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A gravimetric method that draws air at a known flow rate through a pre-weighed filter for 24 hours. Particles in air are collected on the filter which is then weighed in a laboratory and reported as micrograms per cubic metre based on the total mass divided by the volume of air sampled. Because the filters need to be manually changed every 24-hours this method is not practical for NES-AQ compliance monitoring. Normally high volume samplers are operated on a 1-in-3-day frequency.