2B Tech Instrument Calibrations Now Implement the New Value…

The value of a basic parameter that underlies ozone measurements of many ozone monitors, including those made by 2B Tech, has recently been updated by the U.S. Environmental Protection Agency and regulatory agencies worldwide: the ozone cross section.

Absorption of UV light (wavelength 254 nanometers) has long been used to make measurements of atmospheric ozone with high precision and accuracy. A beam of 254nm light passed through a gas sample will be absorbed by ozone molecules that are present. Comparing the resulting intensity of light after the sample (I) with the intensity of the incident light (light measured with no ozone present, I₀), Beer’s Law gives the ozone concentration in the sample:  

C_O3 = [1/(path length x cross section)] x [ln (I₀/I)]

As shown in the equation, the value of the cross section is inversely related to the concentration of ozone calculated through Beer’s Law.

Until this update, the cross section value used in ozone measurements has been 1.1476 x 10^-17 cm²molecule^-1 (308.32 atm^-1cm^-1 at standard temperature and pressure) based on a 1961 study, with an estimated uncertainty in this value of 1.4%.  Since then, research advances have improved the accuracy of the ozone absorption cross section and rigorously assessed the bias and uncertainty in the value.  In 2020, an international group of stakeholders agreed to implement a globally coordinated change to the new value of 1.1329 x 10^-17 cm²molecule^-1 (304.39 atm^-1cm^-1 at STP) for surface ozone monitoring.  This value has a greatly reduced uncertainty of 0.31%.  The reduction of 1.29% in the cross section value leads to a 1.29% increase in the ozone values calculated via Beer’s Law.   

In the U.S., the new value is finalized in the ruling (88FR 70595) and required to be fully implemented by 1 January 2026.  2B Technologies has already updated its calibrations to reflect the new cross section value, through re-calibrations of our working standards carried out in early January 2025.  Our working standards are referenced to a primary working standard, which is in turn calibrated against a standard reference photometer at the National Institute of Standards and Technology (NIST).

Click on the this link to see the video developed by 2B Tech, showing Beer’s Law and how an ozone instrument works!