Investigating the Effect of Fine Particulate Matter (PM2.5) Emission Reduction on Surface-Level Ozone (O3) during Summer across the UK

Abstract

UK air pollutant data collected over a 10-year period (2010–2019) from 46 sites with Urban Traffic, Urban Background, Suburban Background, Rural Background, and Urban Industrial environmental types were analysed to study the relationships between [NO] vs. [PM2.5] and [O3] vs. [PM2.5] during the summer for each site type. These results were used to describe the consequence of recent PM2.5 reductions on NO and O3 concentrations at different site types across the UK. The strongest positive [NO] vs. [PM2.5] correlation was observed for the Urban Traffic site type overall, but it displayed the weakest positive [O3] vs. [PM2.5] correlation. Analysis of individual Urban Traffic sites revealed an overall negative [O3] vs. [PM2.5] gradient at the London Marylebone Road (LMR) site. A sharp 35% PM2.5 decrease occurred at LMR between 2011 and 2015 before annual mean concentrations plateaued. Further examination of annual correlations revealed negative [O3] vs. [PM2.5] gradients in each year directly proceeding the sharp 35% PM2.5 decrease at LMR. NOx fluctuations were minimal and accompanied by comparable volatile organic compound (VOC) decreases; thus, VOC-limited chemistry at LMR was deemed to not be the primary cause of O3 increases. Instead, PM2.5 reductions are suggested to be a more significant factor in causing O3 increases, as suppression of O3 production by PM2.5 chemistry decreases with declining [PM2.5]. The remaining two Urban Traffic sites in Birmingham did not display a negative [O3] vs. [PM2.5] correlation in the years studied. This was partly ascribed to the Birmingham measurement sites not being under the influence of the street canyon effect like LMR. Principal attribution was to the lower-average absolute initial PM2.5 concentrations and absence of a significant (>26%) continuous mean PM2.5 decline of greater than 2 years. This study therefore proposed a threshold initial PM2.5 concentration (t) above which O3 suppression by PM2.5 chemistry is sufficient to induce O3 increases when average PM2.5 concentrations significantly decline (by >26% across >2 years), where 17 μg m−3 < t < 26 μg m−3. Extending this analysis to additional cities across the UK as sufficient data become available would allow refinement of the proposed threshold and improved understanding of the influence from the street canyon effect. These results inform future air pollution policies, in the UK and across the globe, in which further joint reductions of PM2.5 and O3 are crucial to achieve maximum benefits to human health.