Indoor air purifiers are advertised as safe household products for health-conscious individuals, especially for those suffering from allergies and asthma. However, certain air purifiers produce ozone (O 3 ) during operation, either intentionally or as a byproduct of air ionization. This is a serious concern, because O 3 is a criteria air pollutant regulated by health-related federal and state standards. Several types of air purifiers were tested for their ability to produce ozone in various indoor environments at 40 -50% relative humidity, including office rooms, bathrooms, bedrooms, and cars. O 3 levels generated by personal wearable air purifiers were also tested. In many cases, O 3 concentrations were well in excess of public and/or industrial safety levels established by U.S. Environmental Protection Agency, California Air Resources Board, and Occupational Safety and Health Administration. Simple kinetic equations were obtained that can predict the steady-state level of O 3 in a room from the O 3 emission rate of the air purifier and the first-order decay rate of O 3 in the room. The additivity of O 3 levels generated by independent O 3 generators was experimentally demonstrated.

Several complementary experimental and theoretical methodologies were used to explore water uptake on sodium chloride (NaCl) particles containing varying amounts of sodium dodecyl sulfate (SDS) to elucidate the interaction of water with well-defined, environmentally relevant surfaces. Experiments probed the hygroscopic growth of mixed SDS/NaCl nanoparticles that were generated by electrospraying aqueous 2 g/L solutions containing SDS and NaCl with relative NaCl/SDS weight fractions of 0, 5, 11, 23, or 50 wt/wt %. Particles with mobility-equivalent diameters of 14.0(±0.2) nm were size selected and their hygroscopic growth was monitored by a tandem nano-differential mobility analyzer as a function of relative humidity (RH). Nanoparticles generated from 0 and 5 wt/wt % solutions deliquesced abruptly at 79.1(±1.0)% RH. Both of these nanoparticle compositions had 3.1(±0.5) monolayers of adsorbed surface water prior to deliquescing and showed good agreement with the Brunauer−Emmett−Teller and the Frenkel−Halsey−Hill isotherms. Above the deliquescence point, the growth curves could be qualitatively described by Köhler theory after appropriately accounting for the effect of the particle shape on mobility. The SDS/NaCl nanoparticles with larger SDS fractions displayed gradual deliquescence at a RH that was significantly lower than 79.1%. All compositions of SDS/NaCl nanoparticles had monotonically suppressed mobility growth factors (GFm) with increasing fractions of SDS in the electrosprayed solutions. The Zdanovskii−Stokes−Robinson model was used to estimate the actual fractions of SDS and NaCl in the nanoparticles; it suggested the nanoparticles were enhanced in SDS relative to their electrospray solution concentrations. X-ray photoelectron spectroscopy (XPS), FTIR, and AFM were consistent with SDS forming first a monolayer and then a crystalline phase around the NaCl core. Molecular dynamics simulations of water vapor interacting with SDS/NaCl slabs showed that SDS kinetically hinders the initial water uptake. Large binding energies of sodium methyl sulfate (SMS)−(NaCl)4, H2O−(NaCl)4, and SMS−H2O−(NaCl)4 calculated at the MP2/cc-pVDZ level suggested that placing H2O in between NaCl and surfactant headgroup is energetically favorable. These results provide a comprehensive description of SDS/NaCl nanoparticles and their properties. Disciplines Chemistry CommentsReprinted (adapted) ReceiVed: October 8, 2009; ReVised Manuscript ReceiVed: January 5, 2010 Several complementary experimental and theoretical methodologies were used to explore water uptake on sodium chloride (NaCl) particles containing varying amounts of sodium dodecyl sulfate (SDS) to elucidate the interaction of water with well-defined, environmentally relevant surfaces. Experiments probed the hygroscopic growth of mixed SDS/NaCl nanoparticles that were generated by electrospraying aqueous 2 g/L solutions containing SDS and NaCl with relative NaCl/SDS weight fractions of 0, 5, 11, 23, or 50 wt/wt %. Particles with mobility-equivalent diameters of ...

Aerosolized nanoparticles of NaCl coated with variable amounts of surfactant AOT were generated by electrospraying AOT/NaCl aqueous solutions, followed by neutralizing and drying the resulting particles. A tandem differential mobility analyzer was used to select a narrow size distribution of particles with mobility equivalent diameters below 20 nm and monitor their hygroscopic growth as a function of relative humidity. Effects of the particle size and relative amount of surfactant on the hygroscopic growth of NaCl were studied. For pure NaCl nanoparticles, the deliquescence relative humidity (DRH) increased as the particle size was decreased, in full agreement with previous measurements. Below the DRH the NaCl nanoparticles had an equivalent of one-four monolayers of water adsorbed on the surface. The addition of a sub-monolayer AOT coating reduced the DRH and suppressed the hygroscopic growth of the NaCl core. At AOT coverage levels exceeding one monolayer, a clear deliquescence transition was no longer discernible. The Zdanovskii-StokesRobinson (ZSR) model failed to predict the observed growth factors of mixed AOT/NaCl nanoparticles reflecting a large contribution of the interfacial interactions between NaCl and AOT to the total free energy of the particles. There were indications that AOT/NaCl nanoparticles prepared by the electrospray aerosol source were enhanced in the relative mass fraction of AOT in comparison with the solution from which they were electrosprayed.

Ionization air purifiers are increasingly used to remove aerosol particles from indoor air. However, certain ionization air purifiers also emit ozone. Reactions between the emitted ozone and unsaturated volatile organic compounds (VOC) commonly found in indoor air produce additional respirable aerosol particles in the ultrafine (<0.1 microm) and fine (<2.5 microm) size domains. A simple kinetic model is used to analyze the competition between the removal and generation of particulate matter by ionization air purifiers under conditions of a typical residential building. This model predicts that certain widely used ionization air purifiers may actually increase the mass concentration of fine and ultrafine particulates in the presence of common unsaturated VOC, such as limonene contained in many household cleaning products. This prediction is supported by an explicit observation of ultrafine particle nucleation events caused by the addition of D-limonene to a ventilated office room equipped with a common ionization air purifier.