Stage-resolved in-cloud scavenging of submicron and BC-containing particles: A case study
Introduction
Primarily originating from incomplete combustion of fossil fuel and biomass, black carbon (BC) shows remarkable impacts on global climate (Bond et al., 2013; Koch and Del Genio, 2010; Nordmann et al., 2014). BC can be removed via dry deposition and wet scavenging, including in-cloud and below-cloud processes. On a global scale, wet scavenging towards the ocean surface is five folds higher than dry deposition for BC aerosols (Jurado et al., 2008). Previous studies have shown that BC is ubiquitous in liquid cloud (Bi et al., 2016; Lin et al., 2017; Liu et al., 2018) and below-cloud processes usually contribute smaller in particle scavenging (Emerson et al., 2018; Ervens, 2015).
In-cloud scavenging mechanisms are generally represented as impaction/coagulation and nucleation (Ervens, 2015). Impaction/coagulation scavenging dominates in a size range of ≤50 nm whereas nucleation plays a crucial role for larger size particles (>140 nm) (Hoose et al., 2008; Levin et al., 2003). Croft et al. (2016) found that the coagulation/impaction scavenging sharply lower the number concentration of particles smaller than 200 nm. Taking this mechanism into account, the number concentration of particles larger than 10 nm would decrease 15–21% globally, whereas particles larger than 80 nm (a proxy for cloud condensation nuclei, CCN) only drop 10–12% (Pierce et al., 2015). Although numerous studies stated that nucleation is the dominating scavenging mechanism for BC (Schroder et al., 2015; Taylor et al., 2014), Baumgardner et al. (2008) revealed that impaction/coagulation is of great importance to scavenge BC in upper tropospheric or cirrus cloud.
Scavenging efficiency (SE), defined as the mass/number fraction (MSE or NSE) of components in cloud droplets, reflecting the ability of species to form cloud droplets.where CRES and CINT denote the mass or number concentration of BC or detected number of BC-containing particles in cloud droplet residues (RES) and interstitial particles (INT), respectively.
Most current studies focused on the mean MSE of BC (Herckes et al., 2013; Yang et al., 2019). The reported MSEs varied from 6% at urban city (Hallberg et al., 1992) to ~ 80% at a remote marine site near the Arctic (Heintzenberg and Leck, 1994), showing significant regional discrepancies ((Yang et al., 2019) and references therein). For NSE, Zhang et al. (2017) reported a 5–45% of NSE at Mt. Tianjing, southern China, while the NSE was only 1–10% for BC at Mt. Jungfraujoch (Schroder et al., 2015). Ding et al. (2019) found that 80% of BC-containing particles were removed through coagulation in mixed-phase cloud at the north of Taihang ridge. The discrepancy may originate from the cloud types (liquid, mixed-phase, or ice cloud) and the environmental conditions.
The metrological conditions, including supersaturation of water vapor, liquid water content (LWC), temperature, and physicochemical properties of BC (e.g., size, mixing state, thickness of coating, chemical composition, and so forth), play complex roles in the SE of BC (Browse et al., 2012; Ching et al., 2012; Cozic et al., 2007; Moteki et al., 2012; Motos et al., 2019b; Ohata et al., 2016; Schroder et al., 2015; Zhang et al., 2017). Generally, SE increases with growing particle size, LWC, and supersaturation (Ching et al., 2018; Hitzenberger et al., 2001; Matsui, 2016; Motos et al., 2019a; Sellegri et al., 2003; Wu et al., 2019). Nevertheless, key factors and mechanisms remain ambiguous and may differ under various conditions. Zhang et al. (2017) revealed that mixing state is the key factor in in-cloud scavenging of BC under low LWC conditions (<0.1 g m−3). An investigation in Chile revealed that the MSE of BC varied dramatically (13–50%) with different air masses (Heintzenberg et al., 2016).
Considering the ever-changing environmental conditions, understanding the evolution of SE of BC in a cloud event is crucial to evaluate their impacts on climate properly. Targino et al. (2009) found that the SE of aerosol particles at the Mt. Åreskutan in Swedish is not identical even in just one day during a cloud event. By and large, a cloud event can be divided into stages including formation, development, stability and dissipation (Dupont et al., 2012; Koracin et al., 2005).
Therefore, the aims of the present study are (1) to reveal a more detailed evolution of the SE of BC throughout a complete cloud event, (2) to recognize the key factors influencing the in-cloud scavenging of BC particles in different stages. Besides, discrepancies between BC-containing and submicron particles are also discussed.
Section snippets
General characterization
Field observation was carried out from 30 May to 3 Jun 2017. The sampling site is located in National Air Background Monitoring Station at Mt. Tianjing (112°53′56″ E, 24°41′56″ N; 1690 m a.s.l.) in southern China (Lin et al., 2017). It is surrounded by a national forest park (273 km2) and scarcely affected by anthropogenic activities.
During the sampling period, southwestern air mass was dominated (Fig. S1 in the Supplementary Material). The cloud event, identified by a ground-based counterflow
Results and discussion
To characterize the evolution of SEs, Fig. 2 displays the time series of (a) number concentration and NSE of submicron particles by SMPS, (b) mass concentration and MSE of EBC by AE-33 and (c) number concentration and NSE of BC-containing particles by SPAMS, respectively. Three kinds of SEs show a similar trend with the cloud stages. In the formation stage of the cloud event, visibility dropped dramatically from 18 km to 200 m and bounced back to 800 m at about 12:00 on 31 May. In the
Conclusions
The evolution of SE of BC and the stage-resolved main influencing factors were firstly investigated. The MSE of EBC and NSE of BC-containing particles varied from 4.7% to 52.6% and 11.3%–59.6%, with a mean value of 35.1% and 37.3%, respectively. The SEs were the highest in the development and stability stage, followed by the dissipation stage and the formation stage, respectively. The NSEs of different types of particles increased with particle size (0.4–1.2 μm) and LWC, despite the fact that
CRediT authorship contribution statement
Yuxiang Yang: Writing - review & editing, Funding acquisition, Writing - original draft. Qinhao Lin: Formal analysis. Yuzhen Fu: Formal analysis. Xiufeng Lian: Software, Validation. Feng Jiang: Software, Validation. Long Peng: Software, Validation. Guohua Zhang: Writing - review & editing, Funding acquisition, Methodology, Resources, Writing - original draft, Conceptualization. Lei Li: Methodology, Resources. Duohong Chen: Formal analysis, Formal analysis, Formal analysis, Methodology,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Nature Science Foundation of China (No. 41775124 and 41877307), Natural Science Foundation of Guangdong Province (2019B151502022), the National Key Research and Development Program of China (2017YFC0210104), and Guangdong Foundation for Program of Science and Technology Research (No. 2017B030314057). The authors also gratefully acknowledge the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion model (//ready.arl.noaa.gov
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