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

Abstract

Light-absorbing iron oxide (FeO x ) 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 FeO x 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 FeO x 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 FeO x concentrations and high FeO x /BC concentration ratios when the air mass passed through the plant. From the observed FeO x 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 FeO x concentrations and FeO x /BC ratios showed clear diurnal variations and depended little on wind direction. This indicates that other human activities also locally produce FeO x aerosols in Tokyo. Our data imply that although steel plant activities emit a large amount of FeO x , emissions from other anthropogenic sources, for example, motor vehicles, have a major contribution to the abundance of FeO x aerosols at the regional and global scales.

Classification of iron oxide aerosols by a single particle soot photometer using supervised machine learning

“…This suggests that the algorithm identifies iron oxides sourced from anthropogenic emissions with the laboratory samples of pure iron oxides. This application demonstrates that a significant feature of FeO x is present in the atmosphere in Boulder, as has previously been observed in urban areas in East Asia (Yoshida et al, 2016;Moteki et al, 2017;Ohata et al, 2018).…”

“…We designate this case as the "base case" and use only the color ratio and incandescent peak height to differentiate rBC from FeO x , as well as the additional criteria of the core scattering to differentiate anthropogenic FeO x from mineral dust with metallic inclusions. This is based on the method that has previously been used in, e.g., Moteki et al (2017), Ohata et al (2018), and Yoshida et al (2018). Figure 6.…”

“…Since we cannot optically size these aerosols outside of this range (Region 3), we need an additional assumption in order to estimate classification performance. Previous work has only designated FeO x as anthropogenic or dust-like for aerosols in Region 2 and used this (and offline techniques) to provide context for interpreting atmospheric observations of FeO x in East Asia as dominated by anthropogenic emissions (Moteki et al, 2017;Ohata et al, 2018;Yoshida et al, 2018).…”

“…In general, FeO x as quantified by the SP2 in atmospheric observations in past studies potentially included aerosols from both anthropogenic and nonanthropogenic sources. The mass mixing ratio and size distribution of FeO x has been quantified in East Asia, where observations suggested these aerosols were mainly from anthropogenic sources, and were also observed to be significantly more prevalent than previously believed (Yoshida et al, 2016Moteki et al, 2017;Ohata et al, 2018). These measurements have important implications for the climatic effects associated with these aerosols: the direct radiative climate effects of anthropogenic FeO x may be as important as brown carbon in some regions (Moteki et al, 2017;Matsui et al, 2018), and modeling studies based on these measurements indicate these aerosols could also be an important source of particulate iron for the oceanic biogeochemical cycle (Matsui et al, 2018;Ito et al, 2018).…”

“…An SP2 was used in one study to estimate the hematite content in Saharan dust measured off the west coast of Africa and demonstrated good closure between the hematite concentration associated with single dust particles and the optical properties observed in the dust plume (Liu et al, 2018). While previous work focusing on anthropogenic FeO x has relied on using the optical size of these aerosols after any volatile coatings have evaporated as additional criteria to differentiate anthropogenic and nonanthropogenic aerosols with metallic components (Yoshida et al, 2016Moteki et al, 2017;Ohata et al, 2018), this method is limited to the range over which the SP2 optically sizes these aerosols (generally limited to ∼ 170-350 nm volume equivalent diameter for FeO x ). Previous studies using the SP2 to quantify FeO x associated with anthropogenic sources have also not provided quantitative measures of classification performance for these aerosols.…”