COVID19 lockdown effects on air quality
| Authors | Hao Wu, David Carslaw, Louisa Kramer |
| Compilation date | 05 January 2021 |
| Customer | Newcastle City Council |
| Approved by | David Madle |
| Copyright | Ricardo Energy & Environment |
| EULA | http://ee.ricardo.com/cms/eula/ |
| Contract reference | Report reference |
This note has been produced for Newcastle City Council 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 through December 2020, 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.
With a reduction in NOx, we may expect to see an increase in ozone at roadside locations, as there will be limited freshly emitted NO available to scavenge ozone. Additionally, during the lockdown period, there were a number of regional ozone pollution episodes which may also have an effect on any changes observed. High ozone episodes typically occur during the summer season, when weather conditions are warm and sunny.
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 pink shading indicates the period when social distancing was first advised, and the green shading represents the period when the UK-wide Lockdown was in place. The light blue shading indicates the period after the UK-wide Lockdown, during which various restrictions on a regional level have been implemented.
Newcastle Centre
NO2
NOx
O3
Newcastle Cradlewell Roadside
NO2
NOx
Newcastle Percy Street
NO2
NOx
Newcastle Pilgrim Street
NO2
NOx
Road Traffic Data
We can also take a look at the change in road traffic volumes over the past few months. 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. The red line indicates the start of lockdown (23rd March) and shaded grey areas represent weekends.
The percentage of total motor vehicles across Great Britain dropped sharply in March reaching a minimum around mid-April. The number of vehicles on the roads then slowly increased, and by September are comparable to the vehicle volumes observed in the first week of February. Traffic volumes then slowly dropped during October followed by a sudden decrease at the start of November.
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 were similar, suggesting that the 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 the measured ones from 23rd March to July, which suggests that reduced emissions from traffic and industry are being seen in the measurements.
Newcastle Centre
NO2
NOx
O3
Newcastle Cradlewell Roadside
NO2
NOx
Newcastle Percy Street
NO2
NOx
Newcastle Pilgrim Street
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.
Newcastle Centre
NO2
NOx
O3
Newcastle Cradlewell Roadside
NO2
NOx
Newcastle Percy Street
NO2
NOx
Newcastle Pilgrim Street
NO2
NOx
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, up to the date of this report.
NO2
NOx
O3
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 |