Improving Predictions of Indoor Aerosol Concentrations of Outdoor Origin by Considering the Phase Change of Semivolatile Material Driven by Temperature and Mass-Loading Gradients

Abstract

Outdoor aerosols experience environmental changes as they are transported indoors, including outdoor-toindoor temperature and mass-loading gradients, which can reduce or enhance their indoor concentrations due to repartitioning driven by changes in thermodynamic equilibrium states. However, the complexity required to model repartitioning typically hinders its inclusion in studies predicting indoor exposure to ambient aerosols. To facilitate exposure predictions, this work used an explicit thermodynamic indoor aerosol model to simulate outdoor-toindoor aerosol repartitioning typical for residential and office buildings across the 16 U.S. climate zones over an annual time horizon. Results demonstrate that neglecting repartitioning when predicting indoor concentrations can produce errors of up to 80− 100% for hydrocarbon-like organic aerosol, 40−60% for total organic aerosol, 400% for ammonium nitrate, and 60% (typically 3 μg/ m 3 ) for the total PM 2.5 aerosol. Underpredictions were more likely for buildings in hotter than colder regions, and for residences than offices, since both cooler indoor air and more meaningful residential organic aerosol concentrations encourage condensation of semivolatile organics. Furthermore, a method for computing correction factors to more easily account for thermodynamic repartitioning is provided. Applying these correction factors to mechanical-only aerosol predictions significantly reduced errors to <0.5 μg/m 3 for the total indoor PM 2.5 while bypassing explicit thermodynamic simulations.

Temperature and indoor environments

“…Partitioning factor F i for residential settings as a function of indoor–outdoor temperature differences for (a) hydrocarbon‐like organic aerosol and (b) oxidized organic aerosol. Shaded area represents 95% confidence intervals, provided by Cummings and based on model results presented in Cummings et al 340 Red lines represent trends observed by Avery et al 341 …”

“…Indoor sources of SVOCs contribute mass as well, especially if indoor air is cooler than outdoor air. Therefore, models that only consider mechanical removal (e.g., filtration and deposition) of particles can over‐ or underpredict the mass concentration of particles and their components 337–340 . As an example of this effect, Cummings et al 340 predicted the concentration of various chemical components in the aerosol phase for different indoor–outdoor temperatures and other factors for typical aerosols across 16 different US climate zones.…”

“…Therefore, models that only consider mechanical removal (e.g., filtration and deposition) of particles can over‐ or underpredict the mass concentration of particles and their components 337–340 . As an example of this effect, Cummings et al 340 predicted the concentration of various chemical components in the aerosol phase for different indoor–outdoor temperatures and other factors for typical aerosols across 16 different US climate zones. They report a partitioning factor, F i , defined as the concentration of component i in the aerosol phase when considering thermodynamics and mechanical removal divided by the concentration of component i when only considering mechanical removal processes.…”

Phase state of organic aerosols may limit temperature-driven thermodynamic repartitioning following outdoor-to-indoor transport

“…Indoor OA models have also employed these assumptions. [9][10][11][12][13][14] Recent laboratory and atmospheric measurements have challenged the validity of this assumption with evidence of OA that possess either a semisolid or a glassy, amorphous solid phase state. [15][16][17] Subsequent modeling work has since estimated that the annual-average phase state of atmospheric SOA is most oen semisolid in the troposphere over mid-latitude continental regions.…”

Simulating indoor inorganic aerosols of outdoor origin with the inorganic aerosol thermodynamic equilibrium model ISORROPIA

“…Therefore, is only subject to physical losses indoors, while and may additionally experience thermodynamic transformations. Temperature and relative humidity (RH) gradients between ambient and indoor conditions drive the partitioning behavior of semi‐volatile inorganic aerosol (IA) and OA constituents through volatilization or condensation, impacting indoor composition 3,18 . Furthermore, changes between outdoor and indoor RH affect the water content of aerosols via condensation and uptake by hygroscopic components, which further perturbs equilibria with gas phase compounds.…”