Increasing ambient surface ozone levels over the UK accompanied by fewer extreme events

摘要

We report a statistical analysis of surface ozone (O-3) measurements over the UK, 1999-2019. Data are from the Automatic Urban and Rural Network (AURN) that forms the basis of checking nationwide compliance with ambient air quality directives. We determine the mean O-3 trend across all AURN sites to be 0.41 +/- 0.07 mu g m(-3)yr(-1), composed of larger trends in suburban and urban environments (0.36 +/- 0.11 mu g m(-3)yr(-1)-0.75 +/- 0.16 mu g m(-3)yr(-1)) than in the rural environment (0.16 +/- 0.07 mu g m(-3)yr(-1)); consequently the gap between urban and rural surface O-3 across the UK has steadily declined from 20 mu g m(-3) to 12 mu g m(-3) until 2012 when it has since stalled. These trends in urban surface O-3 are coincident with and likely caused by a pronounced decline in surface concentrations of NO2 (proxy for NOx), which we link to reduced emissions from the transport sector. Using a gamma probability distribution to describe changes in surface O-3, we find an increase in the probability of high mean O-3 and an associated decrease in the probability of extreme (elevated) O-3 events. We find no discernible trend in the number of days per year that exceed the 8-h mean O-3 value of 100 mu g m(-3) set by the UK Department for Environment, Food and Rural Affairs (Defra) or a similar metric of 120 mu g m(-3) set by the EU. These exceedances, typically occurring during summer months across the network, range from 499 exceedances in 2010 to in excess of 2300 exceedances in 2003 and 2006. We find that 50% of these exceedances occur at temperatures greater than 18 degrees C (21 degrees C) in rural (urban) environments. With 50% (95%) of the 66 million UK inhabitants living and working within 12 km (50 km) of an AURN site, our analysis is relevant to a large fraction of the UK population. Consequently, we may need to re-prioritize epidemiological studies to understand the impact of chronic exposure to elevated mean O-3 concentrations on human health.