Gas/solid partitioning of semivolatile organic compounds (SOCs) to air filters. 3. An analysis of gas adsorption artifacts in measurements of atmospheric SOCs and organic carbon (OC) when using Teflon membrane filters and quartz fiber filters

Adsorption of gaseous semivolatile organic compounds (SOCs) onto the filter (s) of a filter/sorbent sampler is a potential source of measurement error when determining specific, SOCs as well as organic carbon (OC)levels in the atmosphere. This work examines partitioning to both Teflon membrane filter s (TMFs) and quartz fiber filters (QFFS) for purposes of predicting the mag nitude of the compound-dependent gas adsorption artifact as a function of v arious sampling parameters. The examination is based on values of K-p,K-fac e (m(3) cm(-2)), the gas/filter partition coefficient expressed as [ng sorb ed per cm(2) of filter face]/[ng per m(3) in the gas phase]. Values of K(p, face)were calculated based on literature values of the gas/solid partition coefficient K-p,K-s [ng sorbed per m(2) of filter]/[ng per m(3) in gas phas e] for the adsorption of various polycyclic aromatic hydrocarbons (PAHs), p olychlorinated dibenzodioxins (PCDDs), and polychlorinated dibenzofurans (P CDFs) to TMFs, and for the adsorption of PAHs to QFFs. At relative humidity (RH) values below approximate to 50%, the K-p,K-face values for PAHs are l ower on TMFs than on ambient-backup QFFs. The gas adsorption artifact will therefore be lower for PAHs with TMFs than with QFFs under these conditions . In the past, corrections for the gas/filter adsorption artifact have been made by using a backup filter, and subtracting the mass amount of each com pound found on the backup filter from the total (particle phase + sorbed on filter) amount found on the front filter. This procedure assumes that the ng cm(-2) amounts of each SOC sorbed on the front and backup filters are eq ual. That assumption will only be valid after both filters have reached equ ilibrium with each of the gaseous SOCs in the incoming sample air. The fron t filter will reach equilibrium first. The minimum air sample volume V-min, V-f+b required to reach gas/filter sorption equilibrium with a pair of filt ers is 2K(p,face)A(filter) where A(filter) (cm(2)) is the per-filter face a rea. K-p,K-face values, and therefore V-min,V-f+b values, depend on the com pound, relative humidity (RH), temperature, and filter type. Compound-depen dent V-min,V-f+b values are presented for PAHs and PCDD/Fs on both TMFs and QFFs. Compound-dependent equations which give the magnitude of the filter adsorption artifact are presented for a range of different sampling arrange ments and circumstances The equations are not intended for use in actually correcting field data because of uncertainties in actual field values of re levant parameters such as the compound-dependent K-p,K-face and gas/particl e K-p values, and because of the fact that the equations assume ideal step- function chromatographic movement of gas-phase compounds through the adsorb ing filter. Rather, the main utility of the equations is as guidance tools in designing field sampling efforts that utilize filter/sorbent samplers an d in evaluating prior work. The results indicate that some backup-filter-ba sed corrections described in the literature were carried out using sample v olumes that were too small to allow proper correction for the gas adsorptio n artifact for some specific SOCs of interest. Similar conclusions are reac hed regarding artifacts associated with the measurement of gaseous and part iculate OC.