Constraints on Anthropogenic Iron Oxide Aerosols on a Global Scale

A recent characterization near a source region indicates that anthropogenic iron oxide aerosol (FeO_x) may be an important absorber in the atmosphere. Here we apply a similar methodology to single particle soot photometer measurements made over the remote Pacific Ocean during the HIPPO 2009-2011 and the AToM 2016-2017 aircraft campaigns to constrain FeO_x abundance on a global scale. These measurements of FeO_x were made far from sources in several seasons and across seven years, constraining both the potential radiative importance of these aerosols as a short-term climate forcer, and their possible impact as a source of ice nuclei in the remote atmosphere. We find indications that FeO_x can be efficiently transported over long distances and may represent a non-negligible source of atmospheric absorption in the Northern Hemisphere. Iron oxide is typically found to be correlated with black carbon (BC), implying similar source regions. In conjunction with previous (near-source) measurements of FeO_x in Asian outflow, these observations can constrain the deposition of anthropogenic FeO_x to the oceans, where it provides a source of bio-available iron to the oceanic biogeochemical cycle.