Increased Radiative Forcing from Anthropogenic Ammonia Due to Participation in New Particle Formation

While around half of atmospheric cloud condensation nuclei (CCN) are formed via new particle formation (NPF), most climate models do not represent NPF mechanisms realistically. Focusing on inorganic ternary nucleation involving sulfuric acid, ammonia, and ions, we implemented a New Particle Formation parameterization into the UK Met Office Unified Model based on experimental results from the CERN CLOUD chamber. We evaluated global simulations of total particle number concentration, focusing in the USA . We also evaluate a CCN proxy of particles greater than 100nm in diameter (N100); these are critical to cloud formation processes and indirect effects on climate. This work provides insights into understanding the global variability of aerosol particle concentrations and the importance of recently developed nucleation parameterizations to cloud droplet number concentrations.

In the USA, the updated NPF parameterization causes a substantial increase in CCN proxy(N100) concentrations, particularly in the Northern US (around 25% on an annual average). The concentrations are highly spatial heterogeneous, suggesting a complex interplay of agricultural emissions and chemical processes across northern US regions. Globally, East Asia has the highest N100 concentrations (about 10⁴ cm⁻³) due to intense industrial activity and urban emissions, but there is no evident contribution of inorganic ternary nucleation to large particles. However, North America and Europe display large increases in N100 concentrations when ternary nucleation is included.

We calculate aerosol indirect radiative forcing by comparing simulations nudged to reanalysis with present-day and pre-industrial aerosol concentrations, with and without the new mechanism included. We use these simulations to estimate the contribution of ammonia to anthropogenic radiative forcing via its participation in NPF. With additional simulations that do not include ammonia, we are also able to evaluate and update the overall contribution of ammonium aerosol to anthropogenic aerosol indirect radiative forcing.