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, hour of the day, 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 model now also takes into some account the long-term reduction in NOx concentrations due to the ongoing decrease in NOx emissions.

The method for simulating the normal concentration is our current best estimate. However as things progress this method will evolve and get refined.

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, when lockdown was enforced, the measured and modelled are similar, suggesting that the measured concentrations are 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 the measured concentrations from 23^rd March to July, which suggests that reduced emissions from traffic and industry are being seen in the measurements.

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.

Annual Average Pollutant Concentrations

The annual average concentrations for 2018 and 2019 are shown below for each site and pollutant, along with the BAU and measured averages for 2020.