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Passive NO2 Palmer diffusion tubes - these are "small plastic tubes containing a chemical reagent to absorb the pollutant to be measured directly from the air. In the case of Palmes-type nitrogen dioxide diffusion tubes, the absorbent used is triethanolamine (TEA). Stainless-steel mesh grids at the closed end of the tube are coated with a water-based or acetone-based solution of this absorbent.