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

• We use the model ECHAM6.3-HAM2.3 to simulate the 36 Cl and resulting 10 Be/ 36 Cl ratio. • The polar regions are more sensitive to 36 Cl state (as gas or aerosol particle). • Changes in 36 Cl polar deposition to production rate are independent of 36 Cl state. The cosmogenic radionuclide 36 Cl is a valuable tracer for studying Earth system processes, solar variability and geomagnetic field changes. These applications rely on a comprehensive understanding of 36 Cl 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 36 Cl. We configured 36 Cl as gas in the stratosphere and as aerosol particles in the troposphere (E63H23CTL). Two sensitivity simulations were performed, with 36 Cl configured solely as aerosol particles (E63H23AER) and solely as gas (E63H23GAS). The E63H23CTL simulation agrees well with global 36 Cl measurements in terms of absolute values and temporal variability. E63H23AER significantly underestimates polar 36 Cl 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 36 Cl compared to aerosol-bound 36 Cl. This is further supported by comparison with the other cosmogenic radionuclide 10 Be, which is exclusively aerosol-bound. The stratospheric contribution is dominant (65–70 %) in 36 Cl 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, 36 Cl deposition varies proportionally to global production rate changes. However, as response to geomagnetic field changes, 36 Cl shows latitudinal-dependent deposition enhancements/decreases (10–33 %) relative to global production rate changes. This deposition response is insensitive to the forms of 36 Cl (gaseous or aerosol-bound) and is similar to that shown by 10 Be. The result indicates that differences in transport and deposition between 10 Be and 36 Cl may play a minor role when jointly using these two radionuclides for geomagnetic and solar reconstructions.