Gas-Wall Partitioning of Organic Compounds in a Teflon Film Chamber and Potential Effects on Reaction Product and Aerosol Yield Measurements

Abstract

Gas-wall partitioning of organic compounds (OC) that included C 8 -C 16 n-alkanes and 1-alkenes and C 8 -C 13 2-alcohols and 2-ketones was investigated in two Teflon FEP chambers whose walls were either untreated, oxidized in sunlight, or previously exposed to secondary organic aerosol (SOA). Partitioning was nearly independent of chamber treatment, reversible, and obeyed Henry's law. The fraction of an OC that partitioned to the walls at equilibrium ranged from 0 to ∼65%. Values increased with increasing carbon number within an OC class and for OC with similar vapor pressures increased in the order n-alkanes <1-alkenes <2-alcohols <2-ketones. Estimated time constants for achieving partitioning equilibrium ranged from ∼60 min for n-alkanes to ≤8 min for 2-ketones. The observations are consistent with a sorption mechanism in which OC dissolve into the film but are restricted to the near-surface region by a sharp permeability gradient that develops in response to OC-induced stresses in polymer chains.When the results were analyzed using a model analogous to one commonly employed for gas-particle partitioning, it was estimated that the sorption properties of the chamber walls were equivalent to organic aerosol mass concentrations of 2, 4, 10, and 24 mg m -3 with respect to the partitioning of n-alkanes, 1-alkenes, 2-alcohols, and 2-ketones. These values are up to ∼4 orders of magnitude larger than concentrations used in most laboratory studies of SOA, which are typically 1-10 3 µg m -3 , meaning that if full partitioning equilibrium is established in the chamber then semi-volatile OC will reside overwhelmingly in the chamber walls. Model simulations of gas-particle-wall partitioning were also carried out using the experimental data, and demonstrate quantitatively the large potential effects of Teflon walls on measured yields of gas-phase OC products and SOA.

Size distribution dynamics reveal particle-phase chemistry in organic aerosol formation

“…The model treats the following processes explicitly: gas-phase diffusion, reversible adsorption, bulk diffusion, and chemical reactions in the gas and particle phases. The model also accounts for the loss of gas-phase SVOCs to the chamber Teflon wall (14,15) based on measurements for representative compounds in separate experiments (SI Materials and Methods, Gas-phase wall loss, Table S1 and S2). The physical state of the particle bulk is assumed to be semisolid with an average bulk diffusivity of 10 −12 cm 2 ·s −1 [a typical value for a semisolid state (16) (SI Materials and Methods)], consistent with observations that long-chain alkane-derived SOA particles bounce moderately on the smooth plates of an inertial impactor indicating behavior between that of liquid and glassy particles (17).…”

Influence of vapor wall loss in laboratory chambers on yields of secondary organic aerosol

“…k w reflects the combined effects of turbulent mixing in the chamber, molecular diffusion of vapor molecules through the near-wall boundary layer, and any penetration into chamber walls. k w is likely to be chamber specific, as the extent of turbulent mixing depends on specific chamber operating conditions, but is reasonably independent of compound identity (11). Values of k w for a range of gases were estimated in one study (17) to range from ∼2 × 10 −5 to 10 −3 s −1 , corresponding to timescales of many hours to 10 min.…”

“…15, 16; SI Appendix, The Statistical Oxidation Model). The SOM accounts for vapor wall losses based on observations showing that wall losses of semivolatile species in Teflon chambers are reversible (11). Such wall loss is modeled dynamically and depends on the equivalent OA mass of the chamber walls (C w , mg m −3 ), the first-order vapor wall loss rate coefficient (k w , s −1 ), and the vapor saturation concentration of compounds i (C p i , μg m −3 ).…”

“…Recent experiments have demonstrated that losses of organic vapors to the typically Teflon walls of a laboratory chamber can be substantial (11), but the effects on SOA formation have not yet been quantitatively established. In essence, the walls serve as a large reservoir of equivalent OA mass that compete with the particulate SOA for SOA-forming compounds.…”

Evaporation kinetics and phase of laboratory and ambient secondary organic aerosol

“…However, comparison between the evaporation kinetics observed in Grieshop et al (17) and in our laboratory indicates that 2.5 h is most likely the point at which the evaporation rate slows down and not where it ends. It has also been suggested that this dilution-induced evaporation type of experiment could be strongly affected by material deposited on smog chamber walls (18).…”