Toward real-time measurement of atmospheric mercury concentrations using cavity ring-down spectroscopy

Abstract

Abstract. Cavity ring-down spectroscopy (CRDS) is a direct absorption technique that utilizes path lengths up to multiple kilometers in a compact absorption cell and has a significantly higher sensitivity than conventional absorption spectroscopy. This tool opens new prospects for study of gaseous elemental mercury (Hg 0 ) because of its high temporal resolution and reduced sample volume requirements (<0.5 l of sample air). We developed a new sensor based on CRDS for measurement of (Hg 0 ) mass concentration. Sensor characteristics include sub-ng m −3 detection limit and high temporal resolution using a frequency-doubled, tuneable dye laser emitting pulses at ∼253.65 nm with a pulse repetition frequency of 50 Hz. The dye laser incorporates a unique piezo element attached to its tuning grating allowing it to tune the laser on and off the Hg 0 absorption line on a pulse-to-pulse basis to facilitate differential absorption measurements. Hg 0 absorption measurements with this CRDS laboratory prototype are highly linearly related to Hg 0 concentrations determined by a Tekran 2537B analyzer over an Hg 0 concentration range from 0.2 ng m −3 to 573 ng m −3 , implying excellent linearity of both instruments. The current CRDS instrument has a sensitivity of 0.10 ng Hg 0 m −3 at 10-s time resolution. Ambient-air tests showed that background Hg 0 levels can be detected at low temporal resolution (i.e., 1 s), but also highlight a need for high-frequency (i.e., pulse-to-pulse) differential on/off-line tuning of the laser wavelength to account for instabilities of the CRDS system and variable background absorption interferences. Future applications may include ambient Hg 0 flux measurements with eddy covariance techniques, which require measurements of Hg 0 concentrations with sub-ng m −3 sensitivity and sub-second time resolution.

Cavity ring-down spectroscopy sensor development for high-time-resolution measurements of gaseous elemental mercury in ambient air

“…A further use of high-frequency GEM measurements is in surface-atmosphere flux measurement techniques such as eddy covariance, a method that requires sensor time resolution of 10 Hz or faster to resolve high-frequency concentration deviations induced by turbulent eddies that occur across time scales ranging from 10 −3 to 10 4 s (Arya, 2001). Faïn et al (2010) describe a prototype strictly designed for laboratory use, based on cavity ring-down spectroscopy (CRDS) with high-reflectivity mirrors to create long absorption path lengths in a compact cell (1 m length). The authors present manual wavelength scans including the GEM absorption wavelength of 253.65 nm to characterize GEM concentrations in the laboratory using Hg-free air (i.e., charcoalfiltered air) spiked with GEM to reach concentrations between 0.2 and 573 ng m −3 (Faïn et al, 2010).…”

“…Faïn et al (2010) describe a prototype strictly designed for laboratory use, based on cavity ring-down spectroscopy (CRDS) with high-reflectivity mirrors to create long absorption path lengths in a compact cell (1 m length). The authors present manual wavelength scans including the GEM absorption wavelength of 253.65 nm to characterize GEM concentrations in the laboratory using Hg-free air (i.e., charcoalfiltered air) spiked with GEM to reach concentrations between 0.2 and 573 ng m −3 (Faïn et al, 2010). In addition, they present preliminary measurements of GEM in ambient air and identify potential interferences by ambient air constituents such as O 3 and particulate matter that absorb in the same wavelength region as GEM.…”

Cavity-enhanced absorption using an atomic line source: application to deep-UV measurements

“…Although Faïn et al 19 have achieved a much better detection limit with cavity ringdown spectroscopy, they did so using powerful pulsed lasers and 99.895% mirror reflectivities. They attained a detection limit of about 0.10 ng m −3 in the same averaging time of 10 s. On the other hand, the CEAS detection limit is already better than commercial optical absorption based devices for monitoring elevated Hg levels (e.g., LOD = 100 ng m −3 for the Mercury Tracker 3000IP).…”

“…Nevertheless, this cross-section value is in fair agreement with prior measurements, although we note that there is a considerable spread in literature values. 18,19 Based on the smallest detectable change in signal (around 0.003 at 3σ, Figure 2), the estimated limits of detection to Hg, O 3 , and several other compounds are presented in Table 1.…”

Deployment of a sequential two-photon laser-induced fluorescence sensor for the detection of gaseous elemental mercury at ambient levels: fast, specific, ultrasensitive detection with parts-per-quadrillion sensitivity

“…Pierce et al (2013) provide a review of CRDS and other mercury sensors that might offer an alternative to CVAFS. Faïn et al (2010) proposed that CRDS is capable of fast in situ measurements and reported a detection sensitivity of 0.1 ng m −3 with a 10 s time resolution. In more recent work, the same group appears to have acknowledged that, because of O 3 interference, in situ measurements of Hg(0) are not feasible using CRDS and that the sample needs to be stripped of ozone before introduction into the CRDS cavity (Pierce et al, 2013).…”