Abstract
Abstract. Sea salt aerosols (SSA) are generated via air bubbles bursting at the ocean surface as well as by wind mobilization of saline snow and frost flowers over sea-ice-covered areas. The relative magnitude of these sources remains poorly constrained over polar regions, affecting our ability to predict their impact on halogen chemistry, cloud formation, and climate. We implement a blowing snow and a frost flower emission scheme in the GEOS-Chem global chemical transport model, which we validate against multiyear (2001–2008) in situ observations of SSA mass concentrations at three sites in the Arctic, two sites in coastal Antarctica, and from the 2008 ICEALOT cruise in the Arctic. A simulation including only open ocean emissions underestimates SSA mass concentrations by factors of 2–10 during winter–spring for all ground-based and ship-based observations. When blowing snow emissions are added, the model is able to reproduce observed wintertime SSA concentrations, with the model bias decreasing from a range of −80 to −34 % for the open ocean simulation to −2 to +9 % for the simulation with blowing snow emissions. We find that the frost flower parameterization cannot fully explain the high wintertime concentrations and displays a seasonal cycle decreasing too rapidly in early spring. Furthermore, the high day-to-day variability of observed SSA is better reproduced by the blowing snow parameterization. Over the Arctic (> 60° N) (Antarctic, > 60° S), we calculate that submicron SSA emissions from blowing snow account for 1.0 Tg yr−1 (2.5 Tg yr−1), while frost flower emissions lead to 0.21 Tg yr−1 (0.25 Tg yr−1) compared to 0.78 Tg yr−1 (1.0 Tg yr−1) from the open ocean. Blowing snow emissions are largest in regions where persistent strong winds occur over sea ice (east of Greenland, over the central Arctic, Beaufort Sea, and the Ross and Weddell seas). In contrast, frost flower emissions are largest where cold air temperatures and open leads are co-located (over the Canadian Arctic Archipelago, coastal regions of Siberia, and off the Ross and Ronne ice shelves). Overall, in situ observations of mass concentrations of SSA suggest that blowing snow is likely to be the dominant SSA source during winter, with frost flowers playing a much smaller role.
Highlights
Breaking waves over the open ocean are recognized as the main mechanism for the global production of sea salt aerosol (SSA) (Lewis and Schwartz, 2004; de Leeuw et al, 2011, and references therein)
We evaluate the ability of these two sources to reproduce multiyear (2001–2008) in situ measurements of Na+ mass concentrations at three Arctic sites (Barrow, Alaska; Alert, Canada; Zeppelin, Svalbard) and two coastal Antarctic sites (Neumayer and Dumont d’Urville), as well as Na+ measurements obtained during the International Chemistry Experiment in the Arctic LOwer Troposphere (ICEALOT) cruise during spring 2008
We find that the GEOS-Chem simulation with open ocean emissions fails to capture the elevated SSA mass concentrations observed at five coastal stations in the Arctic and Antarctic during winter (2001–2008) and during the ICEALOT research cruise in March–April 2008
Summary
Breaking waves over the open ocean are recognized as the main mechanism for the global production of sea salt aerosol (SSA) (Lewis and Schwartz, 2004; de Leeuw et al, 2011, and references therein). Laboratory experiments performed by Roscoe et al (2011) demonstrated that no aerosol were produced when frost flowers were exposed to winds speeds up to 12 m s−1 Their result is consistent with electron microscope imaging by Yang et al (2017), which shows that evaporating frost flowers form a cohesive chunk of salt that is unlikely to be a source of SSA. Many questions remain on the formation, composition, occurrence, and mobility of frost flowers and salty blowing snow Most studies examining these two sources have focused on their potential role as an indirect source of gasphase bromine resulting in ozone depletion events during late winter and early spring, with conflicting results as to which source would be most important. We examine the relative contributions of open ocean, blowing snow and frost flower sources to the distribution of SSA over polar regions
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