title: Variation of the Relative Humidity of Air Released from Canisters after Ambient Sampling.
abstract: Dalton's law of partial pressures and the hypothesis that water vapor equilibrium in a canister is identical to that established above liquid water are used to predict the variation of the percent relative humidity (%RH) of air released from canisters used in ambient air sampling, typically 6-L canisters pressurized with 18 L of air. When (and if) the water vapor partial pressure in a canister exceeds its saturation vapor pressure, water vapor condensation begins and the condensation rate equals the sampling rate of water vapor into the canister. Under constant temperature conditions, the air subsequently released from the canister is less humid than the original sample, following the relationship %RH = 100% (6 L/Vs) for Vs> Vr, where Vsis the residual air volume (referenced to atmospheric pressure), and Vris shown to depend on the %RH of the ambient air sample. Vris the residual air volume at which water is completely removed (except for adsorbed water vapor) from the canister wall. For Vs< Vr, the predicted %RH is constant and equal to its value at Vr. Experimental values agree reasonably well with predictions at both high (90%) and low (34%) RH. However, experimental values are often slightly displaced (usually towards lower values of %RH) for mid-range %RH (61%) and variations in %RH near Vrchange from canister to canister.
authors.affiliation: a United States Environmental Protection Agency , Research Triangle Park , North Carolina , USA.
authors.forename: William A
authors.initials: WA
authors.lastname: McClenny
authors.affiliation: a United States Environmental Protection Agency , Research Triangle Park , North Carolina , USA.
authors.forename: S Mark
authors.initials: SM
authors.lastname: Schmidt
authors.affiliation: b ManTech Environmental Technology Inc. , Research Triangle Park , North Carolina , USA.
authors.forename: Keith G
authors.initials: KG
authors.lastname: Kronmiller