Reducing uncertainties in OH radical measurements using an absolute optical technique

大气氧化能力决定了微量物质从大气中消失的速度，从而控制了它们对空气质量和气候的影响。大气中主要的氧化剂是羟基自由基（OH），其浓度决定了甲烷（CH4）、挥发性有机化合物（VOCs）和氮氧化物（NOx = NO + NO2）等关键物质的寿命，同时还控制着臭氧（O3）和二次有机气溶胶（SOA）等二次污染物的形成速度。因此，了解大气中OH的浓度和行为对于了解影响空气质量和气候的许多微量物种的寿命至关重要。OH的高反应性给其测量带来了重大挑战，用于测量大气OH的技术，如激光诱导荧光（LIF）光谱和化学电离质谱（CIMS）提供所需的灵敏度和特异性，但需要校准。校准可以通过许多方法来实现，但每种方法都涉及许多步骤，每个步骤相关的不确定度会传播到OH自由基观测中的不确定度。即使是最精确的校准方法通常也有高达~ 30%的不确定度。在这项工作中，我们将开发使用腔增强吸收光谱（CEAS）来测量大气模拟室中的OH自由基浓度。CEAS是一种绝对光学技术，只需要知道OH的吸收截面和吸收路径长度，就可以利用比尔-朗伯定律从测量的吸光度中确定浓度。CEAS的使用将大大减少与模拟室中OH自由基浓度相关的不确定性，并为化学机制的室内研究和现场仪器校准方法的验证提供了显著的优势。

Atmospheric oxidising capacity determines the rate at which trace species are removed from the atmosphere, thus controlling their impacts on air quality and climate. The dominant oxidising agent in the atmosphere is the hydroxyl radical (OH), the concentration of which determines the lifetimes of key species including methane (CH4), volatile organic compounds (VOCs), and NOx (NOx = NO + NO2), whilst also controlling the rate at which secondary pollutants such as ozone (O3) and secondary organic aerosol (SOA) are formed. Understanding the concentrations and behaviour of OH in the atmosphere is thus critical to understanding the lifetimes of many trace species which impact air quality and climate. The high reactivity of OH leads to significant challenges in its measurement, with techniques used to measure atmospheric OH such as laser-induced fluorescence (LIF) spectroscopy and chemical ionisation mass spectrometry (CIMS) providing the required sensitivity and specificity, but requiring calibration. Calibration can be achieved by a number of methods, but each method involves a number of steps, with the uncertainties associated with each step propagating through to uncertainties in OH radical observations. Even the most accurate calibration method is typically associated with uncertainties of up to ~30 %. In this work, we will develop the use of cavity enhanced absorption spectroscopy (CEAS) to measure OH radical concentrations in an atmospheric simulation chamber. CEAS is an absolute optical technique, requiring only knowledge of the absorption cross-sections for OH and the absorption path length to determine the concentration from the measured absorbance using the Beer-Lambert law. The use of CEAS will significantly reduce uncertainties associated with OH radical concentrations in the simulation chamber, and offers significant advantages for chamber studies of chemical mechanisms, and for validation of calibration methods for field instruments.