Atmospheric transport and deposition of cosmogenic 36Cl using ECHAM6.3-HAM2.3 model

The cosmogenic radionuclide ³⁶Cl is a valuable tracer for studying Earth system processes, solar variability and geomagnetic field changes. These applications rely on a comprehensive understanding of ³⁶Cl transport and deposition processes, which are still poorly studied. In this study, we apply a state-of-the-art climate model ECHAM6.3-HAM2.3 to investigate the source distribution and deposition of ³⁶Cl. We configured ³⁶Cl as gas in the stratosphere and as aerosol particles in the troposphere (E63H23CTL). Two sensitivity simulations were performed, with ³⁶Cl configured solely as aerosol particles (E63H23AER) and solely as gas (E63H23GAS). The E63H23CTL simulation agrees well with global ³⁶Cl measurements in terms of absolute values and temporal variability. E63H23AER significantly underestimates polar ³⁶Cl deposition compared to measurements, E63H23CTL and E63H23GAS, suggesting that polar regions are more sensitive to the ³⁶Cl state (aerosol or gas phase) than other regions. This is most likely attributed to the predominance of mixed-phase clouds in the polar regions, which have a higher scavenging efficiency for gaseous ³⁶Cl compared to aerosol-bound ³⁶Cl. This is further supported by comparison with the other cosmogenic radionuclide ¹⁰Be, which is exclusively aerosol-bound. The stratospheric contribution is dominant (65–70 %) in ³⁶Cl deposition in polar and subtropical regions, while stratospheric and tropospheric contributions are of similar size (50–51 %) in tropical regions. When responding to changes in solar modulation, ³⁶Cl deposition varies proportionally to global production rate changes. However, as response to geomagnetic field changes, ³⁶Cl shows latitudinal-dependent deposition enhancements/decreases (10–33 %) relative to global production rate changes. This deposition response is insensitive to the forms of ³⁶Cl (gaseous or aerosol-bound) and is similar to that shown by ¹⁰Be. The result indicates that differences in transport and deposition between ¹⁰Be and ³⁶Cl may play a minor role when jointly using these two radionuclides for geomagnetic and solar reconstructions.