Tropospheric ozone is a significant climate gas, in addition to having a major influence on air quality, public health, and on food security. Dry deposition of O3 to the Earth’s surface is estimated to account for about a quarter of overall tropospheric O3 removal. Losses to the ocean surface are believed to be the largest global deposition sink and model calculations show that ocean dry deposition has the potential to reduce surface O3mixing ratios by several parts per billion (ppb). This is of a magnitude where it can influence human exposure and impact on ecosystems. However, O3 deposition velocities to the ocean are highly uncertain, with very poor knowledge of the mechanistic details.
A primary reason for the large uncertainty in oceanic O3 deposition is the lack of direct ocean flux measurements. Eddy covariance is the most direct method for measuring air-sea flux. Flux is derived by correlating rapid fluctuations in vertical wind velocity with fluctuations in gas mixing ratio and requires a fast (sub-second) sensor. Ozone flux measurements over the open ocean are challenging, due to the typically very low deposition velocities. To obtain the desired sensitivity, it is necessary to use custom-built instruments, rather than commercial UV-absorption detectors.
We are currently testing and developing fast response ozone detectors based on the ozone + nitric oxide (NO) chemiluminescence reaction, and have deployed this instrument at a coastal ground-based platform. The detection is based on reacting O3 with NO, forming NO2 in an electronically excited state which then relaxes, emitting a photon which is detected by a photomultiplier tube. The instrument uses an excess of NO gas, making O3 the limiting reactant in the reaction chamber. The short response time of this method allows for the kind of high frequency measurements required for eddy covariance flux measurements.
In this project, you will use a more sensitive version of this instrument and deploy it on a ship-based field campaign, guided by highly experienced postdoctoral associates in WACL. The research cruise will span large gradients of the biogeochemical controlling factors of oceanic O3 deposition, including sea surface temperature and concentrations of iodide, dissolved organic carbon and surfactants, allowing an evaluation of the driving factors of deposition. The cruise will substantially improve both the geographical coverage and the quantity of observational data of oceanic O3 fluxes. You will also deploy the instrument at ground based field stations including at the Tudor Hill Marine Atmospheric Observatory in Bermuda and at the Cape Verde Atmospheric Observatory in the tropical Atlantic ocean.
The second aim of this project is to build a Cavity-Enhanced UV Absorption O3 instrument based on the newly developed NASA Rapid Ozone Experiment (ROZE), alongside one of the technical staff in WACL. This high precision, fast time response instrument is particularly well suited to aircraft measurements of O3 fluxes (using the eddy covariance technique). You will characterise the performance of this instrument in the WACL laboratories to demonstrate its suitability for installation on the UK FAAM aircraft. There will be opportunities to deploy the fast aircraft O3 sensor on at least one airborne field campaign once it has been installed.
Training specific to the project will include in gas handling and sampling, instrument design and characterisation, flux measurements, and atmospheric chemistry. WACL has significant technical support including an Experimental Officer and two technicians, and you will also be supported in your project by postdoctoral research associates working on related studies.