Indoor ozone/human chemistry and ventilation strategies

Abstract

This study aimed to better understand and quantify the influence of ventilation strategies on occupant-related indoor air chemistry. The oxidation of human skin oil constituents was studied in a continuously ventilated climate chamber at two air exchange rates (1 h^-1 and 3 h^-1 ) and two initial ozone mixing ratios (30 and 60 ppb). Additional measurements were performed to investigate the effect of intermittent ventilation ("off" followed by "on"). Soiled t-shirts were used to simulate the presence of occupants. A time-of-flight-chemical ionization mass spectrometer (ToF-CIMS) in positive mode using protonated water clusters was used to measure the oxygenated reaction products geranyl acetone, 6-methyl-5-hepten-2-one (6-MHO) and 4-oxopentanal (4-OPA). The measurement data were used in a series of mass balance models accounting for formation and removal processes. Reactions of ozone with squalene occurring on the surface of the t-shirts are mass transport limited; ventilation rate has only a small effect on this surface chemistry. Ozone-squalene reactions on the t-shirts produced gas-phase geranyl acetone, which was subsequently removed almost equally by ventilation and further reaction with ozone. About 70% of gas-phase 6-MHO was produced in surface reactions on the t-shirts, the remainder in secondary gas-phase reactions of ozone with geranyl acetone. 6-MHO was primarily removed by ventilation, while further reaction with ozone was responsible for about a third of its removal. 4-OPA was formed primarily on the surfaces of the shirts (~60%); gas-phase reactions of ozone with geranyl acetone and 6-MHO accounted for ~30% and ~10%, respectively. 4-OPA was removed entirely by ventilation. The results from the intermittent ventilation scenarios showed delayed formation of the reaction products and lower product concentrations compared to continuous ventilation.

Keywords: ToF-CIMS; air exchange rate; indoor environment; oxygenated volatile organic compounds; ozone; squalene.

Figure 1

Visualization of the formation and loss of geranyl acetone, 6‐MHO, and 4‐OPA (f – branching ratio; 1°, 2°, 3° – primary, secondary, tertiary reaction; λ – air exchange rate)

Figure 2

Time profiles of ozone concentration during continuous ventilation experiments (Conditions 1, 2, 3, 5, 6, and 7). T‐shirts were placed in the chamber at t = 0

Figure 3

Time profiles of ozone concentration under conditions with intermittent ventilation (Conditions 8 (t = 0 h) and 9 (t = −2 h)). Data from the corresponding continuous ventilation experiment (t = −16, Conditions 4 and 7, 60 ppb ozone, 1 h⁻¹ AER) are included for reference. T‐shirts were placed in the chamber at t = 0

Figure 4

Time series of geranyl acetone, 6‐MHO, and 4‐OPA for different target ozone concentrations and AERs during experiments with continuous ventilation. The gap in the time series is due to thermal desorption of aerosols and the subsequent memory effect

Figure 5

Time series of geranyl acetone, 6‐MHO, and 4‐OPA during experiments with intermittent ventilation (Conditions 8 and 9). Condition 7 (continuous ventilation) is shown for comparison. Time (t) is the time when ozone generation began relative to when the t‐shirts were placed in the chamber. Note that the concentrations are not corrected for background mixing ratios in the chamber. Background measurements were unavailable prior to Conditions 8 and 9 due to the nature of the experiments (intermittent ventilation: reduced AER and no ozone generation overnight)