Isotopic composition of aerosol iron from anthropogenic sources: implications for source apportionment of aerosol iron

Abstract. Aerosol iron (Fe) significantly impacts human health, atmospheric chemistry and marine biogeochemistry. The stable isotope ratio of Fe, typically reported as δ56Fe, has emerged as a promising method for source apportionment of total and soluble aerosol Fe. However, the δ56Fe endmember values remain poorly constrained for aerosol Fe from various non-dust sources, impeding the application of Fe isotopes in atmospheric research. This work measured isotopic compositions for aerosol Fe from desert dust and several anthropogenic sources. The average δ56Fe was determined to be +0.14 ± 0.10 ‰ for the seven dust samples we examined, in good agreement with previous work. We found that different anthropogenic aerosols exhibit a wide range of Fe isotopic composition. Compared to desert dust, the average δ56Fe was found to be higher for power plant coal fly ash (+0.26 ± 0.18 ‰, n = 28), slightly lower for steelwork fly ash (−0.07 ± 0.41 ‰, n = 18), and considerably lower for biofuel burning aerosol (−0.28 ± 0.39 ‰, n = 11). In addition, the average δ56Fe was determined to be +0.20 ± 0.12 ‰ (n = 2) for municipal incineration fly ash, +0.38 ± 0.13 ‰ (n = 1) for heavy oil bottom ash, and +0.08 ± 0.13 ‰ for certificated urban particulate matter sample (n = 1). We suggest that not all the anthropogenic aerosol Fe is isotopically lighter than natural dust Fe, in contrast to what is conventionally assumed. Our findings also imply that Fe isotope-based source apportionment must account for the δ56Fe endmember variability both among and within different anthropogenic aerosols. Overall, our work substantially improves our ability to constrain δ56Fe endmember values from various anthropogenic sources.