NYC-WTC_000232959

NYC 9/11 Public Portal Document

Ideally, the particulate matter data would have provided information about the airborne concentration with an aerodynamic diameter of 100 microns and less (PMIOO), of 10 microns and less (PMIO), of 4 microns and less (PM4), and of 2.5 microns and less (PM2.5). These different size fractions (PMIOO, PMIO, PM4, and PM2.5) are collected using slightly different equipment, but following the same basic procedures. A pump draws air into a sampling head, and size specific particulate matter is deposited onto a filter. The difference in filter weight, before and after the sampling, represents the mass of the particulate matter that was captured by the filter. Knowing the presampling and postsampling air flow rates of the pump and the time duration for sample collection allows us to calculate the average volume of air that was drawn through the filter. The concentration of the size fraction in the sampled air is then calculated by dividing the PM mass by the air volume sampled.

hl addition to the filters that were used to measure the concentration of airborne particulate matter, the laboratory also sent filters to be used as field blanks. Two blanks were sent for each PM fraction for each of the two sampling teams for each building. The blanks traveled with the sample filters, went to the sampled building or area, and were treated just like the sample filters—except that they were not used. Ideally, there should be very little difference in the weight of the blanks before the sampling and after the sampling because they were not used. A weight gain in field blanks may indicate improper sample handling in the field or problems in filter weighing in the lab. For the latter a decrease in post field blank filter weight is also an indication of lab weighing error.

The graph shown with this discussion shows the frequency distribution of the air sample filter weight change in aU but seven of the blanks used. Seven blanks were not included because their weight change (from a negative -90 milligrams (mg) to 5.99 mg) was greater than the limits shown in this graph (-0.2 mg to 0.5 mg). The vertical axis represents the number of blanks that had a weight change within 0.01 mg of the weight change shown on the horizontal axis.

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