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Oceanic ozone dry deposition and its chemical controls

Prof Lucy Carpenter (WACL, University of York), Dr James Lee (WACL, University of York)

Contact email: lucy.carpenter@york.ac.uk

Tropospheric ozone (O3) makes a significant contribution to anthropogenic greenhouse gas forcing in addition to having a major influence on air quality, public health, food security and ecosystem viability, and being fundamental to the photochemical processing of atmospheric chemicals.  Increased anthropogenic emissions of nitrogen oxides and hydrocarbons, both precursors of ozone, have led to substantial increases in global, surface-level ozone concentrations. It is estimated that tropospheric ozone has at least doubled since pre-industrial times (Lamarque et al., 2005).  Ozone can be destroyed throughout the atmosphere by photochemical processes and by loss to the Earth’s surface due to dry deposition, which accounts for about 25 % of the total tropospheric O3 removal (Lelieveld and Dentener, 2000). Importantly, dry deposition has a significant influence on both the near surface O3 concentration – determining human exposure - and the input of O3 to ecosystems.

Figure 1:  Simplified schematic of O3 deposition to seawater

Oceanic dry deposition is the largest and most uncertain O3 depositional sink by land cover class, contributing about one third of global annual ozone deposition (Hardacre et al., 2015). Atmospheric models typically apply the same O3 deposition rate to all of the world’s oceans and wind conditions, despite large variability in oceanic ozone deposition measurements, which range from 0.01 to 0.15 cm s-1.  Integrating small differences in estimated O3 deposition over the large global area of ocean leads to substantial differences in the calculated total deposition.    Currently, observations do not provide a consensus on either wind speed dependency or variability due to chemical, biological or physical processes affecting deposition.  However, laboratory studies and calculations show that dissolved iodide (I-) and unsaturated dissolved organic compounds (DOC) in the surface ocean are key factors in driving the oceanic ozone uptake (Chang et al. 2004, Clifford et al. 2008, Ganzeveld et al, 2009, Martino et al., 2009, Carpenter et al., 2013).  There is almost no information on the abundance and availability of the reactivity of O3 towards DOC across the world’s oceans, although organic components are likely as important as I- in controlling O3 uptake to seawater.   There is a pressing need for improved characterization of deposition velocities over the ocean, to reduce the uncertainty in total global O3 dry deposition.

Objectives

The project will develop and test O3 flux instrumentation, develop high-resolution mass spectrometric methodologies to determine the organic reactivity to O3 in seawater, and apply these alongside established techniques for iodide measurement at coastal observatories and onboard research ships.

Specific objectives are:

1. Building on the group’s current technology for NO2 fluxes, this project will develop a state of the art chemiluminescent measurement for fast and sensitive O3 flux determination suitable for observing fluxes over the coastal and open ocean.  The fast response ozone chemiluminescence instrument operates on the basis of reaction of O3 with NO forming excited NO2* with the NO2* returning to the ground state emitting a photon at 600 nm < λ

2. A vast array of organic compounds within surface seawater containing carboxylic and phenolic groups and/or aromatic rings are likely to be reactive towards ozone, due to the presence of double bonds. Currently, the only model study attempting to specifically account for O3 reactivity to global oceanic DOC uses chlorophyll a (Chl a) as a proxy for this interaction (Ganzeveld et al, 2009).   In this project, through a combination of laboratory and field measurements, the aim is to characterize the reactivity of marine DOC towards ozone based upon bulk characteristics, i.e. the degree of unsaturation, aromaticity index, and/or degree of oxygenation. These features can be resolved by ultrahigh resolution mass spectrometry (UHR-MS) available in the WACL laboratories.   Within this objective you will also investigate different seawater sampling and extraction techniques for optimum analyses. O3 uptake kinetics will be measured in the laboratory over artificial solutions containing varying concentrations of added DOC including commercial humic acids and polyunsaturated fatty acids, extracted marine DOC, and real sea surface microlayer samples.

Figure 2: Penlee Point Observatory

3.  Measurement of atmospheric O3 fluxes via the eddy covariance technique, along with upwind oceanic DOC and iodide concentrations at a coastal atmospheric observatory such as Penlee Point near Plymouth.   Our collaborators in this project from PML operate both an atmospheric observatory and a long-term oceanic measurement station upwind of the site.  This infrastructure provides an ideal test bed and measurement location for the developed technologies.  The O3 flux instrumentation will be run for up to a year, and concurrent seawater samples will be shipped to York for analysis of DOM by UHR-MS and for iodide using commercial methods already available at WACL.   If time allows within the project, you will also deploy the O3 flux instrumentation onboard a research ship, for example within the AMT program.

4.  Data analysis of the results.  Drawing on the laboratory and field results, you will investigate how iodide and DOC together determine the chemical reactivity of O3 to the ocean, and how these relationships are modified by meteorological variables. You will evaluate state of the art formulations (the COARE algorithm) for calculating O3 deposition to seawater that incorporate both chemical reactivity and meteorological data, and use these to improve current formulations and reduce uncertainty in total O3 dry deposition.

Potential for high impact outcome:

In addition to the significant scientific outcomes of the project, there are wider impacts on policy for air quality management.  The US EPA for example have recently implemented ozone deposition over sea-water into the CMAQ model which they use for air quality management, and find that these processes improve ozone prediction over coastal areas.  Their research however reveals the need for a further improvement of the ozone-seawater interaction. 

Training

The student will receive full training in the theory, operation and maintenance of the instrumentation used in this project including the chemiluminescent technique for O3, ultrahigh resolution mass spectrometry (UHR-MS) for organic speciation, and voltammetry for measurement of aqueous iodide. The eddy covariance calculations to determine O3 fluxes will be carried out using the MatLabTM software package, which is widely used for analysing environmental and other scientific data.  The project will involve handling significant amounts of data and the student will be trained in a range of data handling and visualisation techniques and software packages.

The University of York and the wider NERC SPHERES DTP provide comprehensive training programmes for PhD students with a range of courses on both hard and soft skills.  Prof. Lee is part of the National Centre for Atmospheric Science (NCAS), and thus the student will have access to the wider resources that NCAS provides. You will also have access to training provided by NCAS such as the Arran instrumental Summer School, the Earth System Science Summer School (ES4), and future further developments in computations and data analysis.

The student will work in the Wolfson Atmospheric Chemistry Laboratories, part of the department of Chemistry, University of York.  These were established in 2013 and comprise a state of the art 800 m2 dedicated research building, the first of its kind in the UK. Supported by a large award from the Wolfson Foundation and a private donor, the Laboratories enable experimental and theoretical studies relating to the science of local and global air pollution, stratospheric ozone depletion and climate change. The Laboratories are operated as collaborative venture between the University of York and the National Centre for Atmospheric Science (NCAS), co-locating around 40 researchers from seven academic groups and from NCAS. The Laboratories are also home to independent research fellows, postdoctoral researchers, PhD students and final year undergraduate research projects.

The student will have the opportunity to present their work to the scientific community at national and international meetings and conferences. They will also be encouraged to take part in outreach events organised by both WACL and NCAS in order to disseminate the research beyond the immediate scientific community (e.g. to policymakers and the general public).

Related undergraduate subjects:

  • Chemistry
  • Physics