Size distributions, mixing state, and morphology of refractory black carbon in an urban atmosphere of northeast Asia during summer

Highlights

• Minimum rBC mass mean diameter and shell-to-core-ratio occurred in the morning rush hour rush hour.

• Mixing state of rBC emitted from vehicles was altered throughout the day and night.

• rBC was coated by photochemically oxidized vapor and underwent hygroscopic growth at night.

• Shell-to-core-ratio reasonably increased with Ox (O3 + NO2) concentration.

• rBC core can play a role in condensational growth of rBC-containing particle, increasing mass.

Abstract

Black carbon (BC) exerts profound impacts on air quality, human health, and climate. Here, we investigated concentrations and size distributions of refractory BC (rBC) and mixing state and morphology of rBC-containing particles in urban Seoul for 2019 summer. Mass concentrations of rBC ranged from 0.02 μgm⁻³ to 2.89 μgm⁻³, and daily maximums of rBC mass, daily minimums of rBC mass median diameter (MMD) (110–130 nm), and shell-to-core ratio (R_shell/core) occurred with NO₂ maximums during morning rush hour. As the first report of ground observations on rBC mixing state, these results indicate that vehicle emission is a major local source of rBC in Seoul. MMDs of 127–146 nm and the greatest mass loadings of ≥1 μg m⁻³ were accompanied by high O₃ and PM_2.5 concentrations, in contrast to the largest MMDs (135–165 nm) associated with transport from upstream regions.

The average R_shell/core was 1.25 for the rBC mass-equivalent diameter (D_rBC) of 140–220 nm. R_shell/core increased gradually through the day and was positively correlated with O_x concentration, indicating photochemical aging of rBC particles. Co-emissions of rBC and volatile organic compounds from vehicles facilitated internal mixing during the daytime. However, R_shell/core tended to be low at temperature >∼30 °C, while 58 % of rBC particles with R_shell/core exceeding 1.25 were found at nighttime under relative humidity >75 %. These results demonstrate that the mixing state of freshly-emitted rBC particles was altered through coating by photochemically oxidized vapors during the day and hygroscopic growth at night. Additionally, the delay-time approach revealed rBC morphological characteristics, the most common being the bare type (74 %), and the attached type (6 %) was relatively large in numbers during morning rush hour. Therefore, it is suggested that during summer, rBC particles from traffic emissions should be considered in parallel to winter pollution mitigation strategies in urban atmosphere of northeast Asia.

Introduction

Black carbon (BC) emitted from fossil-fuel combustion and biomass burning exerts profound impacts on climate and air quality. It has strong light-absorbing ability over a broad range of wavelengths (Bond et al., 2013), and its small particle size allows it to be transported over large distances both horizontally and vertically (Lim et al., 2017; Sun et al., 2021). Climate forcing by BC is of concern on both regional and global scales (Ménégoz et al., 2014; Ramanathan and Carmichael, 2008; Zanatta et al., 2018).

Globally, emissions from combustion of fossil fuels and biomass accounts for >60 % of BC in the atmosphere. Northeast Asia, together with India, is a region of the largest atmospheric BC loading (Chung and Seinfeld, 2005), with multiple source types (residential, transportation, and industrial sectors) contributing to emissions, unlike other continents where there is a single predominant emission source; e.g., transportation in North America and open fires in Africa (Bond, 2004). Identification of dominant emission sources and the quantification of amounts released are crucial for mitigation of BC emissions on city and province scales. Dominated in number by Aitken particles (20–100 nm diameter), BC particle mass distributions depend primarily on the emission source, generally with a smaller mass median diameter (MMD) for urban fossil-fuel emissions (approximately <150 nm; Laborde et al., 2013; Liu et al., 2019; Schwarz et al., 2008a).

Light-absorbing abilities of BC particles differ among sources and combustion regimes (Healy et al., 2015; Knox et al., 2009; Wu et al., 2020) and undergo substantial changes during photochemical aging in the atmosphere (Cho et al., 2019; Lim et al., 2018). Light absorption is determined primarily by microphysical (i.e., size, morphology, and mixing state) and optical properties of BC, which is modified by non-BC components that are mixed internally with BC during aging processes. However, the current results of light-absorption enhancements of ambient BC-containing particles are conflicting (e.g., Cappa et al., 2012; Liu et al., 2015) owing to the complex interplay among physicochemical properties of such particles in the atmosphere. A recent study suggests that discrepancy between the observed and predicted absorption enhancement is due to the mixing state heterogeneity of the BC particles (Zhai et al., 2022).

In terms of morphology, BC-containing particles are classified into two categories: a “coated” type, in which BC is fully embedded (centrally) within a non-BC particle; and an “attached” type, in which BC is attached on or near the surface of a non-BC particle (Adachi et al., 2016; Moteki et al., 2014). The former can cause greater enhancement of mass absorption cross-section (MAC) than the latter (Fuller et al., 1999). Furthermore, in the Mie core-shell model high MAC values are associated with small MMDs (<150 nm). An understanding of the abundances of morphologically different BC-containing particles is crucial for accurately projecting climatic effects of BC particles.

Changes in the mixing state of BC particles also aids the understanding of how secondary aerosol forms on pre-existing particles and how such particles grow in size and mass. However, the elucidation of aging mechanisms is challenging due to the complexity of gas–aerosol–meteorology interaction. In a chamber study with ambient air, a coating thickness of 220 nm BC cores grew by 32–52 nm over 2–5 h in Beijing, China, but more slowly in Houston, USA (Peng et al., 2016), indicating the critical roles of the concentration and/or composition of condensable gases on the formation of BC-coating materials. While chamber studies show the significant aging of BC particle with thick coatings, ambient BC particles are reported to have thinner coatings in Beijing during summer periods (10 nm for a 180 nm BC core; Liu et al., 2020). Synoptic and micro-scale meteorology also play an important role in the evolution of BC mixing states. In addition, recent studies propose that above the planetary boundary layer (PBL) or in the upper PBL, aging of BC particles influences stabilization of the atmosphere and enhances haze development by warming up the air and inducing temperature inversion (Ding et al., 2016; Wang et al., 2013).

Among the single-particle approaches available to date, the single-particle laser-induced incandescence technique allows quantitative in situ assessment of the microphysics of BC-containing particles. With this technique, single-particle soot photometry (SP2) provides mass and size distributions of BC cores in BC-containing particles and their mixing state by focusing on a single particle rather than bulk aerosols typically studied in other techniques. In this respect, the term refractory BC (rBC) specifically refers to BC quantified by the laser-induced incandescence method (Petzold et al., 2013). Recently, there has been an increasing number of studies on the microphysical properties of rBC particles in East Asia using the SP2 method (e.g., Adachi et al., 2016; Gong et al., 2016; Liu et al., 2019, Liu et al., 2020), where the seasonality of ambient rBC particles or their characteristic relationship with PM_2.5 levels have been studies in terms of concentration, size, and mixing state. In addition, there are increasing reports about a link between BC particles and aerosol hygroscopicity and chemical composition (e.g., Li et al., 2018; Yu et al., 2022). However, the current understanding still remains poor, due to the paucity of single-particle observations of rBC. Particularly in Seoul, to our knowledge, this study is the first of its kind to measure the mixing state of rBC particles using SP2 on the ground. During the 2016 Korea-United States Air Quality (KORUS-AQ) field campaign, SP2 was deployed on board the NASA DC-8 and at the Gosan Climate Observatory (GCO in Jeju Island) to study rBC mixing states over the Korean peninsula (Lamb et al., 2019) and at a regional background site (Lim et al., in press).

In the present study, microphysical characteristics of rBC particles were investigated using the SP2 method at an urban site in Seoul during summer, focusing on size distributions and diurnal variations of key parameters indicating mixing state (externally or internally mixed) and morphology (coated or attached) of rBC-containing particles to explore their major emission sources and aging processes in an urban atmosphere of northeast Asia.