Abstract
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, NO(2), NO(x), O(3), PM(10) and PM(2.5) time-series from Automatic Urban Rural Network (AURN) monitoring stations across the UK. NO, NO(2) 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 O(3) increased (7 to 7.4 μg·m(-3) or 14 to 17% at Urban stations) broadly in line with NO(2) reductions, but later changes suggest significant non-lockdown contributions to O(3) during the months that followed. Increases of similar magnitudes were observed for both PM(10) (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.