An unheated permeation device for calibrating atmospheric VOC measurements

Abstract

The development of an unpowered permeation device for continuous calibration of in-situ instruments measuring atmospheric volatile organic compounds (VOCs) is described. Being lightweight and compact, and containing only negligible amounts of chemicals, the device is especially suited for field use such as onboard aircraft. Its speciality is to maintain the permeation process in thermal equilibrium, i.e. the instantaneous permeation rate can be ascribed via a simple temperature measurement. This equilibrium state is maintained by a combination of three features: (i) a thin PTFE membrane as permeation medium which guarantees short stabilization times, (ii) a water bath as heat buffer, and (iii) a vacuum-panel based insulation, in which features (ii) and (iii) minimize temperature drifts. The uncertainty of the permeation rate due to thermal non-equilibrium is kept below 1 %. An extensive theory part details the major permeation processes of gases through porous polymers, being Fick's diffusion, Knudsen flow, and viscous flow. Both the measured stabilization time and the measured temperature dependence of the permeation rate independently indicate that the permeation can be described by a viscous flow model, where diffusion of the gas molecules in large pores (having a diameter of >0.05 μm) dominates

Vehicular Emission Ratios of VOCs in a Megacity Impacted by Extensive Ethanol Use: Results of Ambient Measurements in São Paulo, Brazil

“…A system calibration was performed during the campaign using permeation tubes stabilized at 40 °C for isoprene, benzene, toluene, and p-xylene. 23,24 Furthermore, methanol, acetaldehyde, acetone, isoprene, MVK+MACR, MEK, benzene, toluene and p-xylene were postcampaign calibrated using a set of gravimetrically prepared multicomponent mixtures (Apel Riemer Inc., USA) for a large range of humidity values. Concentration estimates of compounds calibrated by both methods agreed well within instrumental accuracy.…”