Assessing the drop in London road traffic emissions during COVID-19 restrictions
The restrictions imposed by the UK Government to limit the spread of COVID-19 have led to unprecedented reductions in the volume of road traffic; the latest Government figures suggest that vehicle use has dropped to 40% of normal levels. Discerning the impact of this on air quality in our towns and cities is difficult however, because air quality is heavily dependent on the weather. In typical windy UK conditions, poor air is rapidly mixed with cleaner air from above, so only those nearest to the busiest roads experience poor air quality. However, when the wind is light, as it was during the fine sunny weather that immediately followed the imposition of COVID-19 restrictions on 23rd March, traffic pollution emitted into stagnant air builds up, leading to more widespread poor air quality. So how can we tease out the impact of the known recent reduction in vehicle use from measured air quality data?
Scientists at CERC have analysed recent measured air quality data from across London using a mathematical technique (Carruthers et al., 2020) that combines measured air pollution concentration data with CERC's ADMS-Urban air quality model to derive an estimate of the underlying road traffic emissions that caused the measured concentrations. The figure shows how measured concentrations and derived emissions have changed since the 16th March, when the Government first issued social distancing advice (a high-resolution figure is here). The derived emissions first show a marked drop over the weekend of 21st/22nd March, which coincides with the announcement on the evening of Friday 20th March that bars and restaurants should close. Monday 23rd March saw a widely-reported rush hour in London; this can be seen in both the measured concentrations and the derived emissions. The impact of the restrictions announced in the evening of 23rd March can be seen in the derived emissions in the following days, even though measured concentrations continued to rise through Wednesday, due to the sunny and still weather conditions. The lowest derived emissions are seen on the weekend of 28th/29th March, averaging 24% of pre-restriction levels. Weekday derived emissions of consistently around 40% of pre-restriction levels contrasts with a second peak in measured concentrations on 1st April, again due to still weather conditions. From today, CERC's air quality forecasting services for London and the South-East have been adjusted to include this temporary reduction in road traffic.
By combining air quality monitoring with dispersion modelling, CERC scientists have been able to show that as well as limiting the spread of COVID-19, the Government restrictions are leading to a reduction in traffic emissions in London. In more normal times in future, this type of analysis can help uncover the true impact of emissions-reduction strategies on local air quality.