Dry deposition and emission of gaseous atmospheric mercury (Hg) to and from terrestrial ecosystems are currently poorly understood due to limitations imposed by the low temporal resolution of sensors which require pre-concentration of Hg prior to analysis resulting in measurement cycles of several minutes. The low temporal resolution of existing Hg sensors specifically prevents the application of the Eddy Covariance technique that samples turbulent motions of air across the canopy-atmosphere interface at very high temporal resolution.

We present our progress in developing a real-time Hg sensor to measure gaseous atmospheric Hg based on Cavity Ring Down Laser Absorption Spectroscopy (CRLAS) with temporal resolution and detection limits good enough for use in ambient Hg measurements and Eddy Covariance studies. CRLAS measures sample extinction in a closed or open optical cavity by implementation of a long, multiple kilometer path length with highly reflective mirrors which greatly enhances instrument sensitivity as compared to conventional absorption techniques. Our calculations indicate that with a measurement frequency of 10 Hz an instrument noise level of <0.2 ng Hg m-3 is desirable to allow detection of small exchange fluxes commonly measured in terrestrial ecosystems. Using a tunable Ytterbium:YAG laser with 4th harmonic output to generate a wavelength of 253.65 nm and a linewidth of 30 MHz at a pulse repetition rate of 1 kHz, the concentration of mercury can be measured by alternating the laser wavelength between “on” the Hg absorption line and a nearby wavelength “off” the Hg absorption line. No interference from aerosol or other atmospheric constituents is expected as the Hg absorption line is very narrow (i.e., 60 GHz in ambient air) and absorption from other constituents is virtually unchanged over this small wavelength interval.