An increasing role for solvent emissions and implications for future measurements of volatile organic compounds

Abstract

Volatile organic compounds (VOCs) are a broad class of air pollutants which act as precursors to tropospheric ozone and secondary organic aerosols. Total UK emissions of anthropogenic VOCs peaked in 1990 at 2,840 kt yr^-1 and then declined to approximately 810 kt yr^-1 in 2017 with large reductions in road transport and fugitive fuel emissions. The atmospheric concentrations of many non-methane hydrocarbons (NMHC) in the UK have been observed to fall over this period in broadly similar proportions. The relative contribution to emissions from solvents and industrial processes is estimated to have increased from approximately 35% in 1990 to approximately 63% in 2017. In 1992, UK national monitoring quantified 19 of the 20 most abundant individual anthropogenic VOCs emitted (all were NMHCs), but by 2017 monitoring captured only 13 of the top 20 emitted VOCs. Ethanol is now estimated to be the most important VOC emitted by mass (in 2017 approx. 136 kt yr^-1 and approx. 16.8% of total emissions) followed by n-butane (52.4 kt yr^-1) and methanol (33.2 kt yr^-1). Alcohols have grown in significance representing approximately 10% of emissions in 1990 rising to approximately 30% in 2017. The increased role of solvent emissions should now be reflected in European monitoring strategies to verify total VOC emission reduction obligations in the National Emissions Ceiling Directive. Adding ethanol, methanol, formaldehyde, acetone, 2-butanone and 2-propanol to the existing NMHC measurements would provide full coverage of the 20 most significant VOCs emitted on an annual mass basis. This article is part of a discussion meeting issue 'Air quality, past present and future'.

Keywords: air pollution; atmospheric emissions; volatile organic compounds (VOCs).

Figure 1.

UK emissions of VOCs from anthropogenic sources 1990–2017 and projections for 2020 and 2030. The solid black marker lines represent the NECD ceiling for that time period. The 2020–2029 ceiling is applicable to all the sectors included in the series minus emissions from agriculture (light blue bar). The dotted lines indicate the 2020–2029 ceiling of the revised Gothenburg Protocol and is applicable to all sectors, including agriculture. (Online version in colour.)

Figure 2.

Trends in sectoral contributions to national emissions of VOCs as a percentage of the overall annual national total, 1970–2017, data from uk-air.gov.uk and the National Atmospheric Emissions Inventory. (Online version in colour.)

Figure 3.

Estimated trends (1990–2017) in the UK emissions of (by rank order in 2017). 1. Ethanol, 2. n-butane, 3. methanol, 4. ethane, 5. propane, 6. n-pentane, 7. ethylene, 8. m-xylene, 9. benzene and 10. toluene. (Online version in colour.)

Figure 4.

(a) Trends in estimated national emissions of functional group classes of VOCs. Contribution of each functional group class to the overall annual national total, 1970 to 2017. (b) Contribution of each functional group class expressed as the percentage of annual emissions. Legend common to both plots. (Online version in colour.)

Figure 5.

Normalized POCP per average UK unit of VOCs emitted 1990–2017 (left-hand y-axis) and total mass of VOCs emitted (right-hand y-axis).

Figure 6.

Trends in selected ambient NMHCs measured at the Marylebone Road automated hydrocarbon network station in the centre of London. (Online version in colour.)

Figure 7.

Comparison of trends in roadside 1,3-butadiene and benzene concentrations at Marylebone Road in central London with the National Atmospheric Emissions Inventory estimate of total 1,3-butadiene emissions from the road transport sector (solid black line). (Online version in colour.)

Figure 8.

Percentage contribution to the overall emission of VOCs from the ‘solvents and related products' class of emissions in the National Atmospheric Emissions Inventory (2017 edition). Red species are not routinely measured, and green species are included in the UK Defra automated hydrocarbon network. (Online version in colour.)