The House Observations of Microbial and Environmental Chemistry (HOMEChem) study was a large-scale collaborative experimental investigation probing indoor air composition and chemistry.

Human health is affected by indoor air quality. One distinctive aspect of the indoor environment is its very large surface area that acts as a poorly characterized sink and source of gas-phase chemicals. In this work, air-surface interactions of 19 common indoor air contaminants with diverse properties and sources were monitored in a house using fast-response, on-line mass spectrometric and spectroscopic methods. Enhanced-ventilation experiments demonstrate that most of the contaminants reside in the surface reservoirs and not, as expected, in the gas phase. They participate in rapid air-surface partitioning that is much faster than air exchange. Phase distribution calculations are consistent with the observations when assuming simultaneous equilibria between air and large weakly polar and polar absorptive surface reservoirs, with acid-base dissociation in the polar reservoir. Chemical exposure assessments must account for the finding that contaminants that are fully volatile under outdoor air conditions instead behave as semivolatile compounds indoors.

We report elevated levels of gaseous inorganic chlorinated and nitrogenated compounds in indoor air while cleaning with a commercial bleach solution during the House Observations of Microbial and Environmental Chemistry field campaign in summer 2018. Hypochlorous acid (HOCl), chlorine (Cl2), and nitryl chloride (ClNO2) reached part-per-billion by volume levels indoors during bleach cleaningseveral orders of magnitude higher than typically measured in the outdoor atmosphere. Kinetic modeling revealed that multiphase chemistry plays a central role in controlling indoor chlorine and reactive nitrogen chemistry during these periods. Cl2 production occurred via heterogeneous reactions of HOCl on indoor surfaces. ClNO2 and chloramine (NH2Cl, NHCl2, NCl3) production occurred in the applied bleach via aqueous reactions involving nitrite (NO2 –) and ammonia (NH3), respectively. Aqueous-phase and surface chemistry resulted in elevated levels of gas-phase nitrogen dioxide (NO2). We predict hydroxyl (OH) and chlorine (Cl) radical production during these periods (106 and 107 molecules cm–3 s–1, respectively) driven by HOCl and Cl2 photolysis. Ventilation and photolysis accounted for <50% and <0.1% total loss of bleach-related compounds from indoor air, respectively; we conclude that uptake to indoor surfaces is an important additional loss process. Indoor HOCl and nitrogen trichloride (NCl3) mixing ratios during bleach cleaning reported herein are likely detrimental to human health.

Bleach can oxidize volatile organic compounds (VOCs) and contribute to secondary organic aerosol (SOA) indoors. During the House Observations of Microbial and Environmental Chemistry (HOMEChem) campaign, we observed indoor terpene mixing ratios decrease during bleach cleaning periods, with simultaneous increases in oxidized VOC (OVOC) products. Cooking just prior to bleach cleaning significantly increased SOA due to uptake of bleach-related OVOCs onto cooking aerosols. While SOA formation occurred, it was small (<3%) relative to total organic aerosol mass concentrations. Bleach cleaning chemistry also produced several potentially toxic chlorinated and nitrogenated VOCs indoors, including isocyanates, cyanogen chloride, and chlorocarbons. Observed volatile chlorinated organic acids were likely impurities from the bleach. The bleach-induced terpene oxidation, SOA formation, and chlorinated/nitrogenated VOC production were independent of indoor illumination, consistent with dark chemical production. These observations add to previous studies that demonstrate bleach as a source of potentially harmful primary and secondary pollutants to indoor air.

With an ongoing interest in displacing petroleum-based sources of energy with biofuels, we measure and model the formation and composition of secondary organic aerosol (SOA) from organic compounds present in biofuels.