Elucidating the Effects of COVID-19 Lockdowns in the UK on the O[sub.3]-NO[sub.x]-VOC Relationship

The unprecedented reductions in anthropogenic emissions over the COVID-19 lockdowns were utilised to investigate the response of ozone (O[sub.3] ) concentrations to changes in its precursors across various UK sites. Ozone, volatile organic compounds (VOCs) and NO[sub.x] (NO+NO[sub.2] ) data were obtained for a 3-year period encompassing the pandemic period (January 2019–December 2021), as well as a pre-pandemic year (2017), to better understand the contribution of precursor emissions to O[sub.3] fluctuations. Compared with pre-lockdown levels, NO and NO[sub.2] declined by up to 63% and 42%, respectively, over the lockdown periods, with the most significant changes in pollutant concentrations recorded across the urban traffic sites. O[sub.3] levels correspondingly increased by up to 30%, consistent with decreases in the [NO]/[NO[sub.2] ] ratio for O[sub.3] concentration response. Analysis of the response of O[sub.3] concentrations to the NO[sub.x] reductions suggested that urban traffic, suburban background and suburban industrial sites operate under VOC-limited regimes, while urban background, urban industrial and rural background sites are NO[sub.x] -limited. This was in agreement with the [VOC]/[NO[sub.x] ] ratios determined for the London Marylebone Road (LMR; urban traffic) site and the Chilbolton Observatory (CO; rural background) site, which produced values below and above 8, respectively. Conversely, [VOC]/[NO[sub.x] ] ratios for the London Eltham (LE; suburban background) site indicated NO[sub.x] -sensitivity, which may suggest the [VOC]/[NO[sub.x] ] ratio for O[sub.3] concentration response may have had a slight NO[sub.x] -sensitive bias. Furthermore, O[sub.3] concentration response with [NO]/[NO[sub.2] ] and [VOC]/[NO[sub.x] ] were also investigated to determine their relevance and accuracy in identifying O[sub.3] -NO[sub.x] -VOC relationships across UK sites. While the results obtained via utilisation of these metrics would suggest a shift in photochemical regime, it is likely that variation in O[sub.3] during this period was primarily driven by shifts in oxidant (OX; NO[sub.2] + O[sub.3] ) equilibrium as a result of decreasing NO[sub.2] , with increased O[sub.3] transported from Europe likely having some influence.