Flaring efficiencies and NOx emission ratios measured for offshore oil and gas facilities in the North Sea

Gas flaring is a substantial global source of carbon emissions to atmosphere and is targeted as a route to mitigating the oil and gas sector carbon footprint due to the waste of resources involved. However, quantifying carbon emissions from flaring is resource-intensive, and no studies have yet assessed flaring emissions for offshore regions. In this work, we present carbon dioxide (CO2), methane (CH4), ethane (C2H6), and NOx (nitrogen oxide) data from 58 emission plumes identified as gas flaring, measured during aircraft campaigns over the North Sea (UK and Norway) in 2018 and 2019. Median combustion efficiency, the efficiency with which carbon in the flared gas is converted to CO2 in the emission plume, was 98.4% when accounting for C2H6 or 98.7% when only accounting for CH4. Higher combustion efficiencies were measured in the Norwegian sector of the North Sea compared with the UK sector. Destruction removal efficiencies (DREs), the efficiency with which an individual species is combusted, were 98.5% for CH4 and 97.9% for C2H6. Median NOx emission ratios were measured to be 0.003ppmppm-1CO2 and 0.26ppmppm-1CH4, and the median C2H6:CH4 ratio was measured to be 0.11ppmppm-1. The highest NOx emission ratios were observed from floating production storage and offloading (FPSO) vessels, although this could potentially be due to the presence of alternative NOx sources on board, such as diesel generators. The measurements in this work were used to estimate total emissions from the North Sea from gas flaring of 1.4Tgyr-1 CO2, 6.3Ggyr-1 CH4, 1.7Ggyr-1 C2H6 and 3.9Ggyr-1 NOx

Airborne measurements of fire emission factors for African biomass burning sampled during the MOYA campaign

Airborne sampling of methane (CH4), carbon dioxide (CO2), carbon monoxide (CO), and nitrous oxide (N2O) mole fractions was conducted during field campaigns targeting fires over Senegal in February and March 2017 and Uganda in January 2019. The majority of fire plumes sampled were close to or directly over burning vegetation, with the exception of two longer-range flights over the West African Atlantic seaboard (100-300 km from source), where the continental outflow of biomass burning emissions from a wider area ofWest Africa was sampled. Fire emission factors (EFs) and modified combustion efficiencies (MCEs) were estimated from the enhancements in measured mole fractions. For the Senegalese fires, mean EFs and corresponding uncertainties in units of gram per kilogram of dry fuel were 1.8 ± 0.19 for CH4, 1633 ± 171.4 for CO2, and 67 ± 7.4 for CO, with a mean MCE of 0.94 ± 0.005. For the Ugandan fires, mean EFs were 3.1 ± 0.35 for CH4, 1610 ± 169.7 for CO2, and 78 ± 8.9 for CO, with a mean modified combustion efficiency of 0.93 ± 0.004. A mean N2O EF of 0.08 ± 0.002 gkg..1 is also reported for one flight over Uganda; issues with temperature control of the instrument optical bench prevented N2O EFs from being obtained for other flights over Uganda. This study has provided new datasets of African biomass burning EFs and MCEs for two distinct study regions, in which both have been studied little by aircraft measurement previously. These results highlight the important intracontinental variability of biomass burning trace gas emissions and can be used to better constrain future biomass burning emission budgets. More generally, these results highlight the importance of regional and fuel-type variability when attempting to spatially scale biomass burning emissions. Further work to constrain EFs at more local scales and for more specific (and quantifiable) fuel types will serve to improve global estimates of biomass burning emissions of climate-relevant gases

Large Methane Emission Fluxes Observed From Tropical Wetlands in Zambia

Methane (CH4) is a potent greenhouse gas with a warming potential 84 times that of carbon dioxide (CO2) over a 20-year period. Atmospheric CH4 concentrations have been rising since the nineteenth century but the cause of large increases post-2007 is disputed. Tropical wetlands are thought to account for ∼20% of global CH4 emissions, but African tropical wetlands are understudied and their contribution is uncertain. In this work, we use the first airborne measurements of CH4 sampled over three wetland areas in Zambia to derive emission fluxes. Three independent approaches to flux quantification from airborne measurements were used: Airborne mass balance, airborne eddy-covariance, and an atmospheric inversion. Measured emissions (ranging from 5 to 28 mg m−2 hr−1) were found to be an order of magnitude greater than those simulated by land surface models (ranging from 0.6 to 3.9 mg m−2 hr−1), suggesting much greater emissions from tropical wetlands than currently accounted for. The prevalence of such underestimated CH4 sources may necessitate additional reductions in anthropogenic greenhouse gas emissions to keep global warming below a threshold of 2°C above preindustrial levels

Isotopic signatures of methane emissions from tropical fires, agriculture and wetlands : The MOYA and ZWAMPS flights

We report methane isotopologue data from aircraft and ground measurements in Africa and South America. Aircraft campaigns sampled strong methane fluxes over tropical papyrus wetlands in the Nile, Congo and Zambezi basins, herbaceous wetlands in Bolivian southern Amazonia, and over fires in African woodland, cropland and savannah grassland. Measured methane δ 13 C CH 4 isotopic signatures were in the range -55 to -49‰ for emissions from equatorial Nile wetlands and agricultural areas, but widely -60 ± 1‰ from Upper Congo and Zambezi wetlands. Very similar δ 13 C CH 4 signatures were measured over the Amazonian wetlands of NE Bolivia (around -59‰) and the overall δ 13 C CH 4 signature from outer tropical wetlands in the southern Upper Congo and Upper Amazon drainage plotted together was -59 ± 2‰. These results were more negative than expected. For African cattle, δ 13 C CH 4 values were around -60 to -50‰. Isotopic ratios in methane emitted by tropical fires depended on the C3: C4 ratio of the biomass fuel. In smoke from tropical C3 dry forest fires in Senegal, δ 13 C CH 4 values were around -28‰. By contrast, African C4 tropical grass fire δ 13 C CH 4 values were -16 to -12‰. Methane from urban landfills in Zambia and Zimbabwe, which have frequent waste fires, had δ 13 C CH 4 around -37 to -36‰. These new isotopic values help improve isotopic constraints on global methane budget models because atmospheric δ 13 C CH 4 values predicted by global atmospheric models are highly sensitive to the δ 13 C CH 4 isotopic signatures applied to tropical wetland emissions. Field and aircraft campaigns also observed widespread regional smoke pollution over Africa, in both the wet and dry seasons, and large urban pollution plumes. The work highlights the need to understand tropical greenhouse gas emissions in order to meet the goals of the UNFCCC Paris Agreement, and to help reduce air pollution over wide regions of Africa. This article is part of a discussion meeting issue 'Rising methane: is warming feeding warming? (part 2)'

The first airborne comparison of N 2 O 5 measurements over the UK using a CIMS and BBCEAS during the RONOCO campaign

Dinitrogen pentoxide (N 2 O 5 ) plays a central role in nighttime tropospheric chemistry as its formation and subsequent loss in sink processes limits the potential for tropospheric photochemistry to generate ozone the next day. Since accurate observational data for N 2 O 5 are critical to examine our understanding of this chemistry, it is vital also to evaluate the capabilities of N 2 O 5 measurement techniques through the co-deployment of the available instrumentation. This work compares measurements of N 2 O 5 from two aircraft instruments on board the Facility for Airborne Atmospheric Measurements (FAAM) BAe-146 aircraft during the Role of Nighttime Chemistry in Controlling the Oxidising Capacity of the Atmosphere (RONOCO) measurement campaigns over the United Kingdom in 2010 and 2011. A chemical ionisation mass spectrometer (CIMS), deployed for the first time for ambient N 2 O 5 detection during RONOCO, measured N 2 O 5 directly using I - ionisation chemistry and an aircraft-based broadband cavity enhanced absorption spectrometer (BBCEAS), developed specifically for RONOCO, measured N 2 O 5 by thermally dissociating N 2 O 5 and quantifying the resultant NO 3 spectroscopically within a high finesse optical cavity. N 2 O 5 mixing ratios were simultaneously measured at 1 second time resolution (1 Hz) by the two instruments for 8 flights during RONOCO. The sensitivity for the CIMS instrument was 52 ion counts per pptv with a limit of detection of 7.4 pptv for 1 Hz measurements. BBCEAS, a proven technique for N 2 O 5 measurement, had a limit of detection of 2 pptv. Comparison of the observed N 2 O 5 mixing ratios show excellent agreement between the CIMS and BBCEAS methods for the whole dataset, as indicated by the square of the linear correlation coefficient, R 2 = 0.89. Even stronger correlations (R 2 values up to 0.98) were found for individual flights. Altitudinal profiles of N 2 O 5 obtained by CIMS and BBCEAS also showed close agreement (R 2 = 0.93). Similarly, N 2 O 5 mixing ratios from both instruments were greatest within pollution plumes and were strongly positively correlated with the NO 2 concentrations. The transition from day to nighttime chemistry was observed during a dusk-to-dawn flight during the summer 2011 RONOCO campaign: the CIMS and BBCEAS instruments simultaneously detected the increasing N 2 O 5 concentrations after sunset. The performance of the CIMS and BBCEAS techniques demonstrated in the RONOCO dataset illustrate the benefits that accurate, high-frequency, aircraft-based measurements have for improving understanding the nighttime chemistry of N 2 O 5

Airborne observations over the North Atlantic Ocean reveal the importance of gas-phase urea in the atmosphere

Reduced nitrogen (N) is central to global biogeochemistry, yet there are large uncertainties surrounding its sources and rate of cycling. Here, we present observations of gas-phase urea (CO(NH2)2) in the atmosphere from airborne high-resolution mass spectrometer measurements over the North Atlantic Ocean. We show that urea is ubiquitous in the lower troposphere in the summer, autumn, and winter but was not detected in the spring. The observations suggest that the ocean is the primary emission source, but further studies are required to understand the responsible mechanisms. Urea is also observed aloft due to long-range transport of biomass-burning plumes. These observations alongside global model simulations point to urea being an important, and currently unaccounted for, component of reduced-N to the remote marine atmosphere. Airborne transfer of urea between nutrient-rich and -poor parts of the ocean can occur readily and could impact ecosystems and oceanic uptake of carbon dioxide, with potentially important climate implications