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

挥发性有机化合物(VOCs)来自各种来源，包括汽车尾气、工业、农业和植物，据估计，环境空气中存在超过10,000种不同的VOCs。一旦释放到大气中，大多数VOCs的主要命运是被羟基(OH)自由基氧化，导致复杂的级联反应，产生二次污染物，如臭氧(O3)和二次有机气溶胶(SOA)，对人类健康有害。据报道，糟糕的空气质量是英国公共健康面临的最大环境风险，最近被认为与痴呆症有关，据估计，英国每年有超过4万人过早死亡。旨在解决空气质量和气候等问题的政策依赖于对大气成分的准确了解，这要求了解大气中痕量VOCs的排放速率、浓度和化学组成。然而，只能识别和测量大气中大量VOCs中的一小部分的浓度，这阻碍了我们提供对空气质量和气候的准确预测的能力。尽管存在这一挑战，但由于排放到大气中的几乎所有物种都与羟基发生反应，因此有可能通过测量大气中OH自由基的消耗速率来量化未测量物种的存在及其对臭氧和SOA产生的贡献程度。对大气中总的OH损失速率的测量可以用来定义OH的反应性，它是描述损失(KOH)的伪一级速率系数和OH的化学寿命(TOH=1/KOH)的倒数。通过比较对OH反应性的测量和基于对OH汇(包括CO、NO、NO2和VOCs)的观测以及对OH自由基动力学的实验室测量，可以确定观测到的汇的全面性，从而能够评估未测量的物种对空气质量和气候的潜在贡献。虽然已经开发了几种仪器来测量OH的反应性，包括利兹小组的工作，但这些仪器往往局限于短期的密集测量。能够进行长期的OH反应性测量将增强我们对大气组成和化学的了解，以及我们监测污染物排放变化趋势的能力。这项工作将通过开发一个使用时间分辨宽带紫外吸收光谱来检测OH自由基的系统来取代激光诱导的荧光系统，从而降低当前使用泵浦-探测技术的OH反应性仪器的复杂性，从而降低仪器的复杂性和尺寸，从而为长期测量提供可能性。学生将参与仪器的初步开发和表征，然后将其与现有的利兹仪器进行比较，并部署到现场进行测试和长期测量。他们将使用基于主化学机制(MCM)的数值模型来解释和理解测量结果，并确定对空气质量和气候的影响。

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.