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Multiphase processes in the EC-Earth model and their relevance to the atmospheric oxalate, sulfate, and iron cycles

Abstract. Understanding how multiphase processes affect the
iron-containing aerosol cycle is key to predicting ocean biogeochemistry
changes and hence the feedback effects on climate. For this work, the
EC-Earth Earth system model in its climate–chemistry configuration is used
to simulate the global atmospheric oxalate (OXL), sulfate (SO42-),
and iron (Fe) cycles after incorporating a comprehensive representation of
the multiphase chemistry in cloud droplets and aerosol water. The model
considers a detailed gas-phase chemistry scheme, all major aerosol
components, and the partitioning of gases in aerosol and atmospheric water
phases. The dissolution of Fe-containing aerosols accounts kinetically for
the solution's acidity, oxalic acid, and irradiation. Aerosol acidity is
explicitly calculated in the model, both for accumulation and coarse modes,
accounting for thermodynamic processes involving inorganic and crustal
species from sea salt and dust. Simulations for present-day conditions (2000–2014) have been carried out
with both EC-Earth and the atmospheric composition component of the model in
standalone mode driven by meteorological fields from ECMWF's ERA-Interim
reanalysis. The calculated global budgets are presented and the links
between the (1) aqueous-phase processes, (2) aerosol dissolution, and (3)
atmospheric composition are demonstrated and quantified. The model results
are supported by comparison to available observations. We obtain an average
global OXL net chemical production of 12.615 ± 0.064 Tg yr−1 in
EC-Earth, with glyoxal being by far the most important precursor of oxalic
acid. In comparison to the ERA-Interim simulation, differences in
atmospheric dynamics and the simulated weaker oxidizing capacity in
EC-Earth overall result in a ∼ 30 % lower OXL source. On
the other hand, the more explicit representation of the aqueous-phase
chemistry in EC-Earth compared to the previous versions of the model leads
to an overall ∼ 20 % higher sulfate production, but this is still
well correlated with atmospheric observations. The total Fe dissolution rate in EC-Earth is calculated at 0.806 ± 0.014 Tg yr−1 and is added to the primary dissolved Fe (DFe) sources
from dust and combustion aerosols in the model (0.072 ± 0.001 Tg yr−1). The simulated DFe concentrations show a satisfactory comparison
with available observations, indicating an atmospheric burden of ∼0.007 Tg, resulting in an overall atmospheric deposition flux into the
global ocean of 0.376 ± 0.005 Tg yr−1, which is well within the range
reported in the literature. All in all, this work is a first step towards
the development of EC-Earth into an Earth system model with fully
interactive bioavailable atmospheric Fe inputs to the marine biogeochemistry
component of the model.
