Trace metal-containing aerosols in the atmosphere of the Indian Ocean

Atmospheric deposition of aerosols constitutes a major source of iron and other micronutrients to remote ocean regions, where nutrient limitation constrains primary productivity and carbon sequestration. However, large uncertainties persist due to sparse observational data and the lack of sensitive techniques capable of resolving metal solubility at low aerosol loadings. Here we present first results from a shipborne campaign conducted aboard R/V Sonne across the Indian Ocean in late 2024 within the framework of the GEOTRACES program.

Aerosol particles were characterized using a novel single-particle mass spectrometer (SPMS) employing resonant laser ionization, enabling the analysis of the chemical composition of several hundred thousand individual particles. While sea spray aerosols dominated the overall particle population, thousands of iron-containing particles were detected, primarily associated with long-range transported mineral dust. Notably, a subset of sea spray aerosol particles exhibited detectable iron signals, suggesting in-cloud mixing or surface re-emission processes as potential sources.

For mineral dust particles, nitrate represented the dominant secondary component even in air masses without continental influence for more than ten days. Elevated iron contents within dust particles frequently coincided with the presence of dicarboxylic acids, whereas Mg/Ca-rich particles were preferentially associated with sulfate, indicating distinct atmospheric processing pathways, transport histories, and likely differences in iron solubility. By resolving such internal mixtures at the single-particle level, the SPMS provides a powerful approach for source attribution and for assessing the potential bioavailability of aerosol-derived metals. These observations reveal an unexpectedly high abundance and chemical diversity of iron-containing aerosols over the Indian Ocean, underscoring their importance for ocean biogeochemistry and nutrient cycling in this understudied region.