COVID19 lockdown effects on air quality
| Authors | Hao Wu, David Carslaw, Louisa Kramer |
| Compilation date | 04 August 2021 |
| Customer | States of Jersey |
| Approved by | David Madle |
| Copyright | Ricardo Energy & Environment |
| EULA | http://ee.ricardo.com/cms/eula/ |
| Contract reference | Report reference |
This report has been produced for States of Jersey and examines the impact of lockdown measures on evolving ambient air quality data. This analysis focuses on NOx, NO2 (and ozone data where measured) from January 2020 through July 2021, and uses proven modelling techniques to discount the influence of weather on ambient pollutant concentrations. Details of this methodology can be found in this blog. At roadside locations NOx concentrations will be closely linked to primary emissions and should show the direct impact of reduced local traffic on air pollution. NO2 will be from a mixture of primary emissions and secondary chemical reactions but should again be closely linked to local traffic reduction.
N.B. Both measured and modelled data reported here are provisional pending full QA/QC processes.
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.
Jersey Halkett Place Roadside
NO2
NOx
Road Traffic Data
We can also take a look at the change in road traffic volumes. The Department for Transport (DfT) publishes statistics (~ weekly) on estimated transport use during the COVID-19 pandemic. Details on the methodology and full statistics can be found here: (https://www.gov.uk/government/statistics/transport-use-during-the-coronavirus-covid-19-pandemic). The blue line in the figure below shows the estimated daily percentage of all motor vehicles, with respect to the equivalent day in the first week of February, across Great Britain, since 1st March 2020. Grey shaded areas represent weekends.
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 23rd 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 23rd March to July, which suggests that reduced emissions from traffic and industry are being seen in the measurements.
Jersey Halkett Place Roadside
NO2
NOx
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.
Jersey Halkett Place Roadside
NO2
NOx
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 (25th percentile) to the upper quartile (75th percentile). The horizontal line within each box represents the median (50th 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.
Jersey Halkett Place Roadside
NOx
NO2
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.
NO2
NOx
For further information, specific questions or bespoke analysis, please use the contact below.
| Name | David Madle |
| Address | Ricardo Energy & Environment, Gemini Building, Harwell, Didcot, OX11 0QR, UK |
| Telephone | +44 1235 753257 |
| david.madle@ricardo.com |