Household humidification is widely practiced to combat dry indoor air. While the benefits of household humidification are widely perceived, its implications to the indoor air have not been critically appraised. In particular, ultrasonic humidifiers are known to generate fine particulate matter (PM). In this study, we first conducted laboratory experiments to investigate the size, quantity, and chemical composition of PM generated by an ultrasonic humidifier. The mass of PM generated showed a correlation with the total alkalinity of charge water, suggesting that CaCO 3 is likely making a major contribution to PM. Ion chromatography analysis revealed a large amount of SO 4 2− in PM, representing a previously unrecognized indoor source. Preliminary results of organic compounds being present in humidifier PM are also presented. A whole-house experiment was further conducted at an actual residential house, with five low-cost sensors (AirBeam) monitoring PM in real time. Operation of a single ultrasonic humidifier resulted in PM 2.5 concentrations up to hundreds of μg m −3 , and its influence extended across the entire household. The transport and loss of PM 2.5 depended on the rate of air circulation and ventilation. This study emphasizes the need to further investigate the impact of humidifier operation, both on human health and on the indoor atmospheric chemistry, for example, partitioning of acidic and basic compounds.

Atmospheric aqueous phases (cloud and fog droplets, aerosol liquid water) are important reaction media for the processing of water-soluble organic acids (OAs). The photochemistry of these species is known to...

Biomass burning is a major source of short-lived climate forcers, e.g., brown carbon (BrC), that absorb solar radiation and cause warming of the Earth’s atmosphere. A large fraction of the...

Peroxides (H 2 O 2 , ROOR, and ROOH) are an important reaction intermediate involved in a number of natural processes, including atmospheric autoxidation and lipid peroxidation in oils and animal tissues. Iodometry is an established spectroscopic technique that has been widely used to quantify total peroxide concentration in food, indoor, and outdoor samples. Iodometry provides selectivity toward peroxides through a quantitative reaction between I − and peroxides to form I 3 − via a molecular iodine (I 2 ) intermediate. However, equilibrium changes caused by a potential interaction between olefinic species and I 2 can suppress I 3 − formation, thereby underestimating peroxide concentration. For the first time in the current study, this unrecognized interference posed by olefins (OEs) is systematically investigated to gauge its effects on the accuracy of iodometry. A number of model molecules were investigated. The interference was observed to be unique to OEs, but universally affecting different peroxide species such as H 2 O 2 , tert-butyl hydroperoxide, and aerosol-bound peroxides. A simple kinetic box model was built to explain this chemistry. The measured rate constant for 3-octenoic acid was found to be 0.84 ± 0.02 M −1 s −1 . Overall, our results show matrix effects induced by OEs can underestimate peroxide concentration determined by iodometry for edible oils, indoor environments, and animal fat, but absent in most of the atmospheric samples. Nonetheless, our results point out the importance of this interfering chemistry in matrices enriched with OEs.

Autoxidation is a widely recognized mechanism known to initiate the degradation of food and lipids and modify organic matter in the atmosphere. Given the low NOx concentration in aqueous media (e.g., cloud water and fog droplets), autoxidation can become vital to facilitate the formation of highly oxygenated molecules such as organic peroxides (ROOH and ROOR). Here, we have identified aqueous-phase autoxidation-initiated hydroperoxides in varying organic precursors, including a laboratory model compound and monoterpene oxidation products. Our results show that autoxidation-initiated ROOHs are suppressed at enhanced precursor and oxidant concentrations. Furthermore, we observed an exponential increase in the yield of ROOHs when UV light with longer wavelengths was used in the experiment, comparing UVA, UVB, and UVC. Water-soluble organic compounds represent a significant fraction of ambient cloud water component (up to 500 µM. Thus, aqueous-phase autoxidation can become an important oxidation pathway for water-soluble species and as such facilitate the formation of ROOHs, thereby adding to the climate and health burden of atmospheric particulate matter.