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Model 211 Scrubberless Ozone Monitor

The World’s Most Accurate Ozone Monitor

Dual beam ozone monitor FEM with a 30-cm pathlength. Patented gas-phase titration scrubber technology used in place of a solid-phase scrubber to minimize interferences. Ideal for ambient ozone monitoring in regions that are in non-attainment for ozone.

  • Dual beam, long path (30 cm) optical bench with high precision (< 0.5 ppb)
  • EPA Federal Equivalent Method (FEM) for compliance monitoring (EQOA–0514–215)
  • “Scrubberless” for interference-free measurement of ambient ozone


  • UV-Absorbance based ozone monitor with our patented (US Patent No. 8,395,776 B2) gas-phase scrubber technology to measure ozone virtually free of interferences
  • Ideally suited for measurements of ozone in heavily polluted air where interference is likely from particulates, mercury, or VOCs
  • Combines the stability of our proven dual beam ozone monitors with our selective scrubber technology to provide accurate ozone measurements in the most polluted air
  • The enhanced optical path length of 30 cm provides a precision of better than 0.5 ppb for 10-second measurements
  • Measurement frequency of every 2 seconds
  • Designated by the EPA as a Federal Equivalent Method (FEM): EQOA–0514–215


Theory Behind the Model 211’s Gas-Phase Scrubber

Conventional ozone monitors remove ozone for the Io measurement by passing the sample air flow through a solid scrubber (e.g., hopcalite or a series of metal oxide screens). Mercury and certain aromatic compounds either adsorb or react at the solid-phase scrubber surface. This causes high ozone values by up to a few ppb on highly polluted days and can cause a region to be out of compliance with the EPA’s ozone standard.
The 2B Tech Model 211 Ozone Monitor removes interferences from all UV-absorbing compounds and mercury by using nitric oxide (NO) in a GPT (gas phase titration) reaction in place of a solid-phase scrubber according to the well known reaction:   NO + O3 → NO2 + O2

The animation above illustrates the principle of a gas-phase NO scrubber for a single-beam instrument. A small amount (~5 ppm) of nitric oxide (NO; blue) is pulsed into the air flow every 2 seconds to destroy the ozone and produce nitrogen dioxide (NO2;green) which doesn’t absorb the UV light.
When ozone (red) is present, the signal at the photodiode is reduced. When ozone is destroyed by the NO, the signal increases. The difference in measured light intensities is proportional to the ozone concentration. Because the potentially interfering species (benzene, toluene, xylenes, aldehydes, mercury, etc.; colored yellow) are always present and not modulated by NO, they do not contribute to the signal difference and thus do not interfere with the ozone measurement.
Nitric oxide can be added directly from a compressed gas cylinder or by photolysis of nitrous oxide (N2O) provided from a liquid N2O cartridge. The use of N2O as a source gas eliminates the need for maintaining cylinders of toxic NO gas. Nitrous oxide is a relatively nontoxic gas sold as a consumer product as “Whippit” cartridges for making whipped cream.
  • Interference-free measurement of ambient ozone
  • DewLine for elimination of water vapor interference, a unique feature of 2B Tech instruments
  • SD flash memory for virtually unlimited portable data storage
  • Dual beam, long path (30 cm) optical bench with high precision (< 0.5 ppb)
  • Internal long-life sample pump (20,000 hours)
  • EPA Federal Equivalent Method (FEM) for compliance monitoring (EQOA–0514–215)
  • Convenient user interface, including remote operation via RS-232 communication
  • Internal NO generator from non-toxic, non-corrosive nitrous oxide (N2O)
  • Regulator assembly (provided) can use “Whippit”-type cartridges (use either 16-gram cartridge for 8 hours of operation, or 8-gram cartridge for 4 hours of operation)
  • Model 211 Scrubberless Ozone Monitor
  • 2.5A power adapter
  • Serial port cable (9PinF-9PinF)
  • External ozone scrubber
  • Hex ball driver (7/64″)
  • N2O regulator assembly
  • Extra o-rings for regulator assembly
  • SD card and SD card reader
  • USB flash drive preloaded with operation manual and other useful documentation
  • NIST-traceable calibration certificate
  • External lithium-ion battery (not included with standard purchase)
  • External particle filter
  • Lamp heater upgrade available for outdoor operation in extreme environments
  • Bluetooth upgrade for wireless data transmission
  • 4-20 mA analog current output upgrade available
  • Serial-to-USB adapter (not included with standard purchase)
  • High-altitude upgrade (~25 km)
  • T. Johnson, J. Capel and W. Ollison (2014) Journal of the Air and Waste Management Association 64, 360-371, “Measurement of microenvironmental ozone concentrations in Durham, North Carolina, using a 2B Technologies 205 Federal Equivalent Method monitor and interference-free 2B Technologies 211 monitor.”
  • Russell Long (2015) “Evaluation and Proposal of a New Federal Reference Method for Ozone: Nitric Oxide Chemiluminescence,” Briefing for NACAA, Office of Research and Development, National Exposure Research Laboratory, Environmental Protection Agency, January 14, 2015.
  • John Birks, Peter Andersen, Craig Williford and Andrew Turnipseed (2015) “2B Tech Model 211 Scrubberless Ozone Monitor for Interference-Free Measurements,” Air Monitoring Conference, Region 4, Environmental Protection Agency, Chattanooga, Tennessee, March 18, 2015.
  • Quantifying Wintertime O3 and NOx Formation with Relevance Vector Machines, D.A. Olson, T.P. Riedel, J.H. Offenberg, M. Lewandowski, R. Long and T.E. Kleindienst, Atmospheric Environment (2021) 259, 118538. ScienceDirect


Spec Sheet
Measurement Principle UV Absorbance at 254 nm; Dual Beam
Federal Equivalent Method (FEM) Yes, 0-500 ppb for 20-30°C: EQOA–0514–215
Ozone Scrubber Technology Gas-phase scrubber using NO, provided directly or by photolysis of N2O (supplied by either compressed gas cylinder or portable liquid N2O cartridge)
Linear Dynamic Range 0-2,000 ppb (0-2 ppm)
Resolution 0.1 ppb
Precision (1σ rms noise) Greater of 0.5 ppb or 1% of reading for 10-s average
Accuracy Greater of 1.0 ppb or 2% of reading
Limit of Detection (2σ) 1.0 ppb for 10-s average
NIST-Traceable Calibration Yes
Flow Rate (nominal) ~2 Liter/min
Flow Rate Requirement >1.2 L/min
Baseline Drift < 1 ppb/day
< 3 ppb/year
Sensitivity Drift < 0.5%/day
< 3%/year
Response Time, 100% of Step Change 20 s for 10-s averaging; 4 s for 2-s measurement
Measurement and Averaging Times 2 s, 10 s, 1 min, 5 min, 1 hr
Internal Data Logger Capacity 16,383 lines (10-s avg. = 1.9 days; 1-min avg = 11 days; 5-min avg = 1.9 mo; 1-hr avg = 1.9 yr)
SD Card Logger Capacity Minimum 2 GB (> 5-year capacity for 10-s measurement mode)
Ozone Units ppb, ppm, pphm, µg m-3, mg m-3
Pressure Units mbar, torr
Temperature Units °C, K
T and P Corrected Yes
Operating Temperature Range 10 to 50 °C
Operating Altitude Range ~0-13.5 km (150-1,013 mbar) standard; optional ~0-25 km (30-1,013 mbar) with upgraded pressure sensor
Power Requirement;
Supplied by Battery or 110/220 VAC Power Pack
11-14 V dc or 120/240 V ac, 1 A at 12 V, 12 watt
Size Rackmount: 17″ w x 14.5″ d x 5.5″ h (43 x 37 x 14 cm)
Weight 14.7 lb (6.7 kg)
Data Outputs RS232, SD data card, LCD, 0-2.5 V Analog, optional 4-20 mA Analog
Data Transfer Baud Rates 2400, 4800, 19200
Output Ranges User-Defined Scaling Factor in Menu
DewLine™ Yes
Long-Life Pump Yes, 20,000 hr
Flow Meter Yes
Options Battery; External Particle Filter; Bluetooth; External Serial-to-USB Adapter; 4-20 mA Current Output; Lamp Heater; High-Altitude Upgrade, ~0-25 km (pressure sensor)


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