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<sup>0</sup>) because of its high temporal resolution and reduced sample volume requirements (&lt;0.5 l of sample air). We developed a new sensor based on CRDS for measurement of (Hg<sup>0</sup>) mass concentration. Sensor characteristics include sub-ng m<sup>&minus;3</sup> 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<sup>0</sup> absorption line on a pulse-to-pulse basis to facilitate differential absorption measurements. Hg<sup>0</sup> absorption measurements with this CRDS laboratory prototype are highly linearly related to Hg<sup>0</sup> concentrations determined by a Tekran 2537B analyzer over an Hg<sup>0</sup> concentration range from 0.2 ng m<sup>&minus;3</sup> to 573 ng m<sup>&minus;3</sup>, implying excellent linearity of both instruments. The current CRDS instrument has a sensitivity of 0.10 ng Hg<sup>0</sup> m<sup>&minus;3</sup> at 10-s time resolution. Ambient-air tests showed that background Hg<sup>0</sup> 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<sup>0</sup> flux measurements with eddy covariance techniques, which require measurements of Hg<sup>0</sup> concentrations with sub-ng m<sup>&minus;3</sup> sensitivity and sub-second time resolution
