We report measurements of the size, concentration, and hygroscopicity of ultrafine particles (UFPs) emitted during thermal spraying of ceramic coatings in an industrial setting. High concentrations (i.e., higher than 10⁶ cm⁻³) of fractal-like UFPs were measured inside the spraying booths of the facility. The emitted UFPs were found to take up small amounts of water when exposed to elevated relative humidity (RH = 87%) within a Hygroscopic Tandem Differential Mobility Analyzer (HTDMA) system. The hygroscopicity of the sampled particles was distinguishably lower compared to those of the atmospheric background aerosol particles present in the breathing air. UFPs smaller than 90 nm that are produced by the thermal spraying process, exhibit hygroscopic factors less than unity in a systematic way. This behavior indicates that the particles were irregularly shaped at dry conditions, and that they underwent a shape change (i.e., restructuring) upon humidification inside the HTDMA. The fractal-like structure of process-emitted UFPs was further corroborated by Transmission Electron Microscopy (TEM) conducted on samples collected at dry conditions on site.

Household air pollution is ranked the 9^th largest Global Burden of Disease risk (Forouzanfar et al., The Lancet 2015). People, particularly urban dwellers, typically spend over 90% of their daily time indoors, where levels of air pollution often surpass those of outdoor environments. Indoor air quality (IAQ) standards and approaches for assessment and control of indoor air require measurements of pollutant concentrations and thermal comfort using conventional instruments. However, the outcomes of such measurements are usually averages over long integrated time periods, which become available after the exposure has already occurred. Moreover, conventional monitoring is generally incapable of addressing temporal and spatial heterogeneity of indoor air pollution, or providing information on peak exposures that occur when specific indoor sources are in operation. This article provides a review of new air pollution sensing methods to determine IAQ and discusses how real-time sensing could bring a paradigm shift in controlling the concentration of key air pollutants in billions of urban houses worldwide. We also show that besides the opportunities, challenges still remain in terms of maturing technologies, or data mining and their interpretation. Moreover, we discuss further research and essential development needed to close gaps between what is available today and needed tomorrow. In particular, we demonstrate that awareness of IAQ risks and availability of appropriate regulation are lagging behind the technologies.