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Radical species play a key role in the atmosphere, yet several decades after they were first measured, quantifying their ambient concentration remains a significant challenge. A recently published review article on methods to measure radicals in the atmosphere (“Techniques for measuring indoor radicals and radical precursors” by Elena Gómez Alvarez et al. 2022) provides a good assessment of the existing methods for radical detection and how our RADICAL project aims to complement current techniques.

Why does radical chemistry matter in the atmosphere?

Every year, millions of tonnes of gases are injected into the atmosphere from both natural and human activities. Fortunately, their accumulation rate is counterbalanced by the atmosphere’s ability to cleanse itself of most of these so-called trace gases. The hydroxyl radical (OH) controls the oxidizing capacity of the atmosphere and thus, profoundly affects the removal rate of pollutants and reactive greenhouse gases. OH oxidizes most of the trace gases in the troposphere into water-soluble products that are washed out by rain and snow and deposited directly onto the earth’s surface. Due its cleansing nature, the OH radical has the nickname, “detergent of the atmosphere”.

Radical levels in the environment are extremely low because radicals react quickly with a wide range of gases. And this is what makes radical measurement both interesting and challenging. We are talking about concentrations lower than one part per trillion (ppt), or the equivalent of detecting a single drop of water in 20 Olympic-sized swimming pools.

Because OH controls the lifetime of trace gases and their impact on air quality and climate, it is not merely an academic question of whether OH concentrations have changed. Unexpected consequences could arise if our atmosphere’s pervasive detergent cannot remove a key pollutant or greenhouse gas or if OH concentrations are significantly depleted. It is thus challenging—but vital—to accurately determine the spatial and temporal variability of tropospheric OH.

Read more: How atmospheric radicals transform the air by John Wenger, UCC

Ground measurement: accuracy and precision

Although OH can be measured quite accurately, radical measurements are performed by only a handful of research institutes. Gómez Alvarez and co-workers counted 14 laboratories with the capacity to measure OH radicals at atmospheric levels worldwide.

The two main techniques for OH radical measurement are based on the Laser Induced Fluorescence (LIF) detection of OH using the FAGE (Fluorescence Assay by Gas Expansion) technique or the chemical conversion of OH to isotopically labelled sulfuric acid followed by Chemical Ionisation Mass Spectrometry (CIMS) detection.

OH CIMS instrument developed and deployed by Dr Kukui Alexander, Researcher at LPC2E/CNRS. Credit: Adrien Gandolfo, 2016.

Both instruments require a high level of operational knowledge and both are about the size of a refrigerator. In addition, The LIF-FAGE method uses an intense laser source, and the CIMS method needs a high vacuum. Both techniques are costly, complex to operate, and difficult to deploy in the field, limiting the range and type of radical measurements that can be made.

Mathematical modelling: a tool for decision making and for revealing knowledge gap

Mathematical modelling provides a complementary, less expensive approach to evaluating OH levels in the atmosphere. Modelling different scenarios can also predict past and future changes in OH concentrations, in order to guide policy decisions. While very practical, it should be noted that modelling results depend on both the  current knowledge of atmospheric chemical mechanisms and an accurate representation of gaseous emission inventories. For instance, when the Atmospheric Chemistry-Climate Model Intercomparison Project (ACCMIP) compared various global models, it found that these models “…disagree ± 30 % in mean OH and in its changes from the preindustrial to late 21st century, even when forced with identical anthropogenic emissions.” (see Murray et al., 2021).

The authors demonstrated that “intermodel differences in OH are best explained by disparate implementations of chemical and physical processes that affect reactive oxides of nitrogen and organic chemical species.” The latter can be overcome by constraining the model with a detailed set of parameters for radical precursors and sinks. Assuming no reactant is missing, it provides an excellent comparison point when used alongside direct OH measurements. Thus, mathematical modelling can reveal unknown or unaccounted mechanisms that support atmospheric oxidizing capacity (see Lelieveld et al., 2008).

Satellite observation: mapping the globe

It is also possible to infer the global OH radical concentration using a top-down approach. Historically, this was achieved through budget closure or by inverting the measurements of a long-lived trace gas (such as methyl chloroform) with known sources and sinks (primarily OH). However, this relies on a few long-lived species concentration measurements across the globe (e.g. approximately 10 measurement sites).

Annual mean OH concentrations near the earth’s surface, calculated with a chemistry-transport model (Lelieveld et al., 2002). The units are 106 radicals/cm3 . These results refer to OH in the boundary layer at low and middle latitudes where mean OH concentrations exceed 105 radicals/cm3

In addition, Li and colleagues remind us that “such models inevitably contain in-built assumptions, including uncertain emissions inventories as well as transport and deposition parameterizations that may differ from real world conditions”.

Recent satellite observations push back some initial limitations and propose new insights into spatial and temporal variability of OH concentrations (e.g. Wolfe et al., 2019), using formaldehyde as a tracer gas. Satellites can now map the planet’s whole surface to provide radical measurements.

E-nose: a new technique for measuring radicals

In our RADICAL project, we are producing a new sensor technology that will enable a fast, direct, and simple way to detect radicals based on the electronic sensor “E-nose” principle. This small sensor can provide new insight into the spatial and temporal variability of OH in the atmosphere. Large areas with extensive spatial coverage can be mapped within a sensor network at a reasonable cost, time, and personal investment. They will help verify the accuracy of our models and support proxy assessment.

Ultimately in RADICAL, we are working towards making radical measurements both more routine and widespread to support ongoing efforts in atmospheric research related to air quality and climate.

About the author: Dr Adrien Gandolfo is a post-doctoral researcher at University College Cork and a member of the Centre for Research into Atmospheric Chemistry. Adrien is an experimentalist in atmospheric chemistry with a research interest in nitrous acid (HONO) formation (a major OH radical precursor) in indoor and forested environments. Within the RADICAL project, he conducts the evaluation and validation tests of sensors in the new Irish Atmospheric Simulation Chamber.

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