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.

Schematic representation of the CO2 eddy measurement.

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 NOx and NOy measurements also use standard addition and have similar plumbing.

 8 inlets are arranged from 30 cm to 30m on or near the tower.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 NOx 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.

Sensor inlet/ instrument altitudes determined quantity (data rate) Dates Active
ATI Sonic Anemometer 30m Three-dimensional wind velocity, temperature, momentum & heat fluxes (30-min avg) Ongoing
NOy (catalyst-->NO) 30m NOy fluxes, 30-minute averages  1992-2002
high-speed CO2-H2O IR-Absorbance 30m CO2 and H2O fluxes (30-min avg) Ongoing
high-speed O3 C2H4 - chemiluminescence 30m Ozone flux (30-min avg) 1992-2002
slow CO2 IR absorbance 30,24,18,12,6,
CO2 concentration along a vertical profile
(8 levels @ 2/hr)
slow O3 UV absorbance 30,24,18,12,6,
3,1,0.05 m
O3 concentration along a vertical profile
(8 levels @ 2/hr)
slow NOx (Photolysis, O3-chemiluminescence) 30,24,18,12,6,
NO, NO2 concentration along a vertical profile (8 levels @ 2/hr) 1992-2002 and intermittently since
Thermistor, thin-film capacitor 30,22,12,6,
Temperature and relative humidity profile (5,60-min avg) Ongoing
Thermistors surface (6 reps),
20cm, 50cm
soil temperatures (5,60-min avg)  Ongoing
PAR sensors 30,12m photosynthetically active radiation
(5,60-min avg) 
Net radiometer 30m net radiative heat flux (5,60-min avg)  Ongoing
CO Gas-filter correlation IR absorbance 30m CO concentrations (4,60-min avg) 

Dasibi 1992-2002

Teledyne 2012-present

GC-FID 30m 1CH4 concentrations(60-min avg) 1992-1994
2-Channel GC-FID 29,24m 
gradient for fluxes
C2-C6 hydrocarbon concentrations (2/hr) 1992-1995
Goldstein et al 1995
4-Channel GC-ECD (FACTS) 29m Halocarbons, N2O, CO, CH4, SF6

Barnes et al. 2003

1-channel GC 30m  PAN suspended
Tuneable Diode laser absorption spectrometer 30m NO2 and HNO3 2000
Quantum Cascade Laser absorption spectrometer 30m NO2 and HONO 2011
See also: Harvard Forest