Smog over Los Angeles. Credit: Mateusz Kudła

In this guest post, we sit down with atmospheric chemist, air sensor expert and RADICAL project advisor Professor Matthew Johnson. Matthew is Professor of Atmospheric Chemistry at the University of Copenhagen, specialising in chemical mechanisms using kinetics, spectroscopy and theory. He is also Chief Science Officer for a number of air pollution monitoring and mitigation companies including AirScape, Devlabs, Rensair, Ambient Carbon, and Luper Tech, several of which he cofounded.

We are delighted to have Matthew as part of the RADICAL team, where his independent advisor role continues to provide an invaluable steer for our electronic gas sensor development.

What’s your research background and how did you get interested in atmospheric science?

I started out working on the laser spectroscopy of clusters formed in molecular beams, as my PhD research, working with Mitchio Okumura at Caltech. These were super interesting systems like SiH₇⁺, AlAr_n, NO⁺(H₂O)_n, I^–(H₂O), and H⁺(HNO₃)(H₂O)_n. These small systems turn out to be very important in atmospheric chemistry, e.g. for new particle formation. It was a wonderful scientific environment. Los Angeles is the Capitol City of air pollution because of the attention the severe air pollution received in the 60s, 70s and 80s, and a lot of the fundamental advances in the field were made at Caltech in the LA suburb of Pasadena. Our lab was across the hall from Rudy Marcus and Ahmed Zewail, who both won the Nobel Prize in Chemistry.

I was interested in atmospheric chemistry because here was something that really mattered in people’s lives, and it was an opportunity to put the theories of reaction dynamics and spectroscopy to good use. Having a real-world application helped me to dig deeper into the abstract theories of the field; it really motivated me. After Caltech I started working on molecules related to stratospheric ozone depletion at the electron storage ring in Lund, Sweden, with support from the Fulbright program and the European Commission, and then started working at the University of Copenhagen where I have been for 25 years now.

What are your current research interests and what science questions excite you?

That’s a broad question! I really believe in people and the absolute best part of the job is having the chance to work with all of the talented students that come through. They teach me a lot and I have to stretch to keep them occupied. Science and research are sometimes misconstrued as being somehow cold and anti-social but my experience is the exact opposite. The good work comes out of working together with people, talking to colleagues, helping students. It is a very engaging and human activity. There is a lot of concern about air pollution and climate change right now and that is what drives our research. We develop new tools, both in the lab and in the startup companies. These could be things like the world’s largest network of pollution sensors that we installed in the Borough of Camden in London – 225 sensors that record air quality once a minute. We have used the sensors to monitor air quality in the Copenhagen and London underground systems and are working together with University College Cork and the Irish Environmental Protection Agency to measure air quality in Cork and at the Port of Dublin. On the fundamental science side, we showed that there is a large isotope effect in the photolysis of carbon dioxide which is the basis of photochemistry in the atmosphere of Mars. The isotope effect that we predicted, using quantum chemistry and laboratory experiments, has just been found in samples of organic matter on Mars. So, you never know where the research is going to go – it is a matter of creating it together with your friends.

Why are atmospheric radicals important?

The atmosphere is far from equilibrium. It contains fuel – hydrocarbon vapors – and oxidants – the main one being atmospheric oxygen. The whole thing should just burn up.  It doesn’t, but it’s on the edge. Radicals are magical little species that can cause a cascade of spontaneous reactions, a low temperature combustion, a chain reaction. These are the reactions that remove pollution from the atmosphere, so that’s good, but these are the reactions that also make some byproducts we don’t like such as atmospheric particles, acid rain, and tropospheric ozone.

The radicals are the key to understanding it all – why the reactions occur, how quickly, which products, all of it. However, that’s not all. Radicals are very good at reacting, which means that they are also good at reacting with you. Atmospheric oxidants cause oxidative stress in the body, inflammation, cancer and hypertension. They are also a cofactor in diabetes and aging and a host of other negative effects.

How has the field of air quality monitoring evolved during your career?

Technologists talk about Moore’s Law. In 1965 Gordon Moore, co-founder of Fairchild Semiconductor and Intel, observed that the number of components on a chip doubled every two years. Even 58 years later, Moore is pretty much exactly right. Sadly, he only just recently passed away at the age of 94 (January 3, 1929 – March 24, 2023). There have been similar trends in related areas, for example the cost per Watt of a solar panel in 1975 was around $100, today it is around 25 cents. Low cost sensors are causing a paradigm shift in air quality monitoring. A leading group of air pollution researchers says that it will be impossible to monitor compliance with the new EC air pollution exposure limits without using low cost sensors. Such networks of pollution sensors would have been beyond the reach of governments – due to the cost of the components and the lack of communication and cloud computing – just a decade ago.

What do you hope the future of air quality monitoring will look like?

We should all be very happy about the improvements in air quality that have been achieved since the 1990s when I was starting out. NOx is down, SO₂ is down, ozone is down. Greenhouse gases are up, that is true, but significant improvements have been made for regional air pollution. However, most people do not spend their time in ‘regional background’ air. People spend most of their time in the human environment, inside buildings, inside vehicles, and in urban areas. This means they are exposed to much higher pollution levels that you might expect. One implication is that there is a significant monitoring task at hand, one that can only be achieved using low cost sensors. The second is that, since pollution exposure occurs in the local environment, people themselves can often have a significant impact on their own pollution exposures. There are specific activities that can cause a majority of a person’s daily exposure including smoking, cooking, and transportation. I think the awareness of air quality is growing every day, leading to smarter designs and behaviours. For example, many buildings have a single ventilation setting that ensures good air quality for maximum occupancy. That’s good in terms of air quality but is bad in terms of energy use because it drives up the bill for heating and cooling. By using sensors we believe it will be possible to install smart ventilation that will achieve both goals – good indoor air quality and reduced energy bills.

Tell us about your work with AirScape

AirScape is giving us a high-resolution map of air pollution in London at one minute time intervals. The level of detail is incredible. We see hot spots when polluting vehicles pass the monitors. One day last June an asphalt roof caught on fire and we could see a huge plume of particles grow across the city, blown by the wind. During the London Tube strike in March we saw particulate matter (PM), NO₂ and CO₂ increase due to increase use of surface transportation. During the Tube strike in August we saw pollution go down because people stayed home. The bigger picture is that AirScape is providing information that is being used by schools and hospitals to involve citizens in improving air quality. We can see how specific events in the city, like changes traffic flows, impact air quality. We’d like to see AirScape used by governments worldwide as a tool to improve air quality.

What do you find exciting about the RADICAL project?

Progress is not inevitable – it only happens when people take an interest.

RADICAL has succeeded in bringing together a diverse group of talented researchers who are interested in creating the technology for the next generation of air quality sensors. These new sensors will be able to see things that we couldn’t see before. Radicals are the key to atmospheric reactivity so just imagine that you had a thermometer that would say, Here! It’s happening here! It might mean that there’s a high voltage wire generating ozone or a leak in a storage flask.

We are learning every day that air quality affects mood, judgement and productivity, in addition to the long-term health affects like lung cancer and heart disease, and so the ultimate result is that we can improve our local environment.

And, as a research geek, I really like seeing the strategies that can be used to convert a chemical interaction into an electrical signal. The first generation of sensors was known to be sensitive but not terribly selective – there was often crosstalk with multiple gases giving a response. By using chemical strategies, similar to how the nose is able to distinguish specific smells using chemical recognition, RADICAL hopes to achieve both sensitivity and selectivity at the same time.

Find out more:

Matthew Johnson’s publications and media articles

How atmospheric radicals transform the air – RADICAL blog post by Professor John Wenger, University College Cork

Tuning the RADICAL sensor to detect free radicals – RADICAL blog post by Professor Victor Chechik

Find our latest RADICAL research on our open access RADICAL project repository on Zenodo

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