Assessing Indoor Emissions and the Composition-Dependent Impact of Outdoor Aerosols on the Indoor Environment in an Urban Classroom
Authors:   , Avery Anita Marie , DeCarlo Peter F. 
Published: July 2021

Aerosols transported to the indoor environment from outdoors undergo changes to their physical and chemical properties, dependent on the original aerosol composition and conditions in each environment. Volatilization or condensation of aerosol components can have a significant impact on the concentration and composition of indoor aerosol. Once indoors, outdoor-originated aerosols interact with indoor-originated aerosols and gases, simply mixing or reacting. Studies conducted in a Drexel University laboratory and classroom space in spring, summer, and winter, measured real-time aerosol composition of both indoor and outdoor air using an Aerodyne aerosol mass spectrometer (AMS) and gas phase measurements of CO, CO₂, CH4, and O3 to assess the impact of outdoor aerosols, third hand smoke, and occupants on the indoor air quality. This study showed that physiochemical properties between species resulted in distinct differences in the indoor-outdoor (I/O) ratio based on the properties of each component, and the conditions (temperature, humidity) in each environment. The organic matrix was further analyzed by positive matrix factorization (PMF). To directly compare between seasons and sampling conditions including air exchange, a sulfate-normalization of the indoor-outdoor ratio (I/O)i/SO4 was used. Volatilization or condensation of semi-volatile aerosol components was quantified using the (I/O)i/SO4 ratio as a function of temperature and humidity gradients. Volatile components, including nitrate, showed the strongest correlation with temperature and humidity gradients. Seasonally-specific trends in each environment were analyzed in detail, including wintertime gas-phase plumes of CO, CO₂, and CH4. Novel evidence of third hand smoke (THS) was observed indoors in summertime experiments. The PMF factor associated with THS was not seen in the outdoor aerosol dataset and contributed 23% of the total submicron aerosol loading indoors. The THS concentration was dependent on the total concentration of (non-THS) components, indicating continuous partitioning behavior from deposited smoke. Furthermore, the THS was related to hygroscopic components, and was only observed when aerosol liquid water (ALW) was predicted (i.e. summer, but not winter indoors). A mechanism for reactive uptake of cigarette-smoke related reduced nitrogen species explains this unexpected indoor emission. A follow-up experiment with deposited smoke confirmed the spectral signatures and partitioning behavior of THS. Impacts of occupancy on submicron aerosol composition were investigated using a CO₂- based categorization of the classroom over time, of occupied and unoccupied conditions. Occupants contributed to loss of ozone indoors, and the products of ozone with squalene, and ozone with other human skin components were investigated as fragment families and as individual fragments. Hydrocarbon fragments were enhanced during occupied by 42%, and the emission of individual fragments were calculated as the solution to a modeled indoor equation, taking into account measured outdoor concentrations and air exchange rates. The total emission was calculated as 6.9 [mu]g [beta]^(-1) h^(-1), or about 25% increase from the average organic mass concentration in an occupied hour. The mass spectrum of emission rates is similar to that of the cooking PMF factor, indicative of the combination of oils and fatty acids that are ubiquitous in both cooking oils and skin oils.