Multiphase chemical processes are important components of the atmospheric system, with significant but complex effects on air quality and Earth’s climate. The goal of the ACRoNNIM project is to investigate the effect of interactions between gas-phase ammonia and organic molecules within aerosol particles on aerosol mass, composition, and optical properties. Due to the climate, health and visibility effects of particulate matter, inclusion of this multiphase chemistry into regional and global weather and climate models is timely. Complicating these efforts are the large number of species and oxidation pathways involved, their seasonally, spatially, and diurnally varying importance, and the complex physical processes controlling their phase partitioning. Regional and global chemical transport models (CTMs) generally under-predict organic aerosol mass in the atmosphere compared to field measurements, suggesting the presence of as-yet unidentified sources or unaccounted for physical processes.
A common approach to treating multiphase chemistry in large-scale atmospheric models is to split individual process across a collection of ‘modules’ that treat, for example, gas-phase chemistry, the partitioning of inorganic species and acid–base chemistry, the partitioning of organic species to the condensed phase, aqueous chemistry in cloud droplets, etc. Progress towards the goal of investigating the multiphase NH3–organics system using results from a recently completed field campaign and a mechanism developed in collaboration with researchers performing laboratory experiments on this system required a rethinking of this approach to treating multiphase chemical systems in atmospheric models. This effort led to the development of the Chemistry Across Multiple Phases (CAMP) framework, which answers several key questions in atmospheric modelling, including:
• How do you describe multiphase atmospheric chemical systems in code in a way that is independent of how a host model treats aerosol systems (e.g. size bins, modes, single particles, etc.)?
• How do you allow a multiphase chemical system to be solved as a single system, thus avoiding artifacts related to operator splitting?
• How do you facilitate the rapid transfer of multiphase chemical knowledge from laboratory results to atmospheric models?
CAMP makes significant progress towards answering these questions in an innovative way.
