Estimating black carbon aging time-scales with a particle-resolved aerosol model

Understanding the aging process of aerosol particles is important for assessing their chemical reactivity, cloud condensation nuclei activity, radiative properties and health impacts. In this study we investigate the aging of black carbon containing particles in an idealized urban plume using a new approach, the particle-resolved aerosol model PartMC-MOSAIC. We present a method to estimate aging time-scales using an aging criterion based on cloud condensation nuclei activation. The results show a separation into a daytime regime where condensation dominates and a nighttime regime where coagulation dominates. For the chosen urban plume scenario, depending on the supersaturation threshold, the values for the aging time-scales vary between 0.06 hours and 10 hours during the day, and between 6 hours and 20 hours during the night.

💡 Research Summary

This paper presents a novel methodology for quantifying the aging time‑scales of black carbon (BC)‑containing aerosol particles using the particle‑resolved model PartMC‑MOSAIC. The authors adopt a cloud condensation nuclei (CCN) activation criterion based on the hygroscopicity parameter κ to distinguish “fresh” from “aged” particles. Each particle’s composition is tracked explicitly; κ values are assigned to inorganic salts (≈0.65), secondary organic aerosol (0.1), primary organic aerosol (0.001), and BC (0). The critical supersaturation S_c for activation is calculated from κ and particle size, and particles that activate at a given supersaturation threshold are classified as aged.

The study employs an idealized urban plume scenario: a Lagrangian air parcel advects over a city, receiving emissions from diesel, gasoline, and cooking sources for 12 h (06:00–18:00 LST) and then evolving for another 12 h without further emissions. The simulation uses roughly 100 000 particles in a 16 cm³ volume, with stochastic treatment of coagulation, emissions, and dilution, while condensation/evaporation are handled deterministically. A binned sampling technique efficiently resolves the Brownian coagulation kernel across the highly non‑uniform size distribution.

Aging time‑scales τ are derived by counting the number of particles transitioning from fresh to aged each time step and fitting the transition rate to a first‑order bulk aging equation dNₐ/dt = (N_f – Nₐ)/τ. The authors explore several supersaturation thresholds (0.1 %–1 %) to assess sensitivity.

Results reveal two distinct regimes. During daytime, high temperature, humidity, and abundant photochemical precursors drive rapid condensation of inorganic and secondary organic species onto BC particles. This increases κ, lowers S_c, and enables CCN activation at low supersaturations. Consequently, τ varies widely from 0.06 h to 10 h depending on the supersaturation cut‑off. At night, photochemistry ceases, condensation is negligible, and coagulation becomes the dominant aging pathway. Coagulation enlarges particles but does not substantially change κ, so activation requires higher supersaturation; τ lengthens to 6–20 h and shows weaker dependence on the supersaturation threshold.

The study demonstrates that a single, globally‑applied aging time‑scale is insufficient to capture the diurnal variability of BC aging. Instead, models should incorporate time‑dependent τ or explicit process‑based representations that distinguish condensation‑driven aging (day) from coagulation‑driven aging (night). The particle‑resolved approach also provides a framework for directly linking mixing state evolution to CCN activity, offering a more physically grounded basis for climate and health impact assessments of BC aerosols. Future work is suggested to validate the methodology against field measurements, extend it to varied atmospheric environments, and integrate the derived τ values into larger‑scale models.