Contribution of combustion Fe in marine aerosols over the northwestern Pacific estimated by Fe stable isotope ratios

The source apportionment of aerosol iron (Fe), including natural and combustion Fe, is an important issue because aerosol Fe can enhance oceanic primary production in the surface ocean. Based on our previous finding that combustion Fe emitted by evaporation processes has Fe isotope ratios (delta Fe-56) that are approximately 4 parts per thousand lower than those of natural Fe, this study aimed to distinguish aerosol Fe sources over the northwestern Pacific using two size-fractionated marine aerosols. The delta Fe-56 values of fine and coarse particles from the eastern or northern Pacific were found to be similar to each other, ranging from 0.0 parts per thousand to 0.4 parts per thousand. Most of them were close to the crustal average, suggesting the dominance of natural Fe. On the other hand, particles from the direction of East Asia demonstrated lower delta Fe-56 values in fine particles (-0.5 parts per thousand to -2.2 parts per thousand) than in coarse particles (on average -0.02 +/- 0.12 parts per thousand). The correlations between the delta Fe-56 values and the enrichment factors of lead and vanadium suggested that the low delta Fe-56 values obtained were due to the presence of combustion Fe. The delta Fe-56 values of the soluble component of fine particles in this region were lower than the total, indicating the preferential dissolution of combustion Fe. In addition, we found a negative correlation between the delta Fe-56 value and the fractional Fe solubility in air masses from the direction of East Asia. These results suggest that the presence of combustion Fe is an important factor in controlling the fractional Fe solubility in air masses from the direction of East Asia, whereas other factors are more important in the other areas. By assuming typical delta Fe-56 values for combustion and natural Fe, the contribution of combustion Fe to the total (acid-digested) Fe in aerosols was estimated to reach up to 50 % of fine and 21 % of bulk (coarse + fine) particles in air masses from the direction of East Asia, whereas its contribution was small in the other areas. The contribution of combustion Fe to the soluble Fe component estimated for one sample was approximately twice as large as the total, indicating the importance of combustion Fe as a soluble Fe source despite lower emissions than the natural. These isotope-based estimates were compared with those estimated using an atmospheric chemical transport model (IMPACT), in which the fractions of combustion Fe in fine particles, especially in air masses from the direction of East Asia, were consistent with each other. In contrast, the model estimated a relatively large contribution from combustion Fe in coarse particles, probably because of the different characteristics of combustion Fe that are included in the model calculation and the isotope-based estimation. This highlights the importance of observational data on delta Fe-56 for size-fractionated aerosols to scale the combustion Fe emission by the model. The average deposition fluxes of soluble Fe to the surface ocean were 1.4 and 2.9 nmol m(-2) d(-1) from combustion and natural aerosols, respectively, in air masses from the direction of East Asia, which suggests that combustion Fe could be an important Fe source to the surface seawater among other Fe sources. Distinguishing Fe sources using the delta Fe-56 values of marine aerosols and seawater is anticipated to lead to a more quantitative understanding of the Fe cycle in the atmosphere and surface ocean.

Automated summary

This study aimed to distinguish aerosol Fe sources in the northwestern Pacific using Fe isotope ratios, finding that combustion Fe contribution is higher in air masses from East Asia and has higher solubility. Combustion Fe plays an important role in controlling the fractional Fe solubility in air masses from East Asia, while other factors are more influential in other areas. Observational data on Fe isotopes for aerosols are necessary for accurate estimation of combustion Fe emissions in atmospheric models.