An Aerosol Flame Photometer to Study Indoor Aerosol Dispersion

JONATHAN SYMONDS, Julie Pongetti, Kristian Hoffman, Chris Nickolaus, Cambustion Ltd

     Abstract Number: 159
     Working Group: Indoor Aerosols

Abstract
The COVID-19 pandemic has underlined the need for good indoor ventilation. Yet many studies of its efficacy rely upon gas tracing even though the transport properties of aerosols and gases are very different; thus results from gases may not apply well to the study of disease transmission. Aerosols can be used as a tracer in indoor environments, and sodium chloride is a good candidate as it is considered safe, but Condensation Particle Counters (CPCs) cannot distinguish between the tracer aerosol and any existing background aerosol, for example from external traffic.

Flame photometry, the detection of the characteristic emission of light when chemical species are heated in a flame, has been used in standards for filter testing where NaCl is used as a test aerosol, and yellow light is emitted from the flame in proportion to the mass concentration of sodium sampled. We have recently developed a refined Aerosol Flame Photometer (AFP) instrument, with high sensitivity to NaCl and a fast temporal response. These properties make it an ideal tool to study dispersion of aerosols in indoor environments.

To test we nebulised and released into a small room NaCl, using 20% CO₂ as a carrier which also allowed gas tracing to occur using a fast response CO₂ analyser. The AFP was pre-calibrated for NaCl mass using an Aerodynamic Aerosol Classifier and a CPC. A CPC is also used in the room to count particles. We show here how the concentrations vary over time and with various ventilation strategies. The ratio of CPC to NaCl concentrations relates to the size of particles upon release, which varies slightly; however even if the size decreases due to evaporation after release, the mass of NaCl contained, and hence the AFP signal, is invariant and thus only depends upon dispersion and dilution in the air.