Metrology and measurements of atmospheric nitrous oxide in the UK

Student thesis: Doctoral ThesisDoctor of Science (DSc)

Abstract

Nitrous oxide (N2O) is the third most important greenhouse gas and has significant importance for stratospheric ozone depletion. Globally N2O emissions have been estimated at 60% from natural sources, and the rest is from anthropogenic sources (40%). In 2020 the global atmospheric N2O mole fraction was reported at 333.2±0.1 nmol mol−1; this was about 123% higher than pre-industrial levels (270 nmol mol−1). High accuracy atmospheric measurements play a vital role in understanding N2O emissions from anthropogenic and natural sources.

Presently, very few reported measurements of N2O from urban environments exist. Our in-situ urban measurement of N2O in Bristol showed pollution events with a clear pattern of weekend N2O enhancements; this elevation was, however, found to decline during the COVID lockdown period. These observations were shown to be caused by the usage of N2O for recreational purposes. We determined that N2O emissions related to recreational drug use published in the National Atmospheric Emission Inventory (NAEI) were underestimated by between 60% to 680% based on the emission scenarios developed in this thesis.

The WMO’s Global Atmosphere Watch (WMO- GAW) programme established a compatibility goal of 0.1 nmol mol−1 (extended to 0.3 nmol mol−1) for network measurements of atmospheric N2O [1]. This challenging goal requires both enhancing the accuracy of reference standards and deploying advanced measurement techniques. This thesis explores the development of high precision N2O primary reference material (PRM) in a synthetic air matrix – this new purity assessment of N2O in matrix gas was deployed using a novel cryogen-free sample preconcentration system coupled with a dual-laser spectrometer. This approach successfully decreased the total gravimetric uncertainty of the standard addition approach from 100% to 7.45%, with the gravimetric uncertainty of atmospheric N2O achieving the extended WMO network compatibility goal with a relative uncertainty of 0.05%.

The advanced optical instruments were deployed in the UK DECC network, at Bilsdale, a tall tower observatory of the UK DECC network; in-situ measurements of atmospheric N2O using co-located instruments GC-µECD and CRDS were performed for two years. Instrumental performance shows significant improvement in precision and linearity with the CRDS compared to the GC-µECD. An inter-comparison campaign within the UK DECC network via round-robin experiments showed better network compatibility, and the CRDS instruments were more in line within the WMO N2O extended compatibility goal. As a result of the improved measurement precision, the higher frequency measurements on the CRDS enabled better characterisation of N2O pollution events, which are often short-lived in comparison to CO2 and CH4. The results presented in this thesis lead to a recommendation to move to measure N2O using optical instruments within the UK DECC network to help better characterise pollution events for inventory verification.
Date of Award20 Jun 2023
Original languageEnglish
Awarding Institution
  • University of Bristol
SupervisorSimon O'Doherty (Supervisor) & Anita L Ganesan (Supervisor)

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