COVID19 lockdown effects on air quality

Time Series

First let’s look at the daily concentrations of NOx and NO2 since March 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.

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 NOx and NO₂ concentrations from 2018-01-01 to 2020-02-29 using wind speed/direction, temperature, hour of the day, weekday and Julian day as predictors. This model is then used to predict NOx and NO₂ concentrations from March 2020, which can be seen as the normal NOx and NO₂ concentrations expected if no intervention had taken place.

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

The differences between simulated and measured NOx and NO₂ concentrations are shown below. The coloured vertical segments represent the difference between modelled and measured NOx and 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 and 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 and 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.