Authors Hao Wu and David Carslaw
Compilation date 15 April 2020
Customer The Scottish Government
Approved by Stuart Sneddon
Copyright Ricardo Energy & Environment
EULA http://ee.ricardo.com/cms/eula/

Contract reference Report reference

This note has been produced for the The Scottish Government and examines the impact of lock down measures on evolving ambient air quality data. This analysis focuses on Scottish data and uses modelling technics to discount the influence of weather on ambient pollutant concentrations. Details of this methodology can be found in this blog. This analysis focuses on NOx/NO2 as it’s a primary pollutant that should show the direct impact due to restricted traffic/industry activities. Other pollutants such as PM and O3 are more complicated due to their secondary formation, which deserves special analysis on their own. Data reported in this note remain provisional pending full QA/QC processes.

Time series

First let’s look at the daily concentrations of NOx and NO2 since the beginning of 2020 and get a general sense of the trend of pollutant concentrations. The pink and green shadings indicate the periods when social distancing was advised and when lockdown was enforced.

Aberdeen Union Street Roadside

Dundee Lochee Road

Fife Dunfermline

Edinburgh St John’s Road

Glasgow Kerbside

Inverness

Perth Atholl Street

Simulated Normal Concentrations

A perennial problem when comparing air pollution data before and after certain intervention is the effect of the weather. Air pollutant concentrations are not only influenced by the anthropogenic activities but also equally by the weather condition. To counteract the effect of weather, a model is built to simulate NOx concentrations from 2018-01-01 to 2020-02-29 using wind speed/direction, temperature, hour of the day, weekday and Julian day. This model is then used to predict NOx concentrations from March 2020, which can be seen as the normal NOx concentration expected if no intervention had taken place.

The differences between simulated and measured NOx/NO2 concentrations are shown below. The coloured vertical segments represent the difference between modelled and measured NOx/NO2. Purple lines mean the model overestimated the concentrations, whereas orange means the model underestimated. Before 23rd March, when lockdown was enforced, there was roughly the same amount of overestimation and underestimation by the model, suggesting that the measured NOx/NO2 was comparable to the usual levels at this time of the year and under normal business activities. The modelled (i.e. ‘business as usual’) NOx/NO2 concentrations are predominately higher than the measured ones from 23rd March, which suggests that reduced emissions from traffic and industry are being seen in the measurements.

Aberdeen Union Street Roadside

NO2

NOx

Dundee Lochee Road

NO2

NOx

Fife Dunfermline

NO2

NOx

Edinburgh St John’s Road

NO2

NOx

Glasgow Kerbside

NO2

NOx

Inverness

NO2

NOx

Perth Atholl Street

NO2

NOx

Cusum plot

Another way to look at how much change has taken place is to plot the cummulative sum (cusum) of the modelled and measured difference. By accumulating these differences, the effect is to amplify the changes and also provide an indication of when changes may have occurred. The cusum lines stay around zero for most sites before 16th Marching. However, all of the sites show a consistent decrease after the lockdown started.

Mean NOx/NO2 reduction

To put the magnitude of the decrease into perspective, the mean of difference between modelled and measured NOx (i.e. NOx reduction) after lockdown is shown below. The percent labelling is the calculated as the mean of differences divided by the mean of modelled ‘normal’ concentrations. In general the roadside sites show larger relative decrease compared to background sites.

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Name Stuart Sneddon
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