Variations in Arctic aerosol iron solubility in relation to leaching methodology, air mass characteristics, and seasonality

Atmospheric deposition of the essential micronutrient, iron (Fe), can have an important influence on primary production and marine biogeochemistry. In the Arctic Ocean, the ongoing shift towards seasonal ice coverage means that summertime atmospheric deposition increasingly takes place directly to the surface ocean, rather than onto sea ice. As a result, atmospheric deposition of material emitted from natural and anthropogenic sources may become a more relevant Fe input to the region. As part of the U.S. GEOTRACES GN01 section, aerosols and precipitation samples were collected to quantify the atmospheric delivery of Fe and other trace elements to the Arctic Ocean. Aerosol Fe solubility was assessed using three different leaching approaches. The readily soluble fraction, determined by rapid exposure leaches with ultrapure water (UPW) and filtered seawater (SW) was low throughout GN01, averaging 0.7 % and 1.6 %, respectively. Solubility determined using a more aggressive acetic acid (HAc) leach as an upper limit estimate of post-deposition aerosol Fe bioavailability averaged 44 %. Comparison to Fe UPW-solubility data from winter (median 6.5 %) and springtime (median 1.9 %) aerosol samples collected during the MOSAiC expedition suggests a strong seasonality to Arctic aerosol Fe solubility, potentially associated with winter/springtime Arctic haze. Summertime bulk deposition fluxes, calculated by applying the beryllium-7 method to GN01 data, were estimated at 0.9 ± 1.3 nmolm-2d-1 UPW-soluble Fe, 2.4 ± 1.9 nmolm-2d-1 SW-soluble Fe, and 46 ± 48 nmolm-2d-1 HAc-soluble Fe. Thus, the UPW-soluble Fe flux during GN01 was around an order of magnitude lower than that measured during the winter months despite bulk deposition velocity being almost twice as fast during the summer.