Examining novel atmospheric chemistry in the marine environment with an iodide chemical ionisation mass spectrometer

  • Emily Matthews

Student thesis: Phd

Abstract

The marine atmosphere and ocean play an important role in controlling the abundance of aerosols and trace gases in the atmosphere. The interaction at the ocean-atmosphere interface can have important climate implications due to their effects on ocean biogeochemistry, cloud formation, atmospheric chemistry and the Earth’s radiation budget. However, there are large uncertainties surrounding the role of marine ecosystems in the cycling of trace gases and aerosols in the atmosphere and ultimately how these interactions affect the climate. In order to address these uncertainties, more observational data are needed to be able to improve our understanding of the chemical composition and processes occurring within the marine environment. This thesis presents observations from several aircraft campaigns over the Eastern North Atlantic Ocean and provide novel measurements of biogenic trace gases measured using an iodide ion chemical ionisation mass spectrometer (I-CIMS). The extensive data presented here includes measurements from the Boreal Spring, Summer, Autumn and Winter and as such allows for the characterisation of marine emissions within the studied region. The measurements have identified novel species important to the marine reduced nitrogen and sulfur cycle, including the first observations of gas-phase urea in the atmosphere. Calibration methods were developed in order to provide humidity-corrected and quantitative measurements. Urea was found to be frequently enhanced within the marine boundary layer (MBL) and mixed well into the free-troposphere. The observations indicate that the surface ocean is a significant source of gas-phase urea to the atmosphere during large periods of the year, although no urea was detected during the Spring flights. The transport of biomass-burning and dust plumes over the remote marine environment are an additional source. Global model simulations suggest that the atmospheric burden of urea is significant to the reduced nitrogen budget. However, there are many uncertainties surrounding the source strength, loss processes and mechanisms responsible for the ocean-atmosphere exchange of urea that need to be addressed in order to constrain the role of urea in the global nitrogen cycle. Dimethyl sulfide (DMS) is the largest natural source of sulfur to the atmosphere and its oxidation products are known to contribute to aerosol and cloud formation. The oxidation of DMS is highly complex and thus not yet fully characterised. Recent experimental and in-situ observations have revealed a previously unidentified DMS oxidation product, hydroperoxy methylthioformate (HPMTF), formed from isomerisation reactions (Berndt et al., 2019; Veres et al., 2020). This thesis supports the original observations by Veres et al., that HPMTF is a reservoir of marine sulfur and provides a detailed account of the distribution of HPMTF across the North Atlantic Ocean. Lakes are another aqueous environment that are often biologically productive and, similarly to marine environments, can be a source of trace gases and aerosols. However, emissions from lakes has largely been ignored but may have important implications for regional air quality and cloud formation. In particular, lake breeze events have been found to contribute to harmful levels of pollutants but these studies have been limited to the Great Lakes in the USA. This thesis presents observations from over Lake Victoria during a lake breeze event and demonstrates the importance of biogenic emissions and low NOx conditions for secondary organic aerosol (SOA) formation.
Date of Award1 Aug 2023
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorMartin Gallagher (Supervisor), James Allan (Supervisor), Hugh Coe (Supervisor), David Topping (Supervisor) & Thomas Bannan (Supervisor)

Keywords

  • atmospheric chemistry
  • mass spectrometry
  • ocean-atmosphere exchange

Cite this

'