Global evaluation and calibration of a passive air sampler for gaseous mercury

Abstract

Abstract. Passive air samplers (PASs) for gaseous mercury (Hg) were deployed for time periods between 1 month and 1 year at 20 sites across the globe with continuous atmospheric Hg monitoring using active Tekran instruments. The purpose was to evaluate the accuracy of the PAS vis-à-vis the industry standard active instruments and to determine a sampling rate (SR; the volume of air stripped of gaseous Hg per unit of time) that is applicable across a wide range of conditions. The sites spanned a wide range of latitudes, altitudes, meteorological conditions, and gaseous Hg concentrations. Precision, based on 378 replicated deployments performed by numerous personnel at multiple sites, is 3.6 ± 3.0 %1, confirming the PAS's excellent reproducibility and ease of use. Using a SR previously determined at a single site, gaseous Hg concentrations derived from the globally distributed PASs deviate from Tekran-based concentrations by 14.2 ± 10 %. A recalibration using the entire new data set yields a slightly higher SR of 0.1354 ± 0.016 m3 day−1. When concentrations are derived from the PAS using this revised SR the difference between concentrations from active and passive sampling is reduced to 8.8 ± 7.5 %. At the mean gaseous Hg concentration across the study sites of 1.54 ng m−3, this represents an ability to resolve concentrations to within 0.13 ng m−3. Adjusting the sampling rate to deployment specific temperatures and wind speeds does not decrease the difference in active–passive concentration further (8.7 ± 5.7 %), but reduces its variability by leading to better agreement in Hg concentrations measured at sites with very high and very low temperatures and very high wind speeds. This value (8.7 ± 5.7 %) represents a conservative assessment of the overall uncertainty of the PAS due to inherent uncertainties of the Tekran instruments. Going forward, the recalibrated SR adjusted for temperature and wind speed should be used, especially if conditions are highly variable or deviate considerably from the average of the deployments in this study (9.89 ∘C, 3.41 m s−1). Overall, the study demonstrates that the sampler is capable of recording background gaseous Hg concentrations across a wide range of environmental conditions with accuracy similar to that of industry standard active sampling instruments. Results at sites with active speciation units were inconclusive on whether the PASs take up total gaseous Hg or solely gaseous elemental Hg primarily because gaseous oxidized Hg concentrations were in a similar range as the uncertainty of the PAS.

Characterization and Quantification of Atmospheric Mercury Sources Using Passive Air Samplers

“…A very low relative percent difference between duplicated PAS in this study (2%) indicates excellent precision across the full range of observed concentrations. Considering this and the good agreement with previously reported concentrations (Vaselli et al, 2013), the high accuracy of the PAS reported previously (McLagan et al, 2018) should likewise apply across this range of concentrations. The PAS uncertainty is similar to that of the industry-standard Tekran 2537 series active instrument (Slemr et al, 2015) and lower than that of the LIDAR technique, which is at best 20% under favorable sampling conditions (Grönlund et al, 2005).…”

“…On the other extreme, background concentration levels were observed in the northwest and southwest corners of the larger Amiata grid across all four seasonal deployments (Figure ). Concentrations incrementally increased moving from these sites at the western edge of the Amiata grid toward the ASSM, confirming the PAS's application to background monitoring (McLagan et al, ). Clearly, the PAS can be applied across a wide range of gaseous Hg concentrations.…”

“…The uncertainty of measurements made with the PAS has been determined previously. The precision of measurements made with this sampler of 2% to 4% has been obtained through the analysis of hundreds of replicated samples in deployments across the globe (McLagan, Mitchell, et al, ; McLagan et al, ). While a quantification of the accuracy of a PAS measurement is not possible in the absence of knowledge of the true concentration of Hg in the atmospheric gas phase, it is feasible to compare concentrations obtained with the PAS with those recorded simultaneously by a state‐of‐the‐art active instrument.…”

“…Analytical quality assurance and control measures can be found in section s4. The relative percent difference between duplicated samples ( n = 9) was 2 ± 2% and is below that reported for the PAS in previous studies (McLagan, Mitchell, et al, ; McLagan et al, ).…”

Application of the Passive Sampler Developed for Atmospheric Mercury and Its Limitation

“…This requires the development of a passive sampler that is inexpensive, easily deployed, and does not need electricity.Because a passive sampler is able to identify concentrations at a fine spatial resolution, it has been widely used for certain pollutants including persistent organic pollutants and carbon dioxide [7]; however, nearly all Hg research studies have relied on an active sampler until recently. Many previous studies using an Hg passive sampler did not provide satisfactory results [7], and only a few groups suggested the reliable data [8][9][10][11][12]. For an Hg passive sampler, a diffusive body including radial [10,13], axial [8], box [14], and two-bowl [15] types has been tested.…”