An absolute calibration for gas-phase hydroxyl measurements

Area of Science:

• Atmospheric Chemistry
• Chemical Kinetics
• Instrument Calibration

Background:

• Tropospheric hydroxyl (OH) radicals are key oxidants in atmospheric chemistry.
• Accurate measurement of OH concentrations is crucial for understanding atmospheric processes.
• Existing calibration methods for OH instruments often rely on photon flux measurements with inherent uncertainties.

Purpose of the Study:

• To introduce a novel, absolute calibration method for atmospheric hydroxyl (OH) instruments.
• To establish a reliable calibration technique using steady-state OH radical concentrations generated in a flowtube reactor.
• To provide an alternative calibration approach independent of photon flux measurements.

Main Methods:

• Utilized ozone-alkene mixtures to generate steady-state hydroxyl (OH) radical concentrations within a flowtube reactor.
• Measured OH radical removal rates by varying alkene concentration to determine OH wall loss rates.
• Delivered the calibrated OH radical mixture to the sampling inlet of an atmospheric OH measurement instrument.

Main Results:

• Achieved a calibration precision of +/-8% (1-sigma) based on reproducibility across various ozone concentrations.
• Demonstrated a calibration method that bypasses the need for photon flux measurements or lamp-dependent absorption coefficients.
• The accuracy of the calibration is currently limited to +/-43% due to uncertainties in kinetic rate coefficients and OH yield, which are areas for future improvement.

Conclusions:

• The developed method provides a robust and reproducible means of calibrating atmospheric OH instruments.
• This technique offers an absolute calibration, reducing systematic uncertainties associated with traditional methods.
• The calibration method has been successfully tested in field experiments, indicating its practical applicability.