of the campaign.…”

Aged boreal biomass-burning aerosol size distributions from BORTAS 2011

“…In global and regional modelling of biomass-burning aerosols, mass-based biomass-burning inventories are the standard and are generally not accompanied by size data (Reid et al, 2009;van der Werf et al, 2010;Wiedinmyer et al, 2011), leaving size-distribution estimates to the individual investigator. Current global and regional atmospheric aerosol models have grid box spatial scales (tens of hundreds of kilometres) much larger than many initial biomassburning plume widths (< 10 km).…”