Decay phase aerosol dynamics of an indoor aerosol source has a significant role in exposure analysis

Abstract. Indoor particle sources have been recognized as major contributors to aerosol particle exposure posing a health risk particularly to people spending much of their time indoors. Previously, most of the studies examining indoor particle sources have been focusing on active periods of the sources instead of the decay phase of the emitted particle concentration. This gives motivation for this study to investigate the decay of particle lung-deposited surface area (LDSA) concentrations following indoor particle emissions, with a focus on cooking activities. Two decay functions were derived to describe these processes. The first function considers ventilation, particle deposition onto surfaces and a stable background particle source, whereas the second function also includes coagulation. These functions were validated using measurements that covered four dwellings equipped with mechanical ventilation systems. Both decay functions accurately fit the measured data, with the more comprehensive function, including coagulation, consistently achieving lower fitting errors, particularly at high LDSA concentrations. Using urban air quality data of LDSA concentrations from the city of Tampere, the decay functions were further applied to estimate the contribution of cooking to the daily LDSA dose. The cooking-related fraction of exposure varied widely, from 17.2 % to 93.9 %, reflecting the influence of cooking styles and ventilation systems. Crucially, from 66.5 to 80.3 % of the cooking-related LDSA dose, using the simpler decay function, and from 72.9 to 82.9 %, using the coagulation-inclusive function, occurred during the decay phase after active cooking. The findings highlight the importance of considering the post-cooking decay phase in total exposure assessments and demonstrate the utility of these functions for interpolating or extrapolating LDSA data. The decay functions derived in this study can be applied to describe other indoor particle sources, distinguish emissions of successive indoor emission events and investigate factors affecting the decay process, such as ventilation.