Measurements were performed on seven days between August 8, 2008 and October 28, 2008.
In the first five measurement periods, the air handlers operated without intervention and most
of the air entering the filter banks was recirculated indoor air. During these five measurement
periods, the concentrations of ozone entering and exiting the filter banks were too small (e.g.,
5 ppb) to allow an accurate determination of the ozone removal efficiency of the filters, given
that the ozone instrument’s output signal fluctuated substantially at these concentrations. The
moderate-to-low concentrations of ozone in outdoor air (typically about 20 ppb), the high
proportion of recirculated air in the airstream entering the filters, and the low concentration of
ozone in recirculated air, presumably because of indoor ozone chemical reactions, explain the
low ozone concentrations at the inlet of the filters. Consequently, for the last two
measurement periods, the air handler was operated with 100% outdoor air supply (no
recirculation) during the periods of data collection. In addition, a different ozone monitor
(same brand and model), with a more stable output signal was utilized.