Field evaluation of a low-uptake VOC passive sampler suitable for long-term deployments

Passive samplers have proven to be effective for continuous monitoring of volatile organic compounds (VOCs) in ambient air in remote, urban and industrial environments. Thermal desorption tubes fitted with endcaps that facilitate passive uptake through diffusion are now routinely used for monitoring fugitive benzene emissions from refineries and petrochemical facilities across North America (EPA Method 325A/B). However, deployment periods of 14 days are typically employed to minimize the risk of poor retention, requiring 26 deployments per year to return an annual average concentration for comparison with chronic exposure health-based standards. Here, we explore extending the deployment duration of these passive samplers to one, two and three months by limiting VOC uptake rates using an alternative diffusive endcap featuring a smaller cross-sectional area. Field testing was performed beside a major highway during two separate three-month campaigns. Uptake rates for benzene, toluene, ethylbenzene and xylenes (BTEX) were observed to remain linear for deployments of up to three months when using the low-uptake endcaps. Application of the low-uptake endcaps and the uptake rates determined here will enable annual average concentrations of BTEX to be calculated using only four tube deployments per year. The cost savings associated with this decreased deployment frequency will facilitate increased spatial resolution during future exposure assessment studies. The stability of selected air toxics within the tubes was also assessed and the results suggest that while aromatic VOCs are stable for storage times of at least 70 days, chloroform and trichloroethylene begin to degrade within two weeks of sampling.