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

气溶胶是悬浮在气体中的液体或固体颗粒，在我们的大气中无处不在，其来源包括燃烧化石燃料产生的人为排放，以及海洋喷雾、沙漠尘埃和野火生物质燃烧等自然来源。这些气溶胶对我们的大气有重大影响，例如通过烟雾事件影响人类健康，通过与阳光和云滴相互作用影响全球气候。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。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的变暖影响。然而，气溶胶-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，可以在管理气候变化影响方面制定更有效的风险缓解战略。气溶胶-光相互作用的限制性估计的重要测量参数是气溶胶的光散射和吸收的大小。特别是，对含碳气溶胶的光吸收研究甚少，对一类称为棕色碳气溶胶（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成液体或固体颗粒，这些有机分子在颗粒表面或液体颗粒内部反应形成吸收光的发色团。随后的BrC粒子具有很强的波长依赖吸收光谱，与较长（红色）波长相比，较短（蓝色）光学波长的吸收更强，呈现棕色外观。此外，大气BrC由多种具有不同光吸收光谱的分子种类组成，而这些发色团的组成随着大气老化而发生显著变化。BrC光学特性的不确定性，以及它们是如何随时间和大气处理而演变的，人们对这些不确定性了解甚少，以至于许多气候模型——包括英国气象局的气候模型——都没有任何BrC的表示。因此，提高我们对BrC光学特性的理解对于在气候模式中更好地表示BrC是至关重要的。由于BrC的吸收相对较弱，传统的测量方法在准确测量BrC光学特性方面存在不足。此外，用于探测气溶胶特性的常用实验室技术无法获得通常与大气BrC相关的bbb - 50小时的长老化时间尺度。这项工作使用了只有在英国才有的最先进的仪器，以无与伦比的准确性，精度和灵敏度提供弱吸收气溶胶的光散射和吸收的测量。这些工具包括单粒子腔衰落光谱（SP-CRDS）和光声光谱，单粒子捕获技术（如SP-CRDS）允许在无限老化时间尺度上测量气溶胶的光学特性，同时粒子受到受控的环境条件。BrC的光学特性将在BrC形成过程中以及随后的老化和大气处理（如湿度变化、暴露于紫外线和与臭氧的反应）中进行测量。此外，该提案解决了常见的BrC形成途径，即水滴中的气体前体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式对测量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，以便与最近的实地研究和下一代气候模式的未来实施进行比较。

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.