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Speciation of VOC emissions related to offshore North Sea oil and gas production

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JournalAtmospheric Chemistry and Physics
DateAccepted/In press - 7 Feb 2021
DatePublished (current) - 12 Mar 2021
Issue number5
Volume21
Number of pages22
Pages (from-to)3741-3762
Original languageEnglish

Abstract

The North Sea is Europe s key oil and gas (O&G) basin with the output currently meeting 3 %-4% of global oil supply. Despite this, there are few observational constraints on the nature of atmospheric emissions from this region, with most information derived from bottom-up inventory estimates. This study reports on airborne measurements of volatile organic compounds (VOCs) emitted from O&G-producing regions in the North Sea. VOC source emission signatures for the primary extraction products from offshore fields (oil, gas, condensate, mixed) were determined in four geographic regions. Measured iso-pentane to n-pentane (iC5 = nC5) ratios were 0.89-1.24 for all regions, used as a confirmatory indicator of O&G activities. Light alkanes (ethane, propane, butane, pentane) were the dominant species emitted in all four regions; however, total OH reactivity was dominated by unsaturated species, such as 1,3-butadiene, despite their relatively low abundance. Benzene to toluene ratios indicated the influence of possible terrestrial combustion sources of emissions in the southern, gas-producing region of the North Sea, seen only during south or south-westerly wind episodes. However, all other regions showed a characteristic signature of O&G operations. Correlations between ethane (C2H6) and methane (CH4) confirmed O&G production to be the primary CH4 source. The enhancement ratio (1C2H6=1CH4) ranged between 0.03-0.18, indicating a spatial dependence on emissions with both wet and dry CH4 emission sources. The excess mole fraction demonstrated that deepwater oil extraction resulted in a greater proportion of emissions of higher carbon number alkanes relative to CH4, whereas gas extraction, typically from shallow waters, resulted in a less complex mix of emissions dominated by CH4. The VOC source profiles measured were similar to those in the UK National Atmospheric Emissions Inventory (NAEI) for oil production, with consistency between the molar ratios of light alkanes to propane. The largest discrepancies between observations and the inventory were for mono-Aromatic compounds, highlighting that these species are not currently fully captured in the inventory. These results demonstrate the applicability of VOC measurements to distinguish unique sources within the O&G sector and give an overview of VOC speciation over the North Sea.

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Funding Information:
Acknowledgements. Airborne data were obtained using the BAe-146-301 Atmospheric Research Aircraft (ARA) flown by Airtask Ltd. and managed by FAAM Airborne Laboratory, jointly operated by UK Research and Innovation (UKRI) and the University of Leeds, and a DCH6 Twin Otter operated by the British Antarctic Survey (BAS). The FAAM aircraft data were collected as part of the Demonstration Of A Comprehensive Approach To Monitoring Emissions From Oil and Gas Installations (AEOG) project, the Improved understanding of accidental releases from oil and gas industries offshore project, and the Methane Observations and Yearly Assessments (MOYA) project, all funded by the Natural Environment Research Council (NERC). The Twin Otter aircraft campaigns were funded under the Climate and Clean Air Coalition (CCAC) Oil and Gas Methane Science Studies (MSS), hosted by the United Nations Environment Programme. Funding was provided by the Environmental Defense Fund, Oil and Gas Climate Initiative, European Commission, and CCAC. We also acknowledge the Offshore Petroleum Regulator for Environment and Decommissioning (OPRED) and Ricardo Energy & Environment for their involvement as project partners on the AEOG project.

Funding Information:
Financial support. This research has been supported by the Nat-

Publisher Copyright:
© 2021 BMJ Publishing Group. All rights reserved.

Copyright:
Copyright 2021 Elsevier B.V., All rights reserved.

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