What does this research mean for the field?

The UK Earth System Model (UKESM1.1) inaccurately simulates marine aerosol size distributions by over-relying on primary emissions and underestimating nucleation in the marine boundary layer, indicating missing secondary aerosol formation pathways involving species like iodine, amines, and organic vapors. Novelty: ClaimNovelty.INCREMENTAL. Consensus alignment: ConsensusAlignment.NEUTRAL.

What question did this study set out to answer?

The study aims to evaluate UKESM1.1's performance in simulating marine aerosol formation and underlying processes.

March 19, 2026Open Access

Evaluation of UKESM aerosol size and composition using ATom measurements indicates missing marine aerosol formation mechanisms

Key Points

The study aims to evaluate UKESM1.1's performance in simulating marine aerosol formation and underlying processes.
Assessed atmospheric aerosols using ATom mission aircraft measurements
Evaluated model performance in simulating aerosol precursor vapours and size distributions
Tested various process improvements including sulfuric acid-ammonia nucleation and ammonium nitrate scheme
Analyzed model output discrepancies between upper troposphere and marine boundary layer
UKESM1.1 overestimates nucleation and Aitken mode particles in the upper troposphere while underestimating accumulation mode
In the marine boundary layer, the model overestimates primary aerosols and precursor gases but underestimates nucleation
Findings suggest missing aerosol formation pathways likely involving iodine, amines, and organic vapours
Model outputs show strong dependence on dimethyl sulfide emissions and vapor condensation schemes

Abstract

Abstract. Atmospheric aerosols influence climate through their interactions with radiation and clouds, yet large uncertainties remain in their simulation by global models. This study evaluates the United Kingdom Earth System Model version 1.1 (UKESM1.1) using global-scale aircraft observations from the Atmospheric Tomography (ATom) mission, focusing on aerosol lifecycle processes in the remote marine atmosphere. We assess model performance in simulating aerosol precursor vapours, number size distributions, chemical composition, and environmental conditions. Several process improvements are tested, including sulfuric acid-ammonia nucleation, ammonium nitrate scheme, methanesulfonic acid condensation, and low-temperature isoprene-derived secondary organic aerosol formation. Model biases differ significantly between the upper troposphere (UT) and the marine boundary layer (MBL). In the UT, UKESM1.1 overestimates nucleation and Aitken mode particles while underestimating accumulation mode, indicating insufficient growth. In the MBL, the model overestimates primary aerosols (e.g. seasalt) and precursor gases but underestimates nucleation and Aitken mode particles, even after incorporating updated nucleation and ammonium nitrate scheme. The persistence of low aerosol number concentrations, despite overestimated precursors, suggests missing formation pathways likely involving other species such as iodine, amines, and organic vapours. These limitations result in an unbalanced cloud condensation nuclei budget that over-relies on primary emissions. Sensitivity tests reveal that model outputs are strongly influenced by dimethyl sulfide emissions and vapour condensation schemes. Our results highlight the need for future model development to prioritise mechanistic representation of currently missing aerosol sources, rather than relying on empirical tuning, to improve aerosol-climate interaction estimates.

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