Seasonal variation in aerosol composition and concentration upon transport from the outdoor to indoor environment.

Outdoor-originated aerosols are an important component impacting indoor air quality. Since outdoor aerosols vary over short (diurnal) and long (seasonal) timescales, we examined how the variation in outdoor aerosol concentration and composition impact indoor aerosol. Measurements of both indoor and outdoor aerosol composition in real time in an urban classroom in winter and summer seasons were performed using an aerosol mass spectrometer (AMS), aethalometer, and a suite of gas phase instruments. Factor analysis of the organic aerosol components identified three factors in common between seasons, including hydrocarbon-like, cooking, and oxidized organic aerosol (HOA, COA, and OOA). Since sulfate is non-volatile, we report a sulfate-normalized indoor-outdoor ratio (I/O)i/SO4 for measured aerosol i components, allowing us to estimate aerosol component-based effects of seasonal and other variations in ventilation and HVAC operation, indoor emission sources, and chemically-based loss processes between outdoor and indoor environments. These chemical loss processes are interpreted in terms of changes in temperature and relative humidity (RH) between environments, which fluctuate on a daily and seasonal basis. The degree to which any effect is observed depends on the particular outdoor aerosol population and the magnitude of temperature or RH change. In wintertime, when aerosols were warmed upon transport indoors and loss of volatile components is favored, median (I/O)i/SO4 values for nitrate, total organics, HOA, and BC were smaller (0.35, 1.00, 1.24, and 1.18, respectively) than summertime values (0.75, 1.17, 1.96, and 1.80). For COA and OOA, however, (I/O)i/SO4 values were higher in the winter than in summer. Calculated aerosol liquid water (ALW), which is a function of temperature and RH and the relative contribution of hygroscopic components, varied significantly by season. Summertime ALW indoors provides a medium for aqueous processing, which is necessary for some hydrophilic gas phase reaction products that are important to indoor air quality and occupant exposure. This work describes the linkages between seasonal variability in aerosol composition outdoors and the subsequent chemically-specific variation observed when that aerosol is brought indoors.