Journal Article

Constraining Black Carbon Aging in Global Models to Reflect Timescales for Internal Mixing

Schutgens, Nick A. J., Feng, Yan, Aiken, Allison C., Wuebbles, Donald, Ma, Po‐Lun, Dubey, Manvendra K., Li, Yinrui, Riemer, Nicole, Fierce, Laura

2025

Overview

The radiative effects of black carbon depend critically on its atmospheric lifetime, which is controlled by the rate at which freshly emitted combustion particles become internally mixed with other aerosol components. Global aerosol models strive to represent this process, but the timescale for aerosol mixing is not easily constrained using observations. In this study, we apply a timescale parameterization derived from particle‐resolved simulations to quantify, in a global aerosol model, the timescale for internal mixing. We show that, while highly variable, the average timescale for internal mixing is approximately 3 hr, which is much shorter than the 24‐hr aging timescale traditionally applied in bulk aerosol models. We then use the mixing timescale to constrain the aging criterion in the Modal Aerosol Module. Our analysis reveals that, to best reflect timescales for internal mixing, modal models should assume that particles transition from the hydrophobic (fresh) to the hydrophilic (aged) class once they accumulate a coating thickness equal to four monolayers of sulfuric acid, as opposed to the model's current aging criterion of eight monolayers. We show that, in remote regions like the Arctic and Antarctic, predictions of black carbon loading and its seasonal variation are particularly sensitive to the model representation of aging. By constraining aging in global models to reflect mixing timescales simulated by the particle‐resolved model, we eliminate one of the free parameters governing black carbon's long‐range transport and spatiotemporal distribution. Plain Language Summary Black carbon exerts a strong, but uncertain, radiative effect on Earth's climate. The magnitude of black carbon's radiative effect depends on its atmospheric lifetime, which depends critically on the formation of cloud droplets by particles containing black carbon and their subsequent removal through precipitation. Black carbon tends to be emitted into the atmosphere in hydrophobic particles that do not efficiently form cloud droplets. After emission, these particles mix with other aerosol components through condensation of semi‐volatile gases and coagulation with hygroscopic particles, processes collectively known as aging. Black carbon's aging timescale strongly influences model predictions of its removal through precipitation, the dominant removal process of black carbon from the atmosphere. In this study, we present a framework for constraining black carbon's aging timescale using detailed model simulations. We show that large‐scale models tend to apply aging approximations that result in black carbon remaining hydrophobic for too long, which strongly impacts the concentration of black carbon in remote regions of the atmosphere. Key Points The timescale for internal mixing varies from less than an hour to longer than a month, depending on regional chemistry The average timescale for internal mixing is 3 hr, much shorter than the 24‐hr aging timescale often used in bulk aerosol models A four‐monolayer aging criterion in modal models best reflects timescales for internal mixing simulated by the particle‐resolved model

• Publisher: John Wiley & Sons, Inc,American Geophysical Union (AGU)
• Subject: Aerosol models / Aerosols / Ageing / Aging / Atmospheric aerosols / Black carbon / Carbon / CCN / ENVIRONMENTAL SCIENCES / Gases / Global aerosols / modal modeling / Monomolecular films / Parameterization / Particle size / particle‐resolved modeling / Seasonal variations / Simulation / Sulfuric acid / Sulphuric acid