Indoor and outdoor sets of sample filters used for particle collection. For each filter set, a pump is used to draw air into copper tubing and through a cyclone before branching to four filter holders: two for teflon/cellulose/nylon filters and two for quartz filters. Teflon, cellulose, and nylon filters (labeled T, C, and N) are mounted in filter holders downstream of glass honeycomb denuders. Pairs of quartz filters (labeled Q) are held in stainless steel filter holders. Solenoid valves are programmed to direct airflow to one pair of filter holders (one with teflon/cellulose/nylon filters in series and one with two quartz filters in series) during the economizer-off periods, and then switch to the other matched pair of identical filters during the economizer-on periods.