Emma L. Mungall

Microlayer source of oxygenated volatile organic compounds in the summertime marine Arctic boundary layer

Significance A biogeochemical connection between the atmosphere and the ocean is demonstrated whereby a marine source of oxygenated volatile organic compounds is identified. Compounds of this type are involved in the formation of secondary organic aerosol, which remains one of the most poorly understood components of Earth’s climate system due in part to the diverse sources of its volatile organic compound precursors. This is especially the case for marine environments, where there are more oxygenated volatile organic compounds than can be accounted for by known sources. Although it was observed in the summertime Arctic, this connection may be widespread and important to our understanding of secondary organic aerosol in other remote marine environments, with implications for our understanding of global climate. Summertime Arctic shipboard observations of oxygenated volatile organic compounds (OVOCs) such as organic acids, key precursors of climatically active secondary organic aerosol (SOA), are consistent with a novel source of OVOCs to the marine boundary layer via chemistry at the sea surface microlayer. Although this source has been studied in a laboratory setting, organic acid emissions from the sea surface microlayer have not previously been observed in ambient marine environments. Correlations between measurements of OVOCs, including high levels of formic acid, in the atmosphere (measured by an online high-resolution time-of-flight mass spectrometer) and dissolved organic matter in the ocean point to a marine source for the measured OVOCs. That this source is photomediated is indicated by correlations between the diurnal cycles of the OVOC measurements and solar radiation. In contrast, the OVOCs do not correlate with levels of isoprene, monoterpenes, or dimethyl sulfide. Results from box model calculations are consistent with heterogeneous chemistry as the source of the measured OVOCs. As sea ice retreats and dissolved organic carbon inputs to the Arctic increase, the impact of this source on the summer Arctic atmosphere is likely to increase. Globally, this source should be assessed in other marine environments to quantify its impact on OVOC and SOA burdens in the atmosphere, and ultimately on climate.

Role of degassing of the Noril’sk nickel deposits in the Permian–Triassic mass extinction event

Significance The Noril’sk deposits represent one of the most valuable metal concentrations on Earth and are associated with the world’s largest outpouring of mafic magma. Mass release of nickel into the atmosphere during ore formation has been postulated as one of the triggers for the Permian–Triassic Mass Extinction Event, by promoting the activity of the marine Archaea methanosarcina with catastrophic greenhouse climatic effects. The missing link has been understanding how nickel, normally retained at depth in magmatic minerals, could have been mobilized into magmatic gases. The flotation of magmatic sulfides to the surface by gas bubbles was suggested as a possible mechanism. Here, we provide evidence of physically attached nickel-rich sulfide droplets and former gas bubbles, frozen into the Noril’sk ores. The largest mass extinction event in Earths history marks the boundary between the Permian and Triassic Periods at circa 252 Ma and has been linked with the eruption of the basaltic Siberian Traps large igneous province (SLIP). One of the kill mechanisms that has been suggested is a biogenic methane burst triggered by the release of vast amounts of nickel into the atmosphere. A proposed Ni source lies within the huge Noril’sk nickel ore deposits, which formed in magmatic conduits widely believed to have fed the eruption of the SLIP basalts. However, nickel is a nonvolatile element, assumed to be largely sequestered at depth in dense sulfide liquids that formed the orebodies, preventing its release into the atmosphere and oceans. Flotation of sulfide liquid droplets by surface attachment to gas bubbles has been suggested as a mechanism to overcome this problem and allow introduction of Ni into the atmosphere during eruption of the SLIP lavas. Here we use 2D and 3D X-ray imagery on Noril’sk nickel sulfide, combined with simple thermodynamic models, to show that the Noril’sk ores were degassing while they were forming. Consequent “bubble riding” by sulfide droplets, followed by degassing of the shallow, sulfide-saturated, and exceptionally volatile and Cl-rich SLIP lavas, permitted a massive release of nickel-rich volcanic gas and subsequent global dispersal of nickel released from this gas as aerosol particles.

Organic condensation and particle growth to CCN sizes in the summertime marine Arctic is driven by materials more semi-volatile than at continental sites

Ship-based aerosol measurements in the summertime Arctic indicate elevated concentrations of ultrafine particles with occasional growth to CCN sizes. Focusing on one episode with two continuously growing modes, growth occurs faster for a large, pre-existing mode (dp ≈ 90 nm) than for a smaller nucleation mode (dp ≈ 20 nm). We use microphysical modeling to show that growth is largely via organic condensation. Unlike results for mid-latitude forested regions, most of these condensing species behave as semi-volatile organics, as lower-volatility organics would lead to faster growth of the smaller mode. The magnitude of the CCN hygroscopicity parameter for the growing particles, ~0.1, is also consistent with organic species constituting a large fraction of the particle composition. Mixing ratios of common aerosol growth precursors, such as isoprene and sulfur dioxide, are not elevated during the episode, indicating that an unidentified aerosol-growth precursor is present in this high-latitude marine environment.