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Assessing the sources and chemistry of nitrogen oxides in the remote oceanic atmosphere

Prof James Lee (YDC), Prof Lucy Carpenter (YDC)

Contact email: james.lee@york.ac.uk

Introduction

Atmospheric nitrogen oxides (NOx = NO + NO2) are crucial controls of oxidants such as ozone (O3) and the hydroxyl radical (OH), and hence play a major role in determining atmospheric composition affecting human and environmental health. The sources and chemical cycling of NOx over the remote oceanic atmosphere remain highly uncertain, despite this region accounting for more than 70% of the Earth’s surface. This leads to uncertainty in the global oxidizing capacity as this region removes a substantial proportion of air pollutants and climate gases such as methane (CH4) and ozone (O3).  For much of the surface of the planet (oceans, un-inhabited regions) there is little or no direct emission of NOx and as such, concentrations are typically very low (of the order of 10 – 100 pptv). This is a particularly crucial NOx range, in which depending on the time of year, location in the troposphere and levels of O3 and H2O, falls the ozone compensation point, a level of NOx at which there is a change from net ozone destruction to net O3 production. It is important to understand the sources (and sinks) of NOx in remote environments as changes in ozone in the background troposphere impacts the ability of countries downwind to achieve their air quality standards (Derwent et al., 2004).

Traditionally, the dominant source of NOx in the remote marine boundary layer (MBL) atmosphere has been assumed to be thermal decomposition of Peroxy Acetyl Nitrate (PAN) transported from polluted areas (Singh and Salas, 1983). Oxidation of NO2 by OH to nitric acid (HNO3), which is then permanently removed by rapid wet and dry deposition, is assumed to be the primary loss mechanism.  Whilst HNO3 also photolyses and reacts with OH to return nitrogen oxides (NOx) to the atmosphere, these processes are slow in comparison to deposition. HNO3 can also be transferred into the aerosol phase through uptake onto a range of particulate types. Recently however, this view of nitric acid as a permanent sink for nitrogen oxides has been challenged by observational evidence that particulate phase nitrate (pNO3) can be efficiently recycled back to NOx (NO + NO2) via a so-called “renoxification” process where particulate nitrate is photolytically reduced to gaseous reactive nitrogen (Ye et al., 2016; Reed et al., 2017). There is also some debate about the importance of direct emissions from shipping on remote marine levels of NOx.

Due to the complex nature of the nitrogen oxide budget and the dominance of fresh emission sources in urban / continental regions, the best place to identify such chemistry is in the remote marine troposphere. The Cape Verde Atmospheric Observatory (CVAO) is situated in the remote marine boundary layer on the island of São Vicente in the tropical North Atlantic Ocean and receives constant, unperturbed trade winds blowing directly off the ocean from the north east. It has an 11 year continuous data series of reactive gases (including NO, NO2 and O3), measured in air with a variety of origins. Levels of NOx are low (typically

It is clear that more work is needed in order to understand the sources and chemistry of NOx in the remote marine environment, with the ultimate aim of allowing models to predict how O3 changes under future emission scenarios.

Aims and Objectives

This project will seek to improve understanding of sources and sinks of NOx in the remote marine environment, specifically at the Cape Verde Atmospheric observatory (see photo above). The specific aims of the project are:

1.) To analyse the 11 year (and  ongoing) dataset of NOx and O3 from the CVAO to assess trends and potential sources of NOx. Data will be analysed according to the air mass origin (e.g. Africa, remote Atlantic, Southern Europe), assessed using back trajectory and dispersion model calculations. The plot to the right shows an example of NOx and O3 diurnal profiles divided into different air mass origins.

2.) To make year-round measurements of total reactive nitrogen (NOy) and PAN at the CVAO. PAN will be measured using a gas chromatography instrument, and compared with previous thermal desorption measurements of total peroxy nitrates. The PAN measurements will be used in a photochemical model to assess the role of PAN as a NOx source. Total NOy will be measure using catalytic conversion to NO followed by chemiluminescence detection, with a switchable particle filter on the inlet to provide a differential measurement of particulate phase nitrate. 

3.) To make measurements of NOx, total NOy and HONO from an aircraft in remote tropical Atlantic. Autumn and spring aircraft campaigns are planned around the Cape Verde region using the Facility for Airborne Atmospheric Measurements (FAAM) BAe 146 aircraft (see picture above). Vertical and horizontal profiles around the CVAO will be measured, allowing the data from the observatory to be put in a wider atmospheric context. A new measurement of HONO on board the aircraft will also provide a wider context for the HONO previously measured at the CVAO (Reed et al., 2017), allowing an assessment of its importance as a NOx source for the entire tropical Atlantic Ocean region.   

4.) Investigate the importance of shipping as a source of NOx at CVAO. During the 11 year time series, regular spikes of NOx have been observed in the data. One possibility of these is the presence of shipping in the area, with Cape Verde being close to a major shipping lane between Europe and South Africa - see figure to the right showing global NOx emissions with the Cape Verde area circled. Previous data will be investigated to assess whether there has been a change in ship originated peaks over the past 11 years and whether this coincides with any change in shipping patterns. In addition, an instrument to measure SO2 (which is a marker for shipping emissions in a marine environment) has recently been installed at CVAO and this data will be used to ascertain whether shipping is indeed the source of the NOx spikes. 

Training

The student will work under the supervision of both Prof. James Lee and Prof. Lucy Carpenter (University of York) and will be based at the Wolfson Atmospheric Chemistry Laboratories, part of the Department of Chemistry, University of York. Here, the student will develop transferrable skills in making atmospheric measurements (e.g. of NOx, total NOy and HONO for both ground and aircraft based measurements), chemical mechanism development and evaluation, numerical and data skills associated with model/measurement comparison and analysis of long term trends of atmospheric gases using a variety of statistical techniques. 

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 works for the National Centre for Atmospheric Science (NCAS), and thus the student will have access to the wider resources that NCAS provides. The student 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).

We appreciate that this PhD project encompasses several different science and technology areas, and we don’t expect applicants to have experience in many of these fields. The project is very well supported with experienced scientists and training in these new techniques and disciplines is all part of the PhD.

Useful References

Derwent, R. G., D. S. Stevenson, W. J. Collins, and C. E. Johnson (2004), Intercontinental transport and the origins of the ozone observed at surface sites in Europe, Atmos. Environ., 38(13), 1891– 1901, doi:10.1016/j.atmosenv.2004.01.008.

Lee, J. D., Moller, S. J., et al., (2009), Year-round measurements of nitrogen oxides and ozone in the tropical North Atlantic marine boundary layer, J. Geophys. Res., 114, D21302, doi:10.1029/2009JD011878.

Reed, C., Evans, M. J., et al., (2017), Evidence for renoxification in the tropical marine boundary layer, Atmos. Chem.. Phys., 17, doi:10.5194/acp-17-4081-2017.

Singh, H. B. and Salas, L. J. (1983), Peroxyacetyl nitrate in the free troposphere, Nature, 302, 326 - 328; doi:10.1038/302326a0.

Ye, C., Zhou, X., et al., (2016), Rapid cycling of reactive nitrogen in the marine boundary layer, Nature, 532, 489–491, doi:10.1038/nature17195. 

Related undergraduate subjects:

  • Atmospheric science
  • Chemistry
  • Earth science
  • Earth system science
  • Environmental science
  • Mathematics
  • Physical geography
  • Physical science
  • Physics