Modelling wintertime sea-spray aerosols under Arctic haze conditions
Résumé
Anthropogenic and natural emissions contribute to enhanced concentrations of aerosols in the Arctic
winter and early spring, with most attention being paid to anthropogenic aerosols that contribute to so-called
Arctic haze. Less-well-studied wintertime sea-spray aerosols (SSAs) under Arctic haze conditions are the focus
of this study, since they can make an important contribution to wintertime Arctic aerosol abundances. Analysis of
field campaign data shows evidence for enhanced local sources of SSAs, including marine organics at Utqia ̇gvik
(formerly known as Barrow) in northern Alaska, United States, during winter 2014. Models tend to underestimate
sub-micron SSAs and overestimate super-micron SSAs in the Arctic during winter, including the base version of
the Weather Research Forecast coupled with Chemistry (WRF-Chem) model used here, which includes a widely
used SSA source function based on Gong et al. (1997). Quasi-hemispheric simulations for winter 2014 including
updated wind speed and sea-surface temperature (SST) SSA emission dependencies and sources of marine sea-
salt organics and sea-salt sulfate lead to significantly improved model performance compared to observations at
remote Arctic sites, notably for coarse-mode sodium and chloride, which are reduced. The improved model also
simulates more realistic contributions of SSAs to inorganic aerosols at different sites, ranging from 20 %–93 %
in the observations. Two-thirds of the improved model performance is from the inclusion of the dependence
on SSTs. The simulation of nitrate aerosols is also improved due to less heterogeneous uptake of nitric acid on
SSAs in the coarse mode and related increases in fine-mode nitrate. This highlights the importance of interactions
between natural SSAs and inorganic anthropogenic aerosols that contribute to Arctic haze. Simulation of organic
aerosols and the fraction of sea-salt sulfate are also improved compared to observations. However, the model
underestimates episodes with elevated observed concentrations of SSA components and sub-micron non-sea-salt
sulfate at some Arctic sites, notably at Utqia ̇gvik. Possible reasons are explored in higher-resolution runs over
northern Alaska for periods corresponding to the Utqia ̇gvik field campaign in January and February 2014. The
addition of a local source of sea-salt marine organics, based on the campaign data, increases modelled organic
aerosols over northern Alaska. However, comparison with previous available data suggests that local natural
sources from open leads, as well as local anthropogenic sources, are underestimate in the model. Missing local anthropogenic sources may also explain the low modelled (sub-micron) non-sea-salt sulfate at Utqia ̇gvik.
The introduction of a higher wind speed dependence for sub-micron SSA emissions, also based on Arctic data,
reduces biases in modelled sub-micron SSAs, while sea-ice fractions, including open leads, are shown to be an
important factor controlling modelled super-micron, rather than sub-micron, SSAs over the north coast of Alaska.
The regional results presented here show that modelled SSAs are more sensitive to wind speed dependence but
that realistic modelling of sea-ice distributions is needed for the simulation of local SSAs, including marine
organics. This study supports findings from the Utqia ̇gvik field campaign that open leads are the primary source
of fresh and aged SSAs, including marine organic aerosols, during wintertime at Utqia ̇gvik; these findings do not
suggest an influence from blowing snow and frost flowers. To improve model simulations of Arctic wintertime
aerosols, new field data on processes that influence wintertime SSA production, in particular for fine-mode
aerosols, are needed as is improved understanding about possible local anthropogenic sources
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