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

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.

This project will assess the sources and chemistry of nitrogen oxides in the remote oceanic atmosphere using a combination of new measurements on-board the BAe 146 aircraft, extending the measurements at the Cape Verde Atmospheric Observatory (CVAO), and analysis of existing CVAO data. The CVAO is situated in the remote marine boundary layer 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 <50 pptv), leading to almost constant daily O3 destruction (Lee et al., 2009). NOx mixing ratios peak around solar noon (at 20–30 pptV depending on season), which is counter to box model simulations that show a midday minimum due to OH conversion of NO2 to HNO3. Recent work (Reed et al 2017), has shown that production of NOx via decomposition of organic nitrogen species and the photolysis of HNO3 appear insufficient to provide the observed noontime maximum. A rapid photolysis of nitrate aerosol to produce HONO and NO2, however, is able to simulate the observed diurnal cycle. During this project, an instrument will be deployed to the CVAO to make year-round measurements of PAN. These measurements will then be used in a photochemical model to assess the role of PAN as a NOx source. The project will also involve making measurements of NOx, total NOy and HONO in the remote tropical Atlantic using the Facility for Airborne Atmospheric Measurements (FAAM) BAe 146 aircraft. 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.

The student will gain experience and training on a range of atmospheric instrumentation (both ground and aircraft based), as well as using the measurement data in photochemical models in order to interpret the results in the context of sources and sinks of NOx in the remote tropical troposphere and the subsequent effect on O3.

Shortlisting will take place as soon as possible after the closing date and successful applicants will be notified promptly.
Shortlisted applicants will be invited for an interview to take place at the University of Leeds between 19 and 22 February 2018. Candidates will be interviewed by an academic panel.

The Department of Chemistry holds an Athena SWAN Gold Award and is committed to supporting equality and diversity for all staff and students. This PhD project is available to study full-time or part-time (50%).