The Impact of Stratospheric Aerosol Injection on Cirrus Clouds: The Overlooked Role of Ammonia in Sulfuric Acid Aerosol Neutralization

PI: Xiaohan Sally Li (UChicago)
Co-Investigators: Vaishali Naik (NOAA Geophysical Fluid Dynamics Laboratory) Mingyi Wang (UChicago)
Collaborators: Daniel Cziczo (Purdue University), Songmiao Fan (NOAA Geophysical Fluid Dynamics Laboratory), Minghuai Wang (Nanjing University), Fabien Paulot (NOAA Geophysical Fluid Dynamics Laboratory), Paul Ginoux (NOAA Geophysical Fluid Dynamics Laboratory)

Cirrus clouds are thin, high-altitude ice clouds that cover approximately 30% of the midlatitudes and up to 80% of the tropics. These clouds have a net warming effect on the Earth’s climate, and their properties may be substantially modified by stratospheric aerosol injection (SAI)—a proposed method of sunlight reflection.

However, current assessments of SAI’s impact on cirrus clouds rely on the understudied assumption that injected sulfuric acid aerosol particles only form ice through “homogenous nucleation,” a process that requires very cold and humid conditions.

This project challenges that assumption. As sulfuric acid particles settle into the upper troposphere or lower stratosphere, they can react with ammonia in the atmosphere to form ice-nucleating particles such as ammonium sulfate. These new particles are more effective at forming ice and can trigger cirrus clouds at warmer temperatures and lower humidity. This suggests SAI’s effects on cirrus may be more regionally variable than previously thought, including possible “hot spots” in relatively clean but ammonia-influenced regions such as agriculturally active areas of South America.

The research team will use chemical and cloud-scale models to study how these particle transformations occur and how efficiently they form ice. They will then estimate the global impact of this process on cirrus formation and contribute essential physical understanding needed for the evaluation of precipitation and hydrological cycle responses to SAI.