Abundance of Light-Absorbing Anthropogenic Iron Oxide Aerosols in the Urban Atmosphere and Their Emission Sources

Abstract

Light-absorbing iron oxide (FeOx) aerosols such as magnetite contribute to shortwave atmospheric heating and possibly affect the biogeochemical cycle. However, their atmospheric abundance and emission sources are poorly understood. In this study, we quantified the abundance and mixing states of FeOx at two urban sites in Tokyo and Chiba, Japan, using a modified single-particle soot photometer and filter-based instruments. At both sites, the majority of the FeOx were of anthropogenic origin in the form of aggregated magnetite nanoparticles, and their concentrations generally correlated with those of black carbon (BC) and carbon monoxide. In Chiba, where the observatory was located near an integrated steel plant, we observed distinctly high FeOx concentrations and high FeOx/BC concentration ratios when the air mass passed through the plant. From the observed FeOx plumes with the mass equivalent diameter range of 170–2,100 nm, we made an estimate of their emission flux to be approximately 0.012% of the crude steel production, assuming that a Gaussian plume approximation was applicable to our data analysis. Meanwhile, in Tokyo, where the observatory was 20–40 km northwest of steel plants, the FeOx concentrations and FeOx/BC ratios showed clear diurnal variations and depended little on wind direction. This indicates that other human activities also locally produce FeOx aerosols in Tokyo. Our data imply that although steel plant activities emit a large amount of FeOx, emissions from other anthropogenic sources, for example, motor vehicles, have a major contribution to the abundance of FeOx aerosols at the regional and global scales.