Non-methane Volatile Organic Compounds in the UK

Volatile organic compounds (VOCs) are a broad class of air pollutants which act as precursors to tropospheric ozone and secondary organic aerosols (a component part of PM2.5). The National Atmospheric Inventory (NAEI) indicates that UK emissions of anthropogenic VOCs peaked around 1990 at 2,840 kt yr-1 and then declined to ~810 kt yr-1 in 2017. Notable has been success in reducing emissions from the tailpipe of gasoline vehicles and other evaporative losses of VOCs from fuels (including natural gas) during their production and distribution. Ambient observations of selected VOCs in the Defra Automated Hydrocarbon Network also show significant declines since the 1990s, including species that are emitted directly from fuel loss, such as alkanes and mono-aromatics, and VOCs that are by-products of incomplete combustion such as alkenes and ethyne. The rates of reduction in ambient concentrations slowed around 2010 and have now plateaued. Benzene and 1,3 butadiene have specific limit and target values in the UK and concentrations of these have been successfully reduced such that the UK has reported no exceedances in recent years. Whilst both emissions and concentrations of VOCs have fallen, further reductions in VOC emissions are anticipated for the UK to meet obligations under the National Emission Ceiling Directive in 2030 and UNECE Convention on Long-Range Transport of Air Pollution.
The relative contribution to UK emissions from solvents is estimated to have increased over the past 20 years, in 2017 representing ~74% of national emissions. Notable has been a post-2000 growth in emissions of oxygenated VOCs, none of which are routinely measured in regulatory networks. Ethanol is now the largest VOC emitted by mass (~136 kt yr-1 in 2017 or ~16.8% of total UK emissions) followed by n-butane (52.4 kt yr-1) and methanol (33.2 kt yr-1). Alcohols more generally have grown in significance, representing ~10% of VOC emissions in 1990 rising to ~30% in 2017. The growth in ethanol is due to increased reported emissions from the whisky industries and in estimated domestic use of ethanol as a solvent, for example contained within personal care, car care and household products. For some simple hydrocarbons there have also been notable changes in the major contributing sources. N-butane for example is currently the second most abundant VOC in the UK inventory; in 1990 n-butane was emitted overwhelmingly from gasoline extraction and fugitive distribution losses (139.8 kt yr-1). In 2017 the largest anthropogenic source of n-butane in the inventory was from its use as a domestic aerosol propellant (25.5 kt yr-1), with the gasoline/fugitive losses having been reduced to 23.3 kt yr-1.
Recent changes in the contributing VOC sources to emissions then impacts on the observational strategies to verify emission reduction policies. In 1992 UK national monitoring in the Defra Automated Hydrocarbon Network quantified 19/20 of the most abundant anthropogenic VOCs emitted (all non-methane hydrocarbons), but by 2017 monitoring captured only 13/20 species. To evaluate progress across Europe towards meeting the future VOC emissions targets requires a revision of ambient monitoring strategies. Adding ethanol, methanol, formaldehyde, acetone, 2-butanone and 2-propanol to existing non-methane hydrocarbon measurements would provide full coverage of the 20 most significant VOCs emitted on an annual mass basis in the UK.