Model algorithm development: clouds

We have developed new fundamental methods for atmospheric chemistry models, principally for cloud chemistry.

In partly cloudy environments, net reaction rates depend on both physical entrainment and reactive chemical uptake, which can be accounted for in a simple kinetic rate expression.

Aqueous reactions in clouds play an important role in atmospheric chemistry, production of acid rain from SO2┬ábeing a prominent example. Rapid heterogeneous (surface and multiphase) reactions can consume reactants within clouds, making the overall reaction rate dependent on entrainment to supply additional reactants from the surrounding air. Since clouds are sub-grid-scale features in many large-scale regional and global atmospheric models, accounting for these processes in chemical transport models is challenging. To address these challenges, Holmes et al. (2019) and Holmes (2022) introduced “entrainment-limited kinetics”, an algorithm to accurately and efficiently account for cloud chemistry occurring in just a fraction of a grid cell. The method incorporates cloud fraction and entrainment into the kinetic rate expression, enabling calculation of concentrations in a partly cloudy model grid cell with very little computational effort. Our new algorithm has subsequently led to improved understanding of how clouds affect the global budgets of several important atmospheric gases, including the following…