LAUSR

Aerosol sampling onto filter media with laboratory weighing before and after drawing air through the filter is the most commonly applied method to determine PM2.5 and PM10 concentrations. Although simple in concept, determining the net gain in weight from an aerosol deposit is complicated by adsorption of gases onto the filter, retention of water by the filter and the deposit, electrostatic charges that result in attraction of the filter to balance surfaces, contamination during filter handling, losses or additions to filter material, evaporation of semi-volatile aerosols, and inhomogeneous sample deposits. Additional restrictions apply when the weighed filters are submitted to subsequent chemical speciation analyses. A precisely controlled environment, with clean air and constant temperature and humidity, is required for filter weighing and processing, and filters must be equilibrated at these conditions. Balances must have sensitivities of ≤1 µg, be calibrated with well-established primary standards, and be regularly serviced. Ionizing sources are used to neutralize electrostatic charges. Regular quality control and quality assurance procedures involve filter inspection for defects before and after sampling, periodic balance zero and span checks, replicate filter weights, and independent system and performance audits.