Human occupant contribution to secondary aerosol mass in the indoor environment

Abstract

Humans impact indoor air quality directly via emissions from skin, breath, or personal care products, and indirectly via reactions of oxidants with skin constituents, or with skin that has been shed. However, separating the influence of the many emissions and their oxidation products from the influence of outdoor-originated aerosols has been a challenge. Indoor and outdoor aerosols were alternatively sampled at 4 minute time resolution with a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) in a classroom with student occupants at regular intervals per university class schedule. Mass spectral analysis showed aerosol enhancements of oxidized and unoxidized hydrocarbon ion families during occupied periods, especially at ion fragments larger than m/z 100 and double bond equivalents consistent with squalene (C₃₀H₅₀) and its oxidized products from reaction with ozone, indicative of the secondary nature of the aerosol mass. Individual hydrocarbon mass fragments consistent with squalene fragmentation, including C₅H₉⁺, and C₆H₉⁺ were especially enhanced with room occupancy. Emissions of individual organic fragment ions were estimated using a model accounting for outdoor aerosols and air exchange. This showed occupancy related emissions at smaller fragments (C₃H₅⁺, C₄H₉⁺) that despite reflecting mostly outdoor-originated aerosols transported indoors, also show enhancements from occupant emissions indoors. Total emission of all fragments was 17.6 μg β⁻¹ h⁻¹ above unoccupied levels, translating to approximately 25% increase in organic aerosol mass concentration in the classroom during an occupied hour with a median occupied ozone loss (β). Human occupants, therefore, represent an additional mass burden of organic aerosol, especially in poorly ventilated or highly occupied indoor spaces.