Morphological features and water solubility of iron in aged fine aerosol particles over the Indian Ocean

Abstract

Atmospheric transport of iron (Fe) in fine anthropogenic aerosol particles is an important route of soluble Fe supply from continental areas to remote oceans. To elucidate Fe properties of aerosol particles over remote oceans, we collected atmospheric aerosol particles over the Indian Ocean during the RV Hakuho Maru KH-18-6 cruise. After aerosol particles were collected using a cascade impactor, particles of 0.3–0.9 µm aerodynamic diameter on the sample stage were analyzed using transmission electron microscopy (TEM) with an energy-dispersive X-ray spectrometry analyzer. The particle shape and composition indicated that most particles collected north of the Equator were composed mainly of ammonium sulfate. Regarding the particle number fraction, 0.6 %–3.0 % of particles contained Fe, which mostly co-existed with sulfate. Of those particles, 26 % of Fe occurred as metal spheres, often co-existing with Al or Si, regarded as fly ash; 14 % as mineral dust; and 7 % as iron oxide aggregates. Water dialysis analyses of TEM samples indicated Fe in spherical fly ash as being almost entirely insoluble and Fe in other morphological-type particles as being partly soluble (65 % Fe mass on average). Global model simulations mostly reproduced observed Fe mass concentrations in particulate matter with a diameter of less than 2.5 µm (PM2.5) collected using a high-volume air sampler, including their north–south contrast during the cruise. In contrast, a marked difference was found between the simulated mass fractions of Fe mineral sources and the observed Fe types. For instance, the model underestimated anthropogenic aluminosilicate (illite and kaolinite) Fe contained in matter such as fly ash from coal combustion. Our observations revealed multiple shapes and compositions of Fe minerals in particles over remote ocean areas and further suggested that their solubilities after aging processes differ depending on their morphological and mineral types. Proper consideration of such Fe types at their sources is necessary for accurately estimating atmospheric Fe effects on marine biological activity.