The atmospheric background of perfluorocarbon compounds used as tracers.

There are seven cyclic perfluoroalkane compounds, which can be detected in extremely low concentrations, that are used to track mass movement and transfer in a variety of research and practical applications. They are used in leak detection in underground storage and pipelines and in atmospheric transport and diffusion research on local, regional, and continental scales. They are likely to be a used globally for monitoring carbon sequestration in geological formations. The atmospheric background levels of these compounds must be accurately known, and trends in their concentrations determined for these compounds to be effective in monitoring CO2 reservoirs and because there are environmental concerns about their release. Results of measurements of perfluorocarbon background concentrations from two recent field programs are presented, and trends in these values examined using data collected over the last 25 years. The current atmospheric concentrations of these compounds are in the low parts per quadrillion levels, and their annual atmospheric growth rate is less than 1 part per quadrillion per year. The environmental effects of these compounds are examined and found to be negligible at current release rates.

Housing characteristics and indoor concentrations of nitrogen dioxide and formaldehyde in Quebec City, Canada.

Concentrations of nitrogen dioxide and formaldehyde were determined in a study of 96 homes in Quebec City, Canada, between January and April 2005. In addition, relative humidity, temperature, and air change rates were measured in homes, and housing characteristics were documented through a questionnaire to occupants. Half of the homes had ventilation rates below 7.5 L/s person. Nitrogen dioxide (NO2) and formaldehyde concentrations ranged from 3.3 to 29.1 microg/m3 (geometric mean 8.3 microg/m3) and from 9.6 to 90.0 microg/m3 (geometric mean of 29.5 microg/m3), respectively. The housing characteristics documented in the study explained approximately half of the variance of NO2 and formaldehyde. NO2 concentrations in homes were positively correlated with air change rates (indicating a significant contribution of outdoor sources to indoor levels) and were significantly elevated in homes equipped with gas stoves and, to a lesser extent, in homes with gas heating systems. Formaldehyde concentrations were negatively correlated with air change rates and were significantly elevated in homes heated by electrical systems, in those with new wooden or melamine furniture purchased in the previous 12 months, and in those where painting or varnishing had been done in the sampled room in the previous 12 months. Results did not indicate any significant contribution of indoor combustion sources, including wood-burning appliances, to indoor levels of formaldehyde. These results suggest that formaldehyde concentrations in Quebec City homes are caused primarily by off-gassing, and that increasing air change rates in homes could reduce exposure to this compound. More generally, our findings confirm the influence of housing characteristics on indoor concentrations of NO2 and formaldehyde.