Nathaniel W. May

Lake Spray Aerosol: A Chemical Signature from Individual Ambient Particles

Aerosol production from wave breaking on freshwater lakes, including the Laurentian Great Lakes, is poorly understood in comparison to sea spray aerosol (SSA). Aerosols from freshwater have the potential to impact regional climate and public health. Herein, lake spray aerosol (LSA) is defined as aerosol generated from freshwater through bubble bursting, analogous to SSA from seawater. A chemical signature for LSA was determined from measurements of ambient particles collected on the southeastern shore of Lake Michigan during an event (July 6-8, 2015) with wave heights up to 3.1 m. For comparison, surface freshwater was collected, and LSA were generated in the laboratory. Single particle microscopy and mass spectrometry analysis of field and laboratory-generated samples show that LSA particles are primarily calcium (carbonate) with lower concentrations of other inorganic ions and organic material. Laboratory number size distributions show ultrafine and accumulation modes at 53 (±1) and 276 (±8) nm, respectively. This study provides the first chemical signature for LSA. LSA composition is shown to be coupled to Great Lakes water chemistry (Ca(2+) > Mg(2+) > Na(+) > K(+)) and distinct from SSA. Understanding LSA physicochemical properties will improve assessment of LSA impacts on regional air quality, climate, and health.

Multiyear study of the dependence of sea salt aerosol on wind speed and sea ice conditions in the coastal Arctic

Thinning of Arctic sea ice gives rise to ice fracturing and leads (areas of open water surrounded by sea ice) that are a potential source of sea salt aerosol. Atmospheric particle inorganic ion concentrations, local sea ice conditions, and meteorology at Barrow, AK, from 2006 to 2009, were combined to investigate the dependence of submicron (aerodynamic diameter < 1 µm) and supermicron (aerodynamic diameter 1–10 µm) sea salt mass concentrations on sea ice coverage and wind speed. Consistent with a wind-dependent source, supermicron sea salt mass concentrations increased in the presence of nearby leads and wind speeds greater than 4 m s−1. Increased supermicron and submicron sea salt chloride depletion was observed for periods of low winds or a lack of nearby open water, consistent with transported sea salt influence. Sea salt aerosol produced from leads has the potential to alter cloud formation, as well as the chemical composition of the Arctic atmosphere and snowpack.

Active molecular iodine photochemistry in the Arctic

Significance We report here the first measurements of molecular iodine (I2) in the Arctic atmosphere and iodide (I−) in the Arctic snowpack. Although iodine chemistry is expected to have significant impacts on Arctic atmospheric ozone destruction and new particle production, sparse measurements of atmospheric iodine have limited our ability to examine sources and impacts. We show, through sunlit and artificially irradiated snowpack experiments, that the coastal Arctic snowpack is capable of photochemical production and release of I2 to the boundary layer. This is supported by enrichment of the snowpack in I− compared with that expected from sea spray influence alone. Through photochemical modeling, we demonstrate that, at observed I2 levels, snowpack production can have a significant impact on Arctic atmospheric chemistry. During springtime, the Arctic atmospheric boundary layer undergoes frequent rapid depletions in ozone and gaseous elemental mercury due to reactions with halogen atoms, influencing atmospheric composition and pollutant fate. Although bromine chemistry has been shown to initiate ozone depletion events, and it has long been hypothesized that iodine chemistry may contribute, no previous measurements of molecular iodine (I2) have been reported in the Arctic. Iodine chemistry also contributes to atmospheric new particle formation and therefore cloud properties and radiative forcing. Here we present Arctic atmospheric I2 and snowpack iodide (I−) measurements, which were conducted near Utqiaġvik, AK, in February 2014. Using chemical ionization mass spectrometry, I2 was observed in the atmosphere at mole ratios of 0.3–1.0 ppt, and in the snowpack interstitial air at mole ratios up to 22 ppt under natural sunlit conditions and up to 35 ppt when the snowpack surface was artificially irradiated, suggesting a photochemical production mechanism. Further, snow meltwater I− measurements showed enrichments of up to ∼1,900 times above the seawater ratio of I−/Na+, consistent with iodine activation and recycling. Modeling shows that observed I2 levels are able to significantly increase ozone depletion rates, while also producing iodine monoxide (IO) at levels recently observed in the Arctic. These results emphasize the significance of iodine chemistry and the role of snowpack photochemistry in Arctic atmospheric composition, and imply that I2 is likely a dominant source of iodine atoms in the Arctic.

Aerosol Emissions from Great Lakes Harmful Algal Blooms

In freshwater lakes, harmful algal blooms (HABs) of Cyanobacteria (blue-green algae) produce toxins that impact human health. However, little is known about the lake spray aerosol (LSA) produced from wave-breaking in freshwater HABs. In this study, LSA were produced in the laboratory from freshwater samples collected from Lake Michigan and Lake Erie during HAB and nonbloom conditions. The incorporation of biological material within the individual HAB-influenced LSA particles was examined by single-particle mass spectrometry, scanning electron microscopy with energy-dispersive X-ray spectroscopy, and fluorescence microscopy. Freshwater with higher blue-green algae content produced higher number fractions of individual LSA particles that contained biological material, showing that organic molecules of biological origin are incorporated in LSA from HABs. The number fraction of individual LSA particles containing biological material also increased with particle diameter (greater than 0.5 μm), a size dependence that is consistent with previous studies of sea spray aerosol impacted by phytoplankton blooms. Similar to sea spray aerosol, organic carbon markers were most frequently observed in individual LSA particles less than 0.5 μm in diameter. Understanding the transfer of biological material from freshwater to the atmosphere via LSA is crucial for determining health and climate effects of HABs.

Laboratory Studies of the Cloud Droplet Activation Properties and Corresponding Chemistry of Saline Playa Dust

Playas emit large quantities of dust that can facilitate the activation of cloud droplets. Despite the potential importance of playa dusts for cloud formation, most climate models assume that all dust is nonhygroscopic; however, measurements are needed to clarify the role of dusts in aerosol-cloud interactions. Here, we report measurements of CCN activation from playa dusts and parameterize these results in terms of both κ-Köhler theory and adsorption activation theory for inclusion in atmospheric models. κ ranged from 0.002 ± 0.001 to 0.818 ± 0.094, whereas Frankel-Halsey-Hill (FHH) adsorption parameters of AFHH = 2.20 ± 0.60 and BFHH = 1.24 ± 0.14 described the water uptake properties of the dusts. Measurements made using aerosol time-of-flight mass spectrometry (ATOFMS) revealed the presence of halite, sodium sulfates, and sodium carbonates that were strongly correlated with κ underscoring the role that mineralogy, including salts, plays in water uptake by dust. Predictions of κ made using bulk chemical techniques generally showed good agreement with measured values. However, several samples were poorly predicted suggesting that chemical heterogeneities as a function of size or chemically distinct particle surfaces can determine the hygroscopicity of playa dusts. Our results further demonstrate the importance of dust in aerosol-cloud interactions.