Long-term Measurements of OH Reactivity: A Potential New Metric for Air Quality

Volatile organic compounds (VOCs) are emitted into the atmosphere from a variety of sources, including vehicle exhausts, industry, agriculture and plants, with estimates of over 10,000 different VOCs present in ambient air. Once released into the atmosphere the dominant fate for the majority of VOCs is oxidation by hydroxyl (OH) radicals, leading to a complex cascade of reactions, generating secondary pollutants such as ozone (O3) and secondary organic aerosol (SOA), which are harmful to human health.
Poor air quality has been reported as the greatest environmental risk to public health in the UK, has recently been linked to dementia, and is estimated to cause over 40,000 premature deaths in the UK each year. Policies designed to address issues such as air quality and climate rely on accurate knowledge of atmospheric composition, requiring understanding of the emission rates, concentrations, and chemistry of trace VOCs in the atmosphere. However, it is only possible to identify and measure the concentrations of a small fraction of the vast array of VOCs present in the atmosphere, which hinders our ability to provide accurate predictions of air quality and climate. Despite this challenge, it is possible to quantify the presence of unmeasured species, and the extent to which they contribute to the production of ozone and SOA, through measurements of the rate at which OH radicals are consumed in the atmosphere, since almost all species emitted into the atmosphere react with OH.

Measurements of the total OH loss rate in the atmosphere can be used to define the OH reactivity, which is the pseudo-first-order rate coefficient describing the loss (kOH) and the inverse of the chemical lifetime of OH (TOH = 1/kOH). Comparison between measurements of OH reactivity and calculations based on observations of OH sinks, which include CO, NO, NO2 and VOCs, and laboratory measurements of OH radical kinetics, provides a means to determine the comprehensiveness of the observed sinks, which enables assessment of the potential contribution of unmeasured species to air quality and climate.

While several instruments have been developed to measure OH reactivity, including work in the Leeds group, these instruments tend to be limited to short-term intensive measurements. The capability to make long-term OH reactivity measurements would enhance our understanding of atmospheric composition and chemistry and our ability to monitor changing trends in pollutant emissions.

This work will reduce the complexity of current OH reactivity instruments using the pump-probe technique by developing a system using time-resolved broadband UV absorption spectroscopy to detect the OH radicals in place of the laser-induced fluorescence system, thereby reducing the complexity and size of the instrument to provide the potential for long-term measurements. The student will be involved in the initial development and characterisation of the instrument, which will then be compared to the existing Leeds instrument and deployed in the field for testing and long-term measurements. They will use numerical models based on the Master Chemical Mechanism (MCM) to interpret and understand the measurements, and to determine impacts on air quality and climate.