Accurate and Direct Measurements of Brown Carbon Aerosol Optical Properties During Formation and Atmospherically-Relevant Ageing Processes

准确、直接测量地层和大气相关老化过程中棕碳气溶胶的光学特性

• 批准号: NE/S014314/2
• 负责人: Michael Cotterell
• 金额: $ 17.63万
• 依托单位: University of Oxford
• 依托单位国家: 英国
• 项目类别: Fellowship
• 财政年份: 2023
• 资助国家: 英国
• 起止时间: 2023 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FS014314%2F2
• 关键词: Accurate; Direct; Measurements; Brown; Carbon

Aerosols are liquid or solid particles suspended in a gas and are pervasive in our atmosphere, with sources including anthropogenic emissions from burning of fossil fuels, and natural sources including from sea spray, desert dust and wildfire biomass burning. These aerosols have significant impacts on our atmosphere, affecting human health through, for example, smog events and global climate through interacting with Sun light and cloud droplets. Indeed, aerosols represent one of the largest uncertainties in predicting future climate change. The net aerosol cooling effect, provided by aerosol scattering sunlight back to space, partially offsets the warming impact of greenhouse gases. However, large uncertainties in this aerosol-light interaction degrade the confidence we have in models of future climate. Improvements to our understanding of aerosol-light interactions could lead to more effective risk mitigation strategies in managing climate change impacts.The important parameters to measure for constraining estimates of aerosol-light interactions are the magnitudes of light scattering and absorption by aerosol. In particular, light absorption is studied poorly for carbonaceous aerosol, with the optical properties of a class of aerosol called brown carbon aerosol (BrC) understood very poorly. BrC particles are formed readily in biomass burning regions where gaseous organic molecules emitted during burning rapidly condense onto liquid or solid particles, with these organic molecules reacting on particle surfaces or inside liquid particles to form light absorbing chromophores. The subsequent BrC particles possess strong wavelength-dependent absorption spectra, with stronger absorption at shorter (blue) optical wavelengths compared to longer (red) wavelengths giving a brown appearance. Also, atmospheric BrC consists of a variety of molecular species with differing light absorption spectra, while the compositions of these chromophores evolve significantly with atmospheric ageing. The uncertainties in BrC optical properties, and how they evolve with time and atmospheric processing, are understood so poorly that many climate models - including UK Met Office climate models - are devoid of any BrC representation. Thus, it is of paramount importance that our understanding of BrC optical properties is improved for better BrC representations in climate models.Traditional measurement approaches have shortcomings in measuring BrC optical properties accurately due to the relatively weak absorption by BrC. Moreover, common laboratory techniques for probing aerosol properties do not access the long ageing timescales of >50 hours that often pertain to atmospheric BrC. This work uses new state-of-the-art instruments available only in the UK to provide measurements of both light scattering and absorption by weakly absorbing aerosol with unrivalled accuracy, precision and sensitivity. Such tools include Single Particle Cavity Ring-Down Spectroscopy (SP-CRDS) and photoacoustic spectroscopy, with single particle trapping techniques such as SP-CRDS allowing measurements of aerosol optical properties on unlimited ageing timescales while particles are subjected to controlled ambient conditions. BrC optical properties will be measured during the BrC formation process and for subsequent ageing and atmospheric processing, such as changes in humidity, exposure to ultraviolet light and reaction with ozone. Furthermore, the proposal addresses both of the common BrC formation pathways, from reaction of gas precursors in aqueous droplets or from the heterogeneous reactions of gas precursors directly on particle surfaces. These results will be used to assess the sensitivity of aerosol-radiation models used at the UK Met Office to measured variations in BrC optical properties and to develop parameterisations of the ageing of BrC optical properties for comparison to recent field studies and future implementation in the next generation of climate models.

气溶胶是悬浮在气体中的液体或固体颗粒，普遍存在于我们的大气中，来源包括化石燃料燃烧的人为排放，以及海浪、沙漠灰尘和野火生物质燃烧等自然资源。这些气溶胶对我们的大气有重大影响，例如通过与太阳光和云滴相互作用而影响烟雾事件和全球气候，从而影响人类健康。事实上，气溶胶是预测未来气候变化的最大不确定因素之一。由气溶胶将阳光散射回太空所提供的净气溶胶冷却效应部分抵消了温室气体造成的变暖影响。然而，气溶胶与光相互作用的巨大不确定性降低了我们对未来气候模型的信心。提高我们对气溶胶-光相互作用的理解可能会导致在管理气候变化影响方面采取更有效的风险缓解策略。用于限制气溶胶-光相互作用估计的重要参数是气溶胶的光散射和吸收的幅度。特别是，对碳质气溶胶的光吸收研究很少，对一类称为棕碳气溶胶（BrC）的气溶胶的光学特性了解甚少。 BrC颗粒很容易在生物质燃烧区域形成，燃烧过程中释放的气态有机分子迅速凝结到液体或固体颗粒上，这些有机分子在颗粒表面或液体颗粒内部反应形成光吸收发色团。随后的 BrC 颗粒具有强的波长依赖性吸收光谱，与较长（红色）波长相比，在较短（蓝色）光学波长处具有更强的吸收，从而呈现棕色外观。此外，大气中的 BrC 由具有不同光吸收光谱的多种分子组成，而这些发色团的组成随着大气老化而显着变化。人们对 BrC 光学特性的不确定性以及它们如何随时间和大气处理而演变的了解甚少，以至于许多气候模型（包括英国气象局气候模型）都缺乏任何 BrC 表示。因此，提高我们对 BrC 光学特性的理解，以更好地在气候模型中表示 BrC 至关重要。由于 BrC 的吸收相对较弱，传统测量方法在准确测量 BrC 光学特性方面存在缺陷。此外，用于探测气溶胶特性的常见实验室技术无法达到通常与大气 BrC 相关的 >50 小时的长时间老化时间尺度。这项工作使用了英国独有的最先进的新型仪器，以无与伦比的准确性、精密度和灵敏度来测量弱吸收气溶胶的光散射和吸收。此类工具包括单粒子腔衰荡光谱 (SP-CRDS) 和光声光谱，其中 SP-CRDS 等单粒子捕获技术允许在粒子受到受控环境条件的同时，在无限的老化时间尺度上测量气溶胶光学特性。 BrC 的光学特性将在 BrC 形成过程以及随后的老化和大气处理过程中进行测量，例如湿度变化、暴露于紫外线以及与臭氧的反应。此外，该提案还解决了两种常见的 BrC 形成途径，即气体前体在水滴中的反应或气体前体直接在颗粒表面上的非均相反应。这些结果将用于评估英国气象局使用的气溶胶辐射模型对测量 BrC 光学特性变化的敏感性，并开发 BrC 光学特性老化的参数化，以便与最近的现场研究和未来的实施进行比较气候模型的生成。