Online water-soluble iron speciation in ambient aerosols under cloud-water-relevant and neutral conditions

Abstract. We present the first online instrument for the speciation of water-soluble iron in ambient aerosols under cloud-water-relevant and neutral conditions, enabling simultaneous quantification of Fe(II) (ws-Fe(II)) and total water-soluble Fe (total ws-Fe). The system combines flow injection analysis with spectrophotometric detection of the Fe(II)–ferrozine complex using a liquid waveguide capillary cell (LWCC) for sensitive detection, thus providing time-resolved measurements of iron solubility with high temporal resolution and good reproducibility. The setup was tested with two different aerosol sampling units during field campaigns in Berlin. In summer 2024, the Metrohm AeRosol Sampler (MARS) was operated with an extraction pH of 6.5, providing neutral reference extraction conditions. In winter 2025, a particle-into-liquid sampler (PILS) was applied using an extraction pH of 4.5 to probe iron solubility and speciation under mildly acidic, cloud-water-relevant extraction conditions. Limits of quantification (LOQs) for Fe(II) determination were 1.6 and 1.0 ng m−3 for the MARS-FIA and PILS-FIA setups, respectively, with ambient ws-Fe concentrations ranging from below the LOQ to 47 ng m−3. Both setups yielded robust online measurements; however, the PILS-FIA working at pH 4.5 underestimated ws-Fe compared to filter sampling and extraction. This discrepancy can be attributed to the shorter extraction time in the PILS system, highlighting the influence of extraction duration on the measured iron concentration. Since soluble iron drives important tropospheric aqueous-phase reactions like hydroxyl radical formation through Fenton chemistry, the speciation data provided by the presented setup could improve model representations of atmospheric iron processes.