COVID19 lockdown effects on air quality

Time Series

First let’s look at the daily concentrations of the pollutants since January 2020 and get a general sense of the trend of pollutant concentrations. The dark green shading represents periods when either a UK-wide or England-wide Lockdown was in place. The light blue shading indicates the periods during which various restrictions on a regional level have been implemented.

Simulated Normal Concentrations

A perennial problem when comparing air pollution data before and after certain interventions is the effect of the weather. To counteract the effect of weather, we use a model to simulate pollutant concentrations using wind speed/direction, temperature, weekday and Julian day as predictors. This model is then used to predict concentrations from March 2020, which can be seen as the normal concentrations expected if no intervention had taken place.

The simulated and measured concentrations are shown below. The orange line represents the modelled concentrations, and the purple line represents the measured concentrations. Before 23^rd March 2020, when lockdown was enforced, measured and modelled concetrations were similar, suggesting that measured concentrations were comparable to the usual levels at this time of the year and under normal business activities. The modelled (i.e. ‘business as usual’) NO₂ and NOx concentrations are predominately higher than measured concentrations during the first lockdown, which suggests that reduced emissions from traffic and industry are being seen in the measurements.

It should be noted that much of the analysis contained in this report provides an estimate of the deviation in air pollutant concentrations from what would have been expected if Covid-19 had not happened. As the time increases from the initial lockdown in March 2020, the calculated deviation becomes more uncertain. Also, concentrations of many pollutants such as NOx and NO₂ would have been expected to decrease anyway, due to ongoing improvements in the vehicle fleet. We have assumed an ongoing 5% annual NOx concentration reduction due to vehicle fleet improvements. As such, there is greater certainty in the changes shown close to the initial lockdown in March 2020 compared with concentrations much later in time. Nevertheless, the results should provide a reasonable overall indication of changes relative to a non-Covid-19 scenario.

Annual Average Pollutant Concentrations

Annual average measured pollutant concentrations for 2018 to 2021 (for each site and pollutant) are shown below. Also shown are the annual average BAU concentrations for 2020 and 2021. These plots are interactive: by clicking on the legend colours, data from different years can be displayed.

Monthly Change in Pollutant Concentrations

To put the magnitude of the decrease into perspective, the monthly mean difference in measured and modelled “business as usual” (BAU) concentrations are shown below. Pink bars represent measurements greater than modelled concentrations and green bars represent measurements lower than modelled concentrations.

Monthly Average Pollutant Concentrations

The plots below are box and whisker plots to show the distribution in monthly concentrations for each pollutant measured. The boxes represent the interquartile range (IQR) ranging from the lower quartile (25^th percentile) to the upper quartile (75^th percentile). The horizontal line within each box represents the median (50^th percentile). The whiskers extend to the maximum and minimum values within the median ± (1.5 x IQR). Values outside the median ± (1.5 x IQR) are not shown here.

These plots are interactive: by clicking on the legend colours, data from different years can be displayed.