Mixing characteristics of refractory black carbon aerosols determined by a tandem CPMA-SP2 system at an urban site in Beijing

Abstract Black carbon aerosols play an important role in climate change by absorbing solar radiation and degrading visibility. In this study, the mixing state of refractory black carbon (rBC) at an urban site in Beijing was studied with a single 15 particle soot photometer (SP2), as well as a tandem observation system with a centrifugal particle mass analyzer (CPMA) and a differential mobility analyzer (DMA), in early summer of 2018. The results demonstrated that the mass-equivalent size distribution of rBC exhibited an approximately lognormal distribution with a mass median diameter (MMD) of 171.2 nm. When the site experienced prevailing southerly winds, the MMD of rBC increased notably by 19%. During the observational period, the ratio of the diameter of rBC-containing particles (Dp) to the rBC core (Dc) was 1.20 on average for Dc=180 nm, 20 indicating that the majority of rBC particles were thinly coated. The Dp/Dc value exhibited a clear diurnal pattern, with a maximum at 1400 LST and an enhancing rate of 0.013/h; higher Ox conditions increased the coating enhancing rate. Bare rBC particles were primarily in a fractal structure with a mass fractal dimension (Dfm) of 2.35, with limited variation during both clean and pollution periods, indicating significant impacts from on-road vehicle emissions. The morphology of rBC-containing particles varied with aging processes. The mixing state of rBC particles could be indicated by the mass ratio of non-refractory 25 matter to rBC (MR). In the present study, rBC-containing particles were primarily found in an external fractal structure when MR < 1.5 and changed to a core-shell structure when MR > 6, at which the measured scattering cross section of rBC-containing particles was consistent with that based on the Mie-scattering simulation. We found only 9% of the rBC-containing particles were in core-shell structures on clean days with a particle mass of 10 fg, and the number fraction of core-shell structures increased considerably to 32% on pollution days. Considering the morphology change, the absorption enhancement (Eabs) was 30 11.7% higher based on core-shell structures. This study highlights the combined effects of morphology and coating thickness on the Eabs of rBC-containing particles, which will be helpful for determining the climatic effects of BC.

Introduction

Black carbon (BC) aerosol is one of the principal light-absorbing aerosols in the atmosphere. It is regarded as the second most 35 important component contributing to global warming only after CO2 (Jacobson, 2000). BC has a much shorter lifetime than CO2. Thus, BC’s radiative perturbation on a regional scale may be distinct from globally averaged estimates. It has been reported that BC’s direct radiative forcing can reach an order of +10 W m-2 over East and South Asia (Bond et al., 2013). BC aerosol can also influence the climate by altering cloud properties, such as the evaporation of cloud droplets, reduction in cloud lifetime and albedo (Ramanathan et al., 2001;Ramanathan and Carmichael, 2008). Ding et al. (2016) determined the existence 40 of BC in the upper mixing layer could absorb downward solar radiation, impeding the development of the boundary layer, which aggravates air pollution. Moreover, BC aerosols have detrimental health effects. BC and organic carbon are regarded as the most toxic pollutants in PM2.5, possibly leading to ~3 million premature deaths worldwide (Apte et al., 2015;Lelieveld et al., 2015).

BC is typically emitted from the incomplete combustion of fossil fuels and biomass. After being emitted into atmosphere, BC 45 particles tend to mix with other substances through coagulation, condensation and other photochemical process, which significantly changes BC’s cloud condensation nuclei activity as well as its light absorption ability (Liu et al., 2013;Bond and Bergstrom, 2006). Model resultssuggest that after BC’s core issurrounded by a well-mixed shell, its direct absorption radiative forcing could be 50% higher than that of BC in an external mixing structure (Jacobson, 2001). Such an absorption enhancement phenomenon is interpreted as exhibiting a “lensing effect”, in which a non-absorbing coating causes more radiation to interact 50 with the BC core and thus more light is absorbed. This absorption enhancement effect has been proven in laboratory studies (Schnaiter et al., 2005). Shiraiwa et al. (2010) reported that the absorption enhancement of BC in a core-shell structure increased with coating thickness and can reach a factor as high as 2. Nevertheless, field observation results demonstrated large discrepancies (6 to 40%) in the absorption enhancement of aged BC particles (Cappa et al., 2012;Lack et al., 2012). The discrepancies could be attributed to the complex mixing state of BC in the real atmosphere, which depends on coating 55 composition, coating amount as well as the size of the BC core and structure. Bond et al. (2013) regarded the mixing state of BC as being one of the most important uncertainties in evaluating BC direct radiative forcing. Furthermore, freshly emitted BC is initially hydrophobic. Mixing BC with other soluble materials will significantly increase BC-containing particles’ hygroscopicity and thus the ability to become cloud condensation nuclei (Zhang et al., 2008;Popovicheva et al., 2011). This ability is associated with the wet deposition rate and consequently influences the lifetime and spatial distribution of BC 60 particles in the atmosphere. For these reasons, more observations are needed to determine the specific spatial and temporal distribution of BC’s mixing state, which would be helpful for minimizing the uncertainty in evaluating BC’s climatic and environmental effects