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

摘要

Cavity ring-down spectroscopy (CRDS) is a direct absorption technique thatutilizes path lengths up to multiple kilometers in a compact absorption celland has a significantly higher sensitivity than conventional absorptionspectroscopy. This tool opens new prospects for study of gaseous elementalmercury (Hg) because of its high temporal resolution and reduced samplevolume requirements (0.5 l of sample air). We developed a new sensorbased on CRDS for measurement of (Hg) mass concentration. Sensorcharacteristics include sub-ng m detection limit and high temporalresolution using a frequency-doubled, tuneable dye laser emitting pulses at~253.65 nm with a pulse repetition frequency of 50 Hz. The dye laserincorporates a unique piezo element attached to its tuning grating allowingit to tune the laser on and off the Hg absorption line on apulse-to-pulse basis to facilitate differential absorption measurements.Hg absorption measurements with this CRDS laboratory prototype are highlylinearly related to Hg concentrations determined by a Tekran 2537Banalyzer over an Hg concentration range from 0.2 ng m to573 ng m, implying excellent linearity of both instruments. Thecurrent CRDS instrument has a sensitivity of 0.10 ng Hg m at10-s time resolution. Ambient-air tests showed that background Hg levelscan be detected at low temporal resolution (i.e., 1 s), but also highlight aneed for high-frequency (i.e., pulse-to-pulse) differential on/off-linetuning of the laser wavelength to account for instabilities of the CRDSsystem and variable background absorption interferences. Future applicationsmay include ambient Hg flux measurements with eddy covariance techniques,which require measurements of Hg concentrations with sub-ng msensitivity and sub-second time resolution.