The development and evaluation of an air handling unit (AHU) module that simulates temperature and humidity dependent organic and inorganic gas-particle repartitioning.

Aerosol constituents transform as they travel through the air handling unit (AHU) due to the air being filtered, heated, and cooled. Though the strength of these processes has direct indoor air quality implications, comprehensive model descriptions of how commercial heating, ventilating, and air conditioning (HVAC) systems affect indoor aerosol composition are lacking. Herein, an AHU module was designed to augment the Indoor Model of Aerosols, Gases, Emissions, and Surfaces (IMAGES) framework, which is a modeling platform that simulates indoor aerosols by incorporating the two-dimensional volatility basis set (2D-VBS) for organic aerosol and ISORROPIA for inorganic aerosol. This AHU module simulates the organic and inorganic concentrations as air travels from the mixing box, through the filter, through the heating and cooling coils, and into the supply duct. It accounts for essential processes inside the AHU, such as particle deposition to the filter and heat exchangers, gas loss to the water condensed on the cooling coil during operation, and temperature-driven repartitioning of organic and inorganic species by using thermodynamic frameworks in the 2D-VBS and ISORROPIA. The module's performance was assessed with measurements taken at various stages of an HVAC system at Johns Hopkins University when periodic cooling occurred. Modeled concentrations post filter and heating coil were slightly over-predicted, which caused over-predictions in the supply duct. However, the module performed well when the processes of aerosols flowing over the cooling coil were isolated. Still, consistent with our previous work that applied ISORROPIA to an indoor setting, a missing condensation sink within ISORROPIA must be accounted for to obtain more accurate inorganic partitioning. Thus, HVAC system impacts on aerosol composition may now be better considered in detailed indoor aerosol models, such as IMAGES.