Variation of the relative humidity of air released from canisters after ambient sampling

Dalton's law of partial pressures and the hypothesis that water vapor equil ibrium in a canister is identical to that established above liquid water ar e used to predict the variation of the percent relative humidity (%RH) of a ir released from canisters used in ambient air sampling, typically 6-L cani sters pressurized with 18 L of air. When (and if) the water vapor partial p ressure in a canister exceeds its saturation vapor pressure, water vapor co ndensation begins and the condensation rate equals the sampling rate of wat er vapor into the canister. Under constant temperature conditions, the air subsequently released from the canister is less humid than the original sam ple, following the relationship %RH = 100% (6 L/V-s) for V-s> V-r, where V- s is the resdual air volume (referenced to atmospheric pressure), and V-r i s shown to depend on the %RH of the ambient air sample. V-r is the residual air volume at which water is completely removed (except for adsorbed water vapor) from the canister wall. For V-s < Vr, the predicted %RH is constant and equal to its value at V-r. Experimental values agree reasonably well w ith predictions at both high (90%) and low (34%) RH. However, experimental values are often slightly displaced (usually towards lower values of %RH) f or mid-range %RH (61%) and variations in %RH near V-r change from canister to canister.