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PubMed2001

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 both controlled and uncontrolled environments. This compound may be subject to reactions with water vapor under elevated temperature to form acidic inorganic compounds such as HF and H2SO4. Thus, in the presence of unvented combustion sources such as kerosene heaters, natural gas heaters, gas log fireplaces, candles, and lamps, the SF6 dissociation may interfere with measurements of the emissions from these sources. Tests were conducted in a research house with a vent-free natural gas heater to investigate these potential interferences. It was observed that the heater operation caused about a 5% reduction of SF6 concentration, which can be an error source for the ventilation 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 the ventilation flow rate. Reducing the tracer gas concentration has no effect on this error. A simple theoretical model is proposed to estimate the magnitude of this error. The second type of interference comes from the primary and secondary products of the SF6 dissociation, mainly H2SO4, SO2, HF, and fine particulate matter (PM). In the presence of approximately 5 ppm SF6, the total airborne concentrations of these species increased by a factor of 4-10. The tests were performed at relatively high SF6 concentrations, which is necessary to determine the interferences quantitatively. The second type of interference can be significantly reduced if the SF6 concentration is kept at a low ppb level.

Air Movements↗

PubMedDate not supplied

Validating existing data in the Environmental Technology Verification Program.

Establishing the credibility of existing data is an ongoing issue, particularly when the data sets are to be used for a secondary purpose, i.e., not the original reason for which they were collected. If the secondary purpose is similar to the primary purpose, the potential user may have little difficulty establishing credibility since the acceptance criteria for both purposes should be similar. If the secondary purpose is different, then data credibility may be more difficult to establish because the experiment generating the data may not have been conducted optimally for the secondary purpose and all of the necessary quality assurance data ("metadata") may not have been collected. In either case, a process will be required to determine the acceptability of the data. For this reason, at the time the U.S. Environmental Protection Agency (EPA) Environmental Technology Verification (ETV) program was established, similar certification and verification programs run by states or foreign countries routinely used existing data sets, for cost reasons, rather than generate new data by testing. The issue of whether existing data could be used in the ETV program immediately surfaced. In response, a policy and a process that addressed existing data were written and published in Appendix C of the ETV Quality and Management Plan (Hayes et al., 1998). This paper discusses how the ETV program determines the credibility of existing data used to verify the performance of environmental technologies.

Data Interpretation, Statistical↗