Introduction

On the 23rd of March, the UK entered a partial lockdown. Although precisely what “lockdown” meant varied somewhat between England, Northern Ireland, Scotland and Wales there was an immediate and sharp reduction in road traffic. As the weeks have progressed road traffic, particularly private vehicle and goods vehicles has inched back towards “normal”. Since poor air quality has been linked with increased mortality rates from coronavirus and Black, Asian and Minority Ethnic (BAME) communities are known to be more heavily impacted by both coronavirus and air pollution, pressure has increased on the UK government to tackle air pollution and to reverse delays to clean air schemes that have been delayed due to the pandemic. The Westminster government has been criticised for failing to even consider the role of air pollution in the increased risk of death for BAME people from coronavirus. The urgency is compounded by the looming threat of Brexit, which will remove the UK from the EU’s regulatory oversight, potentially leaving room for reduced air quality standards.

The UK is well placed to understand national patterns of air pollution over time, having a network of automatic monitoring sites, the Automatic Urban and Rural Monitoring Network (AURN), that have measured a range of pollutants hourly across 228 monitoring sites since 1973. Data from the AURN forms the basis of annual publications from the UK government on air quality. Unfortunately, the timing of the publication means that the most recent version did not contain observations taken during the lockdown period. Observations from selected UK cities have revealed falls in pollution during the lockdown period, leaving open the question of what the pattern has been elsewhere.

AURN data covers pollutants generated directly and indirectly by motor traffic; particulate matter (PM2.5/PM10), sulphur dioxide, carbon monoxide, nitrogen dioxide, nitric oxide and ozone. Unfortunately, the number of monitoring stations recording some pollutants, particularly certain measures of particulate matter levels has reduced steeply since 2018 for reasons that are not clear. Stations that continue to take these measurements are concentrated in cities, reducing their representativeness. Furthermore, data collection itself appears to have been affected by the pandemic, with sharp reductions in the numbers of stations monitoring some pollutants since the start of April 2020. This may be due to maintenance visits to monitoring stations being classed as “non-essential” travel under lockdown rules.

Here I’ve compared the median daily measurement from January 2020 to May 2020 at all monitoring sites for which valid data are available to the four year average from 2015 - 2019 for January to May for pollutants measured by the AURN.

Particulate matter (PM2.5/PM10)

The primary source of particulate matter in European cities is road traffic emissions, and diesel vehicles in particular. Particles of extremely small size in the air (2.5µm and 10 µm) are more readily inhaled than larger particles and, once inhaled, pose health risks due to their deep penetration into the lungs. Particularly at risk are children, the elderly and those with pre-existing heart or lung problems.

Both PM2.5 and PM10 fell after lockdown was announced and reduced to levels below the 4 year average for the period from the start of lockdown to the ed of May. There is a hint that PM10 levels are beginning to rise again towards the end of May as the lockdown eases.

Nitric oxide and nitrogen dioxide

Nitric oxide and nitrogen dioxide are formed directly by combustion for transport and power generation, and nitrogen dioxide is also produced secondarily by oxidisation of nitric oxide by ozone in the atmosphere. It can have both acute and chronic effects on lung function and impacts children and those with respiratory conditions most severely.

The reduction in nitric oxide and nitrogen dioxide levels has been more gradual than in particulate matter concentrations. An immediate effect of the lockdown can be seen but minimal levels are not reached until May.

Ozone

Ozone is not a direct product of emissions but rather of the interaction between nitrogen dioxide and hydrocarbons in the atmosphere. Long term exposure to high ozone levels can affect lung function growth in children and may increase asthma levels.

The number of sites providing ozone concentrations measurements dropped sharply from 74 in April 2020 to 22 in May and so there is greater variability in measurements taken in this month than previously. The remaining monitoring stations are primarily in urban areas so data from May is likely to be less representative of the UK as a whole than previous months. Nonetheless and effect of the lockdown can be seen on ozone concentrations from the end of March 2020 with levels being increased relative to the same time period in previous years. This is likely due to decreased levels of reactions between ozone and nitric oxide in the atmosphere due to reduced levels of nitric oxide.

Sulphur dioxide

The major source of sulphur dioxide in the atmosphere is power generation and atmospheric sulphur dioxide is the major source of acid rain and is associated with losses in lung function in children and those with respiratory illnesses.

Sulphur dioxide concentrations rose after the implementation of lockdown in the UK. It is not clear why this should be so as electricity demand dropped during the same period and a prolonged period of sunny and windy weather early in the lockdown meant that renewables were responsible for a high proportion of energy supplied to the national grid.

Carbon monoxide

Carbon monoxide forms as a result of incomplete combustion and in the outdoor atmosphere is primarily a result of transport emissions. It is known to be acutely toxic in high concentrations, especially indoors. There has been a long term trend of reduction in atmospheric carbon monoxide levels in the UK and levels now rarely rise above those regarded as safe.

This trend of reduction in carbon monoxide levels appears to have been reversed since the lockdown began. However, it is not clear that the increase reflects a breach of safe levels. It is worth noting that only 7 stations in the AURN monitor carbon monoxide levels and all are in urban areas, including London and that London removed its congestion charges during the lockdown period, which may have lead to an increase in traffic in these areas that skews the UK-wide data.