A top-down approach to measuring speciated VOC emissions from North Sea oil and gas facilities

Total offshore emissions (carbon dioxide (CO₂), nitrogen oxides (NO + NO₂ = NO_x), nitrous oxide (N₂O), sulphur dioxide (SO₂), carbon monoxide (CO), methane (CH₄) and total VOCs) from upstream oil and gas production in the UK increased by 7 % from 2016 to 2017. While total greenhouse gas emissions in the UK continue to decrease, total offshore oil and gas emissions of volatile organic compounds (VOCs) increased by 22 % and offshore emissions of NO_x and SO₂ also showed upwards trends. The UK national atmospheric emissions inventory (NAEI) is built from the bottom up by combining source specific emission factors with source location. This approach assumes that there are a common set of emission factors that are equally applicable to all offshore platforms and relies on operators to accurately self-report emissions. Currently offshore atmospheric emissions reporting includes only total VOCs. Emissions of individual VOCs are not reported, but an estimated speciation is made in the NAEI. Here we provide a top-down emissions estimation methodology from which speciated emissions of up to 30 VOCs can be estimated for point-source platforms. We apply a range of techniques including mass flux and Gaussian dispersion methods, and use VOC measurements obtained within downwind plumes as a tool for source identification. A total of 16 research flights were conducted as part of a joint project between the UK National Centre for Atmospheric Science (NCAS), BEIS, the UK Offshore Petroleum Regulator for Environment and Decommissioning (OPRED) and Ricardo Energy & Environment to characterise emissions from platforms in the North Sea. The hydrocarbon to ethane enhancement ratio within downwind plumes, measured under well-mixed boundary layer conditions, was used to scale a 1 Hz ethane measurement from the aircraft to other hydrocarbons collected using whole air samplers and measured using GC-FID. This allowed individual VOC emission rates to be calculated and compared to existing inventories. This work highlights how a top down technique can be used to quantify emissions and also provide insight into specific emission sources, in contrast to existing methods which often fail to achieve both simultaneously.