Students spend at least 8 hours in the office every day. The indoor air quality is tightly related to their health. To further understand the evolution of fine particulate matter in typical student office, the time-of-flight aerosol chemical speciation monitor was deployed in an academic office located at the tower branch of the Institute of Atmospheric Physics to measure the concentration and chemical composition of indoor PM_2.5 for one month. The sources of indoor organic aerosol (OA) are elucidated with positive matrix factorization (PMF). Also, the open-close window experiments and the dampness experiments were performed to figure out the mechanism of indoor/outdoor air exchange, the impact of enhanced ventilation on the time-dependent relationship between indoor and outdoor pollutant levels and the influence of increased humidity on the indoor PM_2.5. Results showed that the indoor and outdoor variation trends are similar for most of aerosol species, indicating that these species were primarily from outdoor air exchange. However, the chemical composition of PM_2.5 between indoor and outdoor environments are found to be different. Since the indoor temperature is significantly higher than that outdoors in winter, the fossil fuel OA and ammonium nitrate will experience evaporation loss or particle-to-gas partitioning upon transport indoors due to the warm of aerosol. Cooking OA might have indoor sources and less oxidized SOA underwent secondary formation (I/O > 1). It was found from the “open-close” windows experiment that the concentrations of indoor aerosol species were consistently lower than outdoors even when the windows were opened, illustrating that the brief windows switch could not allow for a complete exchange of indoor and outdoor air. On the contrary, with PM mass concentration outdoors than in the office, elevated natural ventilation increased PM exposure indoors and this increased exposure might be prolonged when outdoor PM got cleared up. The cleaning activity or use of air humidifier can lead to an increase in indoor humidity. When dampness occurs indoors, aqueous chemistry could alter the chemical composition of indoor air and affect exposure by acting as a sink for certain water-soluble gases in indoor air and a source of other volatile and condensed products. Our results showed that the increase of indoor RH will lead to a significant increase in PM_2.5 mass concentration, especially OA concentration. This might be attributed to the hygroscopic components in organics or water-soluble organic acids in indoor air that partitioned from gas phase to particle phase when RH rose in the office.

aerosol composition,dampness,indoor pollutants,indoor/outdoor exchange,organic aerosol