The single-particle mixing state and cloud scavenging of black carbon: a case study at a high-altitude mountain site in southern China

Guohua Zhang,Long Peng,Weijun Yan,Guoying Sheng,Fred J Brechtel,Xinming Wang,Lei Li,Zhen Zhou,Mei Li,Duohong Chen,Qinhao Lin,Xinhui Bi,Jianxin Chen,Ping'An Peng

doi:10.5194/acp-17-14975-2017

Abstract

Abstract. In the present study, a ground-based counterflow virtual impactor (GCVI) was used to sample cloud droplet residual (cloud RES) particles, while a parallel PM2.5 inlet was used to sample cloud-free or cloud interstitial (cloud INT) particles. The mixing state of black carbon (BC)-containing particles and the mass concentrations of BC in the cloud-free, RES and INT particles were investigated using a single-particle aerosol mass spectrometer (SPAMS) and two aethalometers, respectively, at a mountain site (1690 m a. s. l. ) in southern China. The measured BC-containing particles were extensively internally mixed with sulfate and were scavenged into cloud droplets (with number fractions of 0.05–0.45) to a similar (or slightly lower) extent as all the measured particles (0.07–0.6) over the measured size range of 0.1–1.6 µm. The results indicate the preferential activation of larger particles and/or that the production of secondary compositions shifts the BC-containing particles towards larger sizes. BC-containing particles with an abundance of both sulfate and organics were scavenged less than those with sulfate but limited organics, implying the importance of the mixing state on the incorporation of BC-containing particles into cloud droplets. The mass scavenging efficiency of BC with an average of 33 % was similar for different cloud events independent of the air mass. This is the first time that both the mixing state and cloud scavenging of BC in China have been reported. Our results would improve the knowledge on the concentration, mixing state, and cloud scavenging of BC in the free troposphere.

Highlights

Black carbon (BC), known as soot or elemental carbon, is primarily emitted from incomplete combustion processes (Bond et al, 2013; Petzold et al, 2013)
Hydrophilic black carbon (BC)-containing particles can act as cloud condensation nuclei (CCN) and modify cloud microphysical properties
The number fractions of BC-containing particles that were incorporated within the cloud droplets ranged between those observed at an urban site (70 %) in southern China (Bi et al, 2016) and those observed at a mountain site (∼ 30 %) in Germany (Roth et al, 2016)

Summary

Introduction

Black carbon (BC), known as soot or elemental carbon, is primarily emitted from incomplete combustion processes (Bond et al, 2013; Petzold et al, 2013). Fresh BC-containing particles are generally hydrophilic due to the presence of thin coatings of inorganic or organic materials (Zuberi et al, 2005), and during transport they become more hydrophilic when further coated through coagulation, condensation, and photochemical oxidation (Zuberi et al, 2005; Zaveri et al, 2010; Matsui, 2016). Hydrophilic BC-containing particles can act as cloud condensation nuclei (CCN) and modify cloud microphysical properties The increase in CCN activity enhances the in-cloud scavenging of BC and reduces its lifetime (Zaveri et al, 2010). A more comprehensive understanding of how aerosol particles form cloud droplets is required in order to reduce the uncertainty of the impacts of aerosols on the climate (Furutani et al, 2008)

Methods

Results

Conclusion