The automated Environmental Monitoring Station (EMS) monitors the concentration of a variety of chemical species, in addition to general environmental measurements. These species include NOy, NOx, CO₂, H₂O, O₃, CO, and several different hydrocarbons (For a complete list see the table below). We measure the net flux hourly aggregating to seasons, years, and decades. Methods used to calculate concentrations include infrared and UV-Vis spectrophotometry, chemiluminescence, and gas chromatography.  Measurements of carbon dioxide and trace gases have been nearly continuous since 1990, and data through 1999 is publicly available through our data exchange.

This schematic representation of the CO₂ eddy measurement shows how the CO₂ measurement is calibrated by standard addition. A small flow of calibration gas is added to the sample flow of 5-6 lpm. The pressure in the detector cell is measured and controlled. Total flow in the system is measured in order to identify periods with filter clogging and to do the calibration calculations. Calibration of NO_x and NO_y measurements also use standard addition and have similar plumbing.

8 inlets are arranged from 30 cm to 30m on or near the tower. Each inlet is selected in turn by activating a solenoid on the "hub" pictured above. Teflon is used to minimize alteration of O₃ and NO_x concentrations. The entire sampling line is at a constant reduced pressure to prevent condensation of water vapor.

For more information about our instruments and how we make our measurements contact Bill Munger or John Budney.

List of sensors and quantities measures on eddy-flux tower at Harvard Forest.