Understanding indoor concentrations of outdoor pollutants—lessons from Joint Urban and UDP

Indoor concentrations of pollutants originating outdoors are controlled by a number of variables – ventilation rates, losses due to deposition or sorption, changes in outdoor concentrations, etc. A number of studies have examined differences between indoor and outdoor concentrations of typical ambient pollutants, e.g., particles, oxides of carbon and nitrogen, etc., often without developing a detailed understanding of the processes affecting pollutant concentration behavior. The urban scale tracer gas experiments conducted in Oklahoma City (Joint Urban 2003) and New York City (UDP) provided a unique opportunity to examine the time and spatial behavior of indoor concentrations of tracers released outdoors.

In Oklahoma City, three buildings were instrumented – one intensively – to measure tracer gas concentrations as a function of time at various locations within the buildings, along with other building parameters. In New York City, a single, tall building (>30 floors) was extensively instrumented to provide tracer gas concentration profiles and other building parameters. Short-duration (puff) releases of a gas-phase tracer were done outdoors – typically several blocks upwind of the study buildings. Other tracer gases were released in various HVAC systems within the buildings to provide data on interzonal transport and mixing of air within the building.

The measurements made in these tests were used to assess the effectiveness of sheltering under varying building operating parameters. The effectiveness of sheltering was found to be dependent on both building operation modes and outdoor plume characteristics. In addition, the presumed shape of the toxic load curve (linear, 2nd power, or 4th power, in our examples) had a significant impact on the amount of protection provided by sheltering. This indicates that sheltering is more effective for certain types of toxins. Significant exposure reductions (greater than 95%) were achieved through ventilation system manipulation and optimization of sheltering times.

In this talk, we will show typical results from experiments in both cities and discuss the differences in outdoor and indoor concentrations and how building operations and urban dispersion affect these concentration differences. The results illustrate the importance of improving our understanding of indoor and outdoor concentrations to predict concentrations and provide information on optimum sheltering durations for reducing exposures in emergency situations.