The impact of multi-decadal changes in VOC speciation on urban ozone chemistry: A case study in Birmingham, United Kingdom

Anthropogenic non-methane volatile organic compounds (VOCs) in the United Kingdom have been substantially reduced since 1990, which is, in part, attributed to controls on evaporative and vehicle tailpipe emissions. Over time, other sources with a different speciation (for example, alcohols from solvent use and industry processes) have grown in both relative importance and, in some cases, in absolute terms. The impact of this change in speciation and the resulting photochemical reactivities of VOCs are evaluated using a photochemical box model constrained by observational data during a summertime ozone event (Birmingham, UK) and apportionment of sources based on the UK National Atmospheric Emission Inventory (NAEI) data over the period 1990-2019. Despite road transport sources representing only 3.3% of UK VOC emissions in 2019, road transport continued being the sector with the largest influence on the local O3 production rate (P(O3)). Under case study conditions, the 96% reduction in road transport VOC emissions that has been achieved between 1990 and 2019 has likely reduced daytime P(O₃) by ∼1/41.67ppbvh^-1. Further abatement of fuel fugitive emissions was modeled to have had less impact on P(O₃) reduction than abatement of VOCs from industrial processes and solvent use. The long-Term trend of increased emissions of ethanol and methanol has somewhat weakened the benefits of reducing road transport emissions, increasing P(O₃) by ∼1/40.19ppbvh^-1 in the case study. Abatement of VOC emissions from multiple sources has been a notable technical and policy success in the UK, but some future benefits (from an ozone perspective) of the phase-out of internal combustion engine passenger cars may be offset if domestic and commercial solvent use of VOCs continue to increase.