Responding to a growing need for inexpensive and simple monitoring of persistent organic pollutants (POPs) in the atmosphere, a passive air sampling technique based on the sorption of gaseous pollutants to the sampling resin XAD-2, a styrene-divinylbenzene copolymer, has been developed. A quantitative understanding of the uptake kinetics of the passive air samplers (PAS) was obtained through a combination of field calibration studies, controlled wind tunnel experiments, and flow field simulations. Fortytwo PAS were deployed for varying time periods up to 1 yr at three calibration stations in the Laurentian Great Lakes region and the Canadian High Arctic with ongoing conventional air sampling of organochlorine pesticides. The PAS take up quantifiable levels of POPs within a few weeks of deployment, and the amount of chemical collected increases steadily over a 1-yr sampling period. The uptake of POPs by the PAS is controlled by molecular diffusion and independent of wind velocity. The timeaveraged air concentrations of organochlorine pesticides derived from the PAS data are comparable with those from HiVol sampling. This study suggests that the XAD-2 resinbased PAS can be used to derive at least semiquantitative information on the vapor-phase concentrations of POPs in the atmosphere and are suitable for the measurements of long-term average concentrations at the levels occurring in remote regions.
Reported data on the temperature dependence of atmospheric concentrations of semivolatile organic compounds (SOCs) are compiled and expressed as linear regressions of the logarithm of the partial pressure in air versus reciprocal temperature: ln p A = m/T + b. Two simple models are introduced to explain the dependence of these air concentrations on temperature. The first assumes equilibrium between the atmosphere and the earth's surface. In the second, air concentrations are established as a result of chemical inflow and outflow in advected air and reversible exchange with a soil or water surface. The model equations are rearranged to express the partial pressure of the chemical as a function of temperature. On the basis of these models, it is shown that only under selected circumstances, namely, if surface contamination is high and atmospheric background concentration low, does the slope m of the ln p vs 1/T relationship reflect the thermodynamics of air−surface partitioning. Generally, however, m is a measure of the extent to which air concentrations are controlled by evaporation from surfaces close to the sampling location and by advection of air masses with global background concentrations. A shallow slope or low temperature dependence indicates that long-range transport controls atmospheric levels at a sampling site. Steeper slopes indicate high surface concentrations in the vicinity of the site. This hypothesis is applied to the observed tem perature dependence of the compiled atmospheric concentration data and is found to be capable of explaining differences in slope m (i) between chemicals, (ii) between sampling sites, and (iii) at different seasons. Research efforts should be directed toward quantifying by measure ments and predicting by models the kinetics of exchange of SOCs between the atmosphere and various surfaces.
Annually averaged concentrations and enantiomeric compositions of organochlorine pesticides (OCPs) in air were determined in 2000/2001 at 40 stations across North America using XAD-based passive samplers. Absolute concentration differences acrossthe continent, the skewness and kurtosis of the concentration distribution, the relative abundance of parent compound and metabolites, and the chiral signatures can identify regional sources of OCP to the atmosphere. Specifically, air samples collected in the southeastern United States had elevated concentrations of chlordane-related compounds, higher ratios of trans- to cis-chlordane and heptachlor to heptachlor epoxide, as well as higher enantiomeric fractions of trans-chlordane as compared to other regions, suggesting continued release of microbially unprocessed chlordane to the regional atmosphere. Similarly, greatly elevated concentrations of p,p'-DDT, low relative abundance of the metabolite DDE, and a racemic composition of o,p'-DDT in samples from southern Mexico and Belize indicate recent use of DDT in these regions. Belize is also a potential source region for dieldrin. Reflecting its continued use in North America, endosulfan is now one of the most abundant and ubiquitous OCPs in the continental atmosphere. In contrast to these OCPs, air concentrations of penta- and hexachlorobenzene vary only by factors of 2-4 across the continent, reflecting a long atmospheric residence time and few primary sources. Atmospheric levels of the chlorobenzenes, alpha-endosulfan, and p,p'-DDE increase with elevation in the Canadian Rocky Mountains. Empirical travel distances for the OCPs derived from latitudinal concentration profiles are in good agreementwith model-derived indicators of long-range transport potential and, in particular, lead to a similar categorization of the OCPs. Large-scale passive air sampling networks are suitable for monitoring compliance with, and effectiveness of, regulatory control measures and for establishing experimentally the atmospheric long-range transport behavior of organic air pollutants.
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