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，普遍存在于我们的大气中，其来源包括燃烧化石燃料的人为排放，以及包括海雾、沙漠尘埃和野火生物质燃烧在内的自然来源。这些气溶胶对我们的大气层有重大影响，例如通过烟雾事件影响人类健康，通过与太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性之一。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的变暖影响。然而，这种气溶胶-光相互作用中的巨大不确定性降低了我们对未来气候模型的信心。改善我们对气溶胶-光相互作用的理解可能导致在管理气候变化影响方面更有效的风险缓解策略。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大小。特别是，对碳质气溶胶的光吸收研究得很少，对一类被称为棕色碳气溶胶(BRC)的气溶胶的光学性质了解得很少。BRC颗粒很容易在生物质焚烧区形成，燃烧过程中排放的气态有机分子迅速凝聚成液体或固体颗粒，这些有机分子在颗粒表面或液体颗粒内部反应，形成光吸收发色团。随后的BRC颗粒具有强烈的与波长相关的吸收光谱，与较长的(红色)波长相比，在较短的(蓝色)光学波长下具有更强的吸收，从而呈现棕色外观。此外，大气BRC由具有不同光吸收光谱的各种分子物种组成，而这些生色团的组成随着大气老化而显著演变。BRC光学性质的不确定性，以及它们如何随时间和大气过程演变，人们对此知之甚少，以至于许多气候模型--包括英国气象局气候模型--缺乏任何BRC表示。因此，为了在气候模式中更好地表示BRC，提高我们对BRC光学性质的理解是至关重要的。由于BRC的吸收相对较弱，传统的测量方法在准确测量BRC光学性质方面存在缺陷。此外，用于探测气溶胶特性的普通实验室技术不能达到通常与大气BRC有关的50小时的漫长老化时间尺度。这项工作使用仅在英国提供的最先进的新仪器，以无与伦比的精确度、精密度和灵敏度测量弱吸收气溶胶的光散射和吸收。这些工具包括单粒子空腔衰荡光谱(SP-CRDS)和光声光谱学，单粒子捕获技术(如SP-CRDS)允许在粒子受到受控环境条件影响的情况下，在无限老化时间尺度上测量气溶胶光学特性。在BRC形成过程中以及随后的老化和大气处理过程中，将测量BRC的光学特性，如湿度变化、紫外线照射和与臭氧的反应。此外，该提案涉及两种常见的BrC形成途径，一种是气体前体在水滴中的反应，另一种是气体前体直接在颗粒表面的多相反应。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BRC光学特性测量变化的敏感性，并开发BRC光学特性老化的参数，以便与最近的现场研究和下一代气候模型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.