Early observations on the impact of the COVID-19 lockdown on air quality trends across the UK

Highlights: • Lockdown air pollutant levels across UK analysed using break-point/segment methods. • NO, NO₂ and NO_x decreased (on average) 32% to 50% at roadsides on lockdown. • O₃ concentrations increased by (on average) 20% on lockdown. • Change-points indicate lockdown not a major source of change for UK particulates. • While locked down NO, NO₂ and NO_x gradually increase as vehicles return to roads. UK government implemented national lockdown in response to COVID-19 on the 23–26 March 2020. As elsewhere in Europe and Internationally, associated restrictions initially limited individual mobility and workplace activity to essential services and travel, and significant air quality benefits were widely anticipated. Here, break-point/segment methods are applied to air pollutant time-series from the first half of 2020 to provide an independent estimate of the timings of discrete changes in NO, NO₂, NO_x, O₃, PM₁₀ and PM_2.5 time-series from Automatic Urban Rural Network (AURN) monitoring stations across the UK. NO, NO₂ and NO_x all exhibit abrupt decreases at the time the UK locked down of (on average) 7.6 to 17 μg·m⁻³ (or 32 to 50%) at Urban Traffic stations and 4 to 5.7 μg·m⁻³ (or 26 to 46%) at Urban Background stations. However, after the initial abrupt reduction, gradual increases were then observed through lockdown. This suggests that the return of vehicles to the road during early lockdown has already offset much of the air quality improvement seen when locking down (provisional estimate 50 to 70% by 01 July). While locking down O₃ increased (7 to 7.4 μg·m⁻³ or 14 to 17% at Urban stations) broadly in line with NO₂ reductions, but later changes suggest significant non-lockdown contributions to O₃ during the months that followed. Increases of similar magnitudes were observed for both PM₁₀ (5.9 to 6.3 μg·m⁻³) and PM_2.5 (3.9 to 5.0 μg·m⁻³) at both Rural and Urban stations alike, but the distribution of changes suggests the lockdown was not an obvious direct source of changes in levels of either of these species during this period, and that more complex contributions, e.g. from resuspension and secondary aerosol, may be more likely major drivers for these changes.