Reducing uncertainties in OH radical measurements using an absolute optical technique

Atmospheric oxidising capacity determines the rate at which trace species are removed from the atmosphere, thus controlling their impacts on air quality and climate. The dominant oxidising agent in the atmosphere is the hydroxyl radical (OH), the concentration of which determines the lifetimes of key species including methane (CH4), volatile organic compounds (VOCs), and NOx (NOx = NO + NO2), whilst also controlling the rate at which secondary pollutants such as ozone (O3) and secondary organic aerosol (SOA) are formed.

Understanding the concentrations and behaviour of OH in the atmosphere is thus critical to understanding the lifetimes of many trace species which impact air quality and climate.

The high reactivity of OH leads to significant challenges in its measurement, with techniques used to measure atmospheric OH such as laser-induced fluorescence (LIF) spectroscopy and chemical ionisation mass spectrometry (CIMS) providing the required sensitivity and specificity, but requiring calibration. Calibration can be achieved by a number of methods, but each method involves a number of steps, with the uncertainties associated with each step propagating through to uncertainties in OH radical observations.

Even the most accurate calibration method is typically associated with uncertainties of up to ~30 %.

In this work, we will develop the use of cavity enhanced absorption spectroscopy (CEAS) to measure OH radical concentrations in an atmospheric simulation chamber. CEAS is an absolute optical technique, requiring only knowledge of the absorption cross-sections for OH and the absorption path length to determine the concentration from the measured absorbance using the Beer-Lambert law.

The use of CEAS will significantly reduce uncertainties associated with OH radical concentrations in the simulation chamber, and offers significant advantages for chamber studies of chemical mechanisms, and for validation of calibration methods for field instruments.