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

by X. Faïn, H. Moosmüller, D. Obrist
Atmospheric Chemistry and Physics ()

Abstract

Cavity ring-down spectroscopy (CRDS) is a direct absorption technique that utilizes path lengths up to multi- ple kilometers in a compact absorption cell and has a signif- icantly higher sensitivity than conventional absorption spec- troscopy. This tool opens new prospects for study of gaseous elemental mercury (Hg0) because of its high temporal res- olution and reduced sample volume requirements (<0.5 l of sample air). We developed a new sensor based on CRDS for measurement of (Hg0) mass concentration. Sensor charac- teristics include sub-ngm3 detection limit and high tempo- ral resolution using a frequency-doubled, tuneable dye laser emitting pulses at 253.65nm with a pulse repetition fre- quency 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 Hg0 absorption line on a pulse-to-pulse basis to facilitate differential absorption measurements. Hg0 absorption measurements with this CRDS laboratory proto- type are highly linearly related to Hg0 concentrations deter- mined by a Tekran 2537B analyzer over an Hg0 concentra- tion range from 0.2 ngm3 to 573 ngm3, implying excel- lent linearity of both instruments. The current CRDS instru- ment has a sensitivity of 0.10 ngHg0m3 at 10-s time reso- lution. Ambient-air tests showed that background Hg0 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) dif- ferential on/off-line tuning of the laserwavelength to account for instabilities of the CRDS system and variable background absorption interferences. Future applications may include ambient Hg0 flux measurements with eddy covariance tech- niques, which require measurements of Hg0 concentrations with sub-ngm3 sensitivity and sub-second time resolution.

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