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

Abstract. Atmospheric aerosols can serve as ice-nucleating particles (INPs), influencing the formation and properties of cirrus clouds. While mineral dust has long been considered an effective INP, the role of soot particles remains less explored, limiting our ability to assess their climate impact. Here we use cloud parcel model simulations to examine the competitive ice nucleation behavior of soot and dust, alongside homogeneous nucleation, under a range of meteorological conditions. These simulations provide process-level insights into how soot and dust influence cirrus cloud ice formation. To evaluate their large-scale implications, we integrate these results into the GFDL AM4-MG2 global climate model. We find that on the global scale, soot (represented in the model as black carbon, BC) enhance ice crystal number concentration by ~5 %. However, regional increases are much larger – up to 90 % in the upper troposphere (500–250 hPa). The strongest enhancements are observed during boreal spring across Eurasia and the Maritime Continent, and during austral spring over South America and the South Atlantic. The radiative impacts of BC INPs are also substantial. They enhance the annual global cloud radiative effect in the longwave spectrum by approximately 0.24 W m^-2 and contribute to statistically significant net warming effect during the polar winter in both hemispheres. These results highlight the distinct roles of soot and dust in cloud ice formation and underscore the need to assess the impacts of rising wildfire emissions on atmospheric ice processes and associated climate effects.