A new single particle chemical analysis approach for characterization of atmosphere-marine biosphere interactions: Detection and source apportionment of iron containing Aerosols

The influx of essential elements such as iron into remote ocean areas is an important parameter for the global bio mass production (algae). A major source of Fe in remote ocean areas is the deposition of long-range transported Fe-containing aerosol particles. Increasing desertification and storm activities supposedly increase the natural iron-flux via the aerosol pathway. Fe-induced enhancement of diatom growth impacts the carbon cycle (global warming), as diatoms after dying are efficiently transferring carbon into deep sea sediments. Even intended iron fertilizing of the oceans was discussed in the last years as a potential measure for CO₂-reducion. As particle-ensembles are mixed, agglomerated and heterogeneously grown during the transport to the receptor site, single particle chemical speciation and high sensitivity for iron are required to study sources and mechanisms of Fe-deposition. This motivates the development of improved analytical instrumentation to analyse composition and source of Fe-containing particles in the relevant size range (0.3-3 μm). Recently, a new on-line aerosol mass spectrometer, analysing the single particle-resolved chemical composition in the size range of question was developed (Passig et al., Anal. Chem., 2019). The system added the detectability of organic compounds (polycyclic aromatic hydrocarbons) by laser desorption-resonance-enhanced multiphoton ionisation (LD-REMPI) to the classical laser desorption/ionisation (LDI) analytes spectrum of positive elemental ions (e.g. C⁺, K⁺, Fe⁺ or Ni⁺) and negative inorganic ion species (SO₄^- or NO₃^-). Novel is that the chemical profiling is performed on-line from the very same size classified particles. The fingerprint of organics, positive and negative inorganic ions reveals the source and history of the respective particle. By selecting a laser wavelength which is in resonance with a Fe-atomic transition, the formation of Fe-ions in the LDI-laser plasma could be strongly enhanced. Here the 3d⁶4s²→3d⁶4s4p transition of iron at 248.3 nm is used. For the practical analysis instrument (Photonion GmbH, Germany) a KrF-excimer laser is used, enabling a ca. 5fold increased single particle Fe-detection efficiency. This renders the instrument well for the single particle-based analysis of Fe-containing Aerosols.