Lockdown may have knock-on effect in raising urban ozone

Use of all forms of transport have fallen to a tiny fraction of their pre-lockdown levels, resulting in a sudden drop in traffic pollution across the UK. A study from the Helsinki-based Centre for Research on Energy and Clean Air published recently estimated that drops in air pollution levels across Europe have cut nitrogen dioxide pollution by 40 per cent and particulate matter pollution by 10 per cent, likely saving 11,000 lives.

An analysis of air pollution levels led by researchers at the University of Manchester and submitted to the government in response to a call for evidence has found that falls in nitrogen oxides (nitrogen dioxide and nitric oxide) vary widely across the UK.

Levels of nitrogen oxides have fallen across most of the UK during mid-March and April as the use of private vehicles and public transport has fallen during lockdown, with the level of decline ranging from 20 to 80 per cent. Manchester City Centre, for instance, has seen a 70 per cent reduction in nitrogen oxides.

The analysis also suggests that levels of nitrogen oxides have fallen less in rural areas than urban areas and that they are highest in the morning.

“There is considerable site-to-site variability, with some locations showing far less reduction than others,” said Professor Hugh Coe. “In fact, a small number of sites have even shown a modest increase, for example in parts of Edinburgh.”

“Whether this is due to changes in the number or type of vehicles now travelling in that particular area, changes in driving patterns or other causes is not clear, but the reductions are certainly not uniform.”

There appeared to be no evidence of a decrease in PM2.5 - particulate matter such as soot, dust and atmospheric aerosol particles which are associated with negative health impacts, with a particularly dangerous impact on people with heart and respiratory conditions.

“While these particles are produced by vehicles, they are also known to originate from domestic wood burning and chemical reactions involving emissions from industry and agriculture, so there has been no significant improvement in air quality in that regard,” said Coe.

Coe and his colleagues speculated that the production of ozone – which can be a very dangerous pollutant near Earth’s surface, reducing lung function and worsening respiratory disease – could become more significant in urban areas during summer under conditions with unusually low nitrogen oxides pollution. This is because photochemical ozone production could become more efficient in summer as higher temperatures increase emissions of biogenic hydrocarbon from natural sources, such as trees, significantly altering urban ozone levels.

“Observations in cities across the UK show marked decreases in nitrogen oxides but with corresponding increases in ozone during lockdown,” Coe said.

The researchers recommend that government and local authorities remain alert to the potential increase in this harmful air pollutant as the UK moves towards easing lockdown conditions.