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

挥发性有机化合物（VOC）从各种来源排放到大气中，包括汽车尾气，工业，农业和植物，估计环境空气中存在超过10，000种不同的VOC。一旦释放到大气中，大多数VOC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，空气质量差是英国公共卫生面临的最大环境风险，最近与痴呆症有关，估计每年导致英国超过40，000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，需要了解大气中痕量VOC的排放速率、浓度和化学性质。然而，我们只能识别和测量大气中大量挥发性有机化合物中的一小部分的浓度，这阻碍了我们准确预测空气质量和气候的能力。尽管存在这一挑战，但通过测量OH自由基在大气中消耗的速率，可以量化未测量物质的存在，以及它们对臭氧和SOA产生的贡献程度，因为几乎所有排放到大气中的物质都会与OH反应。大气中总OH损失率的测量可用于定义OH反应性，它是描述损失（kOH）和OH化学寿命倒数（TOH = 1/kOH）的伪一级速率系数。OH反应性的测量值与基于OH汇（包括CO、NO、NO2和VOCs）观测值的计算值以及OH自由基动力学的实验室测量值之间的比较，提供了一种确定观测汇的全面性的方法，从而能够评估未测量物种对空气质量和气候的潜在贡献。包括利兹小组的工作，这些仪器往往限于短期密集测量。进行长期OH反应性测量的能力将提高我们对大气成分和化学的理解，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种使用时间分辨宽带紫外吸收光谱检测OH自由基的系统来代替激光诱导荧光系统，从而减小仪器的复杂性和尺寸，以提供长期测量的可能性。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，并部署在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.