The goal of this project is to develop a new field-usable sensor for measurement of atmospheric mercury (Hg) concentrations and fluxes with high time resolution and sensitivity using cavity-ring down (CRD) spectroscopy. CRD reaches very low detection limits through implementation of long path lengths (>1 km) in a compact cell (<1 m) by confining short laser pulses in a stable, two-mirror resonator. Based on a laboratory prototype developed at the Desert Research Institute (DRI) with NSF EPSCoR funding, the proposed project will implement fundamental design novelties and key technical features to improve sensitivity, increase time resolution, and provide safe and user-friendly field deployments. Proposed novel sensor features will include (1) improved reflectivity of UV mirrors (or prisms) to increase current CRD path lengths and detection limits; (2) optimized cavity design, including implementation of an open-path cavity for applications such as Eddy Covariance (EC) measurements, (3) unique in situ calibration approaches and differential on/off absorption line measurements by detuning and high-frequency (50 Hz) tuning of the laser wavelength; (4) automated wavelength locking and continuous real-time data processing and signal analysis; and (5) integration of the sensor into a mobile rack and fiber coupling of the laser signal into the open- and closed-path cavity, which hence can be placed meters away from the main system allowing measurement of undisturbed air volumes and easy deployment of the sensor on various platforms. The sensor will be comprehensively tested on DRI's ground-based and high-elevation (3200 m asl) research stations and used for EC measurements to determine Hg fluxes.