Indoor aerosol water content and phase state in U.S. residences: Impacts of relative humidity, aerosol mass and composition, and mechanical system operation

Hygroscopic particulate matter (PM) constituents promote uptake of aerosol water (AW), depending on relative humidity (RH), which can constrain qualities such as organic aerosol (OA) phase state and inorganic aerosol (IA) deliquescence and efflorescence. This work provides a first incorporation of AW predictions into residential indoor PM simulations. The indoor model, IMAGES, which simulates factored OA concentrations and thermodynamics using the 2D-volatility basis set, was expanded to predict speciated IA concentrations, AW with κ-Kohler theory of hygroscopic growth, and OA phase state with glass transition temperatures. Since RH is the largest driver of AW and varies with meteorology, simulations were conducted using a database of historical ambient weather and pollution records spanning the sixteen U.S. climate zones, facilitating assessment of seasonal and regional trends. Over this diverse simulation set, the residential indoor AW mass was ∼10 to 100 times smaller than dry PM mass. This relative AW amount indoors was about ∼10 times smaller than outdoors, since indoor-emitted aerosol is likely less hygroscopic. The indoor OA phase state was typically semisolid, suggesting kinetic limitations might inhibit thermodynamic OA partitioning equilibrium from being established indoors. Residences in hot and humid climates during the summertime may have liquid indoor OA, while amorphous solid indoor OA can exist in cold climates. Deliquescence and efflorescence of recirculated IA within HVAC systems during cooling or heating, respectively, was also modeled. Oftentimes, two IA populations with different histories existing as wet or dry aerosol were generated by HVAC operation depending on indoor and outdoor environmental conditions and the HVAC operating mode.