The Role of Indoor Surface pH in Controlling the Fate of Acids and Bases in an Unoccupied Residence

The chemical composition of indoor air is strongly driven by the composition and properties of indoor surfaces. At the Chemical Assessments of Surfaces and Air (CASA) campaign, we performed controlled additions of ammonia (reaching up to 297 ppb to 662 ppb) to investigate the impacts of changing surface basicity on the fate of gaseous and particulate acids and bases in an unoccupied house. In response to ammonia injections, nitrogen-containing compounds (C_2–7H_3–11N₁O_0–3) were emitted from surfaces to the gas phase with signals increasing 10¹% to 10⁴% compared to their signals prior to ammonia addition. At the same time, oxygen-containing compounds (C_1–7H_2–6O_2–3) were removed from the gas phase by indoor surface partitioning. Indoor surface pH and aerosol pH likely increased during these controlled ammonia injections relative to their baseline conditions. We estimate indoor surface pH to be nearly 5 and indoor aerosol pH to range from 2 to 4 during this experiment. At each ammonia injection, we observed ammonium and nitrate concentrations in the aerosol phase to increase due to gas-particle partitioning of ammonia and nitric acid. This gas-particle-surface exchange showed strong dependence on relative humidity; evaporation of gaseous bases was more pronounced at lower relative humidity when surface-associated water volume was reduced, while gas-to-particle partitioning of inorganic species was greater in the presence of more aerosol liquid water at higher relative humidity. From cooking experiments, which represent realistic sources of acids and bases to the indoor environment but which emit 10 times less ammonia than was introduced to the house via pure ammonia injection experiments, we predict that surfaces may still be important sources of these basic gases to indoor air.