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York study: Less traffic in first UK lockdown reduced NO2 pollution but caused increase in surface ozone

Less traffic on the roads during the first COVID-19 lockdown in the UK led to a reduction in air pollution but may have caused potentially damaging surface ozone levels to rise, according to a new study led by researchers at the University of York.

The open-access study, published in the journal Atmospheric Chemistry and Physics, shows levels of nitrogen dioxide (NO2) down on average across the UK by 42%—but surface ozone (O3) increased by 11% on average. Surface, or ground-level ozone, can trigger a variety of health problems, particularly for children, the elderly, and people of all ages who have lung diseases such as asthma.


Mean relative change of deseasonalized UK values of (a) NO2 and (b) O3 for all urban background and traffic sites from 2015 to 2020, with the mean 2015–2019 trend superimposed. Data from 2020 are shown in orange, with the red dashed line denoting the start of the lockdown on 23 March 2020. The 25–75% range is shown by the shaded area. Lee et al.

The report concludes that if the COVID-19 lockdown is taken as an example of how air quality will respond to future reductions in vehicle emissions—with more electric vehicles being introduced—it serves as a warning that the problem of O3 must also be considered.

Professor James Lee from the York Department of Chemistry and the National Center for Atmospheric Science said during the first lockdown levels of O3 were the worst in the South of England.

The problem is being created by the change in chemistry between NOx and O3. The main reason is the change in the nitrogen dioxide levels but the warm sunny weather in April and May also increased the ozone level. As a result we found UV radiation across the UK was higher in 2020 compared to previous years, with the largest increases in southern England.

London, Chilton in Oxfordshire and Camborne in Cambridgeshire saw increases of around 50% compared to previous years, with Glasgow and Inverness showing smaller increases of around 30%.

These results are a cautionary tale. As well as looking at how we reduce levels of nitrogen dioxide by cutting diesel and petrol emissions, we also need to keep an eye on what is happening with ozone so we don’t create other forms of pollution dangerous to human health.

—Professor Lee

The report says nitrogen oxide reductions in China have also led to increases in O3 and air quality abatement strategies are being developed in order to offset the problem. This can be achieved by controlling volatile organic compounds (VOCs)—gases emitted into the air from products or processes of industry and other man-made activity.

Our research shows it will be vital to control man-made VOCs to avoid any health gains made by the reduction of NO2 being offset by O3 increases.

Data was collected from 175 Automatic Urban and Rural Network (AURN) traffic monitoring sites across the UK between 23 March and 31 May 2020 and compared with figures from the previous five years.


  • Lee, J. D., Drysdale, W. S., Finch, D. P., Wilde, S. E., and Palmer, P. I. (2020) “UK surface NO2 levels dropped by 42% during the COVID-19 lockdown: impact on surface O3,” Atmos. Chem. Phys., 20, 15743–15759 doi: 10.5194/acp-20-15743-2020


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