Absorption Enhancement of Black Carbon Aerosols Constrained by Mixing-State Heterogeneity

Abstract

Atmospheric black carbon (BC) has a large yet highly uncertain contribution to global warming. When mixed with non-BC/coating material during atmospheric aging, the BC light absorption can be enhanced through the lensing effect. Laboratory and modeling studies have consistently found strong BC absorption enhancement, while the results in ambient measurements are conflicting, with some reporting weak absorption enhancement even for particles with large bulk coating amounts. Here, from our direct field observations, we report both large and minor absorption enhancement factors for different BC-containing particle populations with large bulk non-BC-to-BC mass ratios. By gaining insights into the measured coating material distribution across each particle population, we find that the level of absorption enhancement is strongly dependent on the particle-resolved mixing state. Our study shows that the greater mixing-state heterogeneity results in the larger difference between observed and predicted absorption enhancement. We demonstrate that by considering the variability in coating material thickness in the optical model, the previously observed model measurement discrepancy of absorption enhancement can be reconciled. The observations and improved optical models reported here highlight the importance of mixing-state heterogeneity on BC's radiative forcing, which should be better resolved in large-scale models to increase confidence when estimating the aerosol radiation effect.

Direct Observation of the Transitional Stage of Mixing‐State‐Related Absorption Enhancement for Atmospheric Black Carbon

“…During the sampling time, a bimodal distribution of SF , which was defined as the ratio of uncoated particle size ( d c ) distribution after heating at 300 o C to the selected size ( d p ) of unheated particles, was observed (Figures 2a and 2b). The bimodal SF distribution implies heterogeneously distributed coating material across the particle population (Zhai et al., 2022). We attribute the two SF modes to two distinct sources: the local traffic emissions from urban Shanghai, which were dominated by the baerly or thinly coated BC particles (i.e., Mode 1), and the long‐range transport air masses from northern China that passed across the densely coal‐burning and industrial areas, taking along BC‐containing particles that are deeply aged with thick coatings (i.e., Mode 2).…”

“…Recently, a particle‐resolved method to predict the E abs was developed by considering the distribution of BC coating thickness, which could successfully predict the E abs consistent with the observations (Zhai et al., 2022). In the current study, the same data set was considered and we extended the previous work to a case study that focused on the relationship between particle aging and the E abs .…”

“…Non‐BC particles that are low‐volatile may exist after heating (Cheung et al., 2016). Thus, number fractions of the non‐BC‐containing particles detected by the single‐particle aerosol mass spectrometer (SPAMS) were taken into consideration to reduce the uncertainty brought by the entire evaporation in the heating process (Zhai et al., 2022).…”

“…The methodology of the particle‐resolved R BC based on the V‐TDMA system and the calculation of the particle‐resolved E abs has been introduced previously (Zhai et al., 2022). Briefly, assuming that the BC‐containing particles exist with a “core‐shell” structure, the mass ratio of the volatile coating and non‐volatile core of BC‐containing particles ( R BC ) is calculated as: $$${R}_{\text{BC}}=\,\frac{{M}_{\text{coating}}}{{M}_{\text{core}}}=\frac{\left({V}_{\text{particle}}-{V}_{\text{core}}\right){\rho }_{\text{coating}}}{{V}_{\text{core}}{\rho }_{\text{core}}}=\frac{\left({d}_{\text{particle}}^{3}-{d}_{\text{core}}^{3}\right){\rho }_{\text{coating}}}{{d}_{\text{core}}^{3}{\rho }_{\text{core}}}$$$ where M x , V x , and ρ x are the mass, volume, and material density of the coating and core for the BC‐containing particles, respectively.…”

Identifying the Fraction of Core–Shell Black Carbon Particles in a Complex Mixture to Constrain the Absorption Enhancement by Coatings

“…The evolution of BC at the level of single particles however raises more complexity as ambient BC contains a range of morphologies, and this could have large spatiotemporal variation. Recent studies using combined field work still found large discrepancies even at the same coating/rBC ratio. , This may be explained by the heterogenicity of particles when the coatings are distributed to a range of individual BC particles in bulk. − To identify the population of single BC in bulk for which the core–shell microstructure should be applied (hereby E abs exists) or not is important to estimate the overall absorbing properties. Previous studies identified a transition regime of BC where hybrid optical models (Mie core–shell approach or without E abs ) should be applied, which corresponded with a coating/rBC mass ratio between 1 and 3 or at other ratio ranges depending on environments .…”