Dissociation of sulfur hexafluoride tracer gas in the presence of an indoor combustion source

As an odorless, nontoxic, and inert compound, sulfur hexafluoride (SF6) is one of the most widely used tracer gases in indoor air quality studies in b oth controlled and uncontrolled environments. This compound may be subject to reactions with water vapor under elevated temperature to form acidic ino rganic compounds such as HF and H2SO4. Thus, in the presence of unvented co mbustion sources such as kerosene heaters, natural gas heaters, gas log fir eplaces, candles, and lamps, the SF6 dissociation may interfere with measur ements of the emissions from these sources. Tests were conducted in a resea rch house with a vent-free natural gas heater to investigate these potentia l interferences. It was observed that the heater operation caused about a 5 % reduction of SF6 concentration, which can be an error source for the vent ilation rate measurement and consequently the estimated pollutant emission rates. Further analysis indicates that this error can be much greater than the observed 5% under certain test conditions because it is a function of t he ventilation flow rate. Reducing the tracer gas concentration has no effe ct on this error. A simple theoretical model is proposed to estimate the ma gnitude of this error. The second type of interference comes from the primary and secondary produc ts of the SF6 dissociation, mainly H2SO4, SO2, HF, and fine particulate mat ter (PM). In the presence of similar to5 ppm SF6, the total airborne concen trations of these species increased by a factor of 4-10. The tests were per formed at relatively high SF6 concentrations, which is necessary to determi ne the interferences quantitatively. The second type of interference can be significantly reduced if the SF6 concentration is kept at a low ppb level.