Comparison of short-term variations (15-minute averages) in outdoor and indoor PM2.5 concentrations

Measurements of 15-min average PM2.5 concentrations were made with a real-t ime light-scattering instrument at both outdoor (central monitoring sites i n three communities) and indoor (residential) locations over two seasons in the Minneapolis-St. Paul metropolitan area. These data are used to examine within-day variability of PM2.5 concentrations indoors and outdoors, as we ll as matched indoor-to-outdoor (I/O) ratios. Concurrent gravimetric measur ements of 24-hr average PM2.5 concentrations were also obtained as a way to compare real-time measures with this more traditional metric. Results indi cate that (1) within-day variability for both indoor and outdoor 15-min ave rage PM2.5 concentrations was substantial and comparable in magnitude to da y-to-day variability for 24-hr average concentrations; (2) some residences exhibited substantial variability in indoor aerosol characteristics from on e day to the next; (3) peak values for indoor shortterm (15-min) average PM 2.5 concentrations routinely exceeded 24-hr average outdoor values by facto rs of 3-4; and (4) relatively strong correlations existed between indoor an d outdoor PM2.5 concentrations for both 24-hr and 15-min averages.