How have surface NO2 concentrations changed as a result of the UK’s COVID-19 travel restrictions?

Restrictions as a result of the COVID-19 pandemic have led to fewer vehicles on UK roads. Since fuel combustion is responsible for a large fraction of UK emissions it is expected that surface NO 2 concentrations would reduce as a result. However, over parts of the UK, surface NO 2 concentrations have increased following the implementation of travel restrictions. NO 2 measurements from 142 Automatic Urban and Rural Network sites are combined with meteorological data from the Met Office high-resolution weather prediction model to build site specific models. These models predict NO 2 concentrations given no change in emissions. It is found that both meteorological and emission changes contribute to the observed changes in NO 2 concentrations. Given no change in emissions, changes in meteorology between pre- and post-lockdown periods would have led to a mean increase in NO 2 concentrations of +6%. Conversely, changes in emissions would have led to a mean reduction in NO 2 concentrations of -18%, resulting in the observed total change in NO 2 concentrations of −12%. However at some sites the reduction in emissions is smaller than the increase in NO 2 concentrations due to meteorology. The largest increases associated with changes in the meteorology are seen at rural sites (+20%) where NO 2 measurements are representative of large areas and thus dominated by the regional advection of secondary NO 2 from Europe. Conversely, the largest decreases associated with reduced emissions are found at urban traffic and urban background sites (−27% and −14% respectively) where NO 2 concentrations are representative of local areas and thus dominated by local reduction in emissions from vehicles. As lockdown measures are relaxed, NO 2 concentrations are likely to return to pre-COVID levels, but these results demonstrate that changes in our behaviour can result in positive impacts on air quality and illustrate the effectiveness of travel-reducing strategies in urban areas.