Resolving the roles of soot and dust in cirrus cloud ice formation at regional and global scales: insights from parcel and climate modeling

Abstract. Atmospheric aerosols can serve as ice-nucleating particles (INPs), influencing cirrus cloud formation and properties. While mineral dust is recognized as an effective INP, the role of soot remains less explored, limiting climate impact assessments. Here we use cloud parcel model simulations to examine the competitive ice nucleation behavior of soot and dust, alongside homogeneous nucleation. These process-level simulations reveal that dust dominates heterogeneous ice nucleation at colder temperatures (T<210 K), whereas soot becomes increasingly more important at warmer temperatures, particularly when dust concentrations are low. To evaluate their global-scale implications, we integrate these results into the GFDL AM4-MG2 climate model. We find that dust shapes the baseline spatial and seasonal ice crystal number concentration (ICNC) patterns, while soot (represented in the model as black carbon, BC) enhances global-mean ICNC by ∼ 5 %. However, BC-driven increases in ICNC can be much larger in the upper troposphere (500–250 hPa), reaching up to 90 %. The strongest enhancements are found during boreal spring across Eurasia and the Maritime Continent, and during austral spring over South America and the South Atlantic. Radiatively, BC INPs can enhance the annual global longwave cloud radiative effect by approximately 0.24 W m−2 and cause statistically significant net warming in both polar regions during their respective winters. These results highlight the coupled roles of dust and soot in cloud ice formation, underscoring the need to assess the impacts of rising wildfire emissions on atmospheric ice processes and associated climate effects.