Light Absorption for Volatile Aerosol Particles: A New Measurement Approach

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。大气气溶胶的来源包括燃烧化石燃料产生的人为排放，以及海洋喷雾、沙漠沙尘等自然来源，以及野火等生物质燃烧来源。这些气溶胶对我们的环境有重大影响，通过与阳光和云滴的相互作用，影响区域空气质量和全球气候。事实上，气溶胶在大气模式中的表现是预测未来气候的最大不确定性之一。在全球范围内，气溶胶粒子将阳光散射回太空所产生的净气溶胶冷却效应部分抵消了温室气体的变暖影响。在区域的基础上，吸收光的气溶胶，如燃烧产生的气溶胶，可以加热大气，推动大气动力学和区域气象学的变化，如云的形成。然而，气溶胶-光相互作用的量化存在很大的不确定性，降低了我们对区域气象和未来气候模式的信心。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。气溶胶光吸收尤其不确定，在气候模式中表现不佳。在某种程度上，这种不确定性源于对气溶胶吸收的不良观测，这妨碍了气溶胶光学特性的准确描述，以及这些特性如何依赖于气溶胶源、寿命和环境湿度等因素。因此，近年来已经看到了光声光谱（PAS）技术的发展，可以从大气研究飞机上部署用于气溶胶吸收的原位采样。Cotterell博士与英国气象局的研究人员合作开发了定制的、国际领先的PAS仪器，这些仪器从FAAM研究飞机上部署，为大气科学界提供了对气溶胶光吸收的广泛观察。然而，与其他直接量化气溶胶吸收的技术一样，PAS测量仅限于不含任何挥发性成分（如水）的颗粒；气溶胶中挥发物的存在在光谱询问过程中由于蒸发效应引起的吸收系数测定中引入了显著的偏差。事实上，在经常遇到的高湿度环境中，气溶胶吸收如何随吸水变化的知识很少，因此在大气模式中，吸收随湿度的变化被忽略了。因此，开发新技术来观察含挥发物颗粒的吸收是至关重要的。该项目开发了一种新的光声光谱实现方法，称为相移光声光谱（PS-PAS），可在气溶胶样品测量期间提供额外的相移信息。通过结合光声能量传递过程建模的一系列实验室实验，我们将证明PS-PAS数据能够校正蒸发效应的测量气溶胶吸收系数。除了提供光吸收的可靠特征外，新的相移数据产品将允许研究气溶胶颗粒的挥发性，这是气候模型预测大气气溶胶数量浓度、大小分布和化学成分演变所需的关键特性。重要的是，这些仪器将随时可以从FAAM研究飞机上部署，为英国提供新的，世界领先的观测能力。这是第一次，大气科学家将有必要的关键数据集开发新的和准确的描述气溶胶吸收，以及挥发性，为下一代气候模式。

Aerosols are liquid or solid particles suspended in a gas and are pervasive in our atmosphere. Sources of atmospheric aerosol include anthropogenic emissions from burning of fossil fuels, and natural sources from sea spray, desert dust, and biomass burning sources such as wildfires. These aerosols have significant impacts on our environment, affecting regional air quality as well as global climate through interacting with sunlight and cloud droplets. Indeed, the representation of aerosols in atmospheric models is one of the largest uncertainties in predicting future climate. Globally, the net aerosol cooling effect provided by aerosol particles scattering sunlight back to space partially offsets the warming impact of greenhouse gases. On a regional basis, light absorbing aerosols such as those from combustion can heat the atmosphere, driving changes in atmospheric dynamics and regional meteorology such as the formation of clouds. However, large uncertainties in quantifications of aerosol-light interactions degrade the confidence we have in models of regional meteorology and of future climate. Improvements to our understanding of aerosol-light interactions could lead to more effective risk mitigation strategies in managing climate change impacts.Aerosol light absorption is particularly uncertain and is represented poorly in climate models. In part, this uncertainty arises from poor observations of aerosol absorption that hamper the development of accurate descriptions of aerosol optical properties and how these depend on factors such as aerosol source, lifetime, and ambient humidity. Thus, very recent years have seen the development of photoacoustic spectroscopy (PAS) techniques that can be deployed from atmospheric research aircraft for in situ sampling of aerosol absorption. Dr Cotterell has collaborated with Met Office researchers in developing bespoke, internationally-leading PAS instruments that are deployed from the FAAM research aircraft, providing the atmospheric science community with extensive observations of aerosol light absorption. However, as with other techniques that directly quantify aerosol absorption, PAS measurements are limited to particles that do not contain any volatile components (such as water); the presence of in-aerosol volatiles introduces significant biases in determined absorption coefficients arising from evaporative effects during spectroscopic interrogation. Indeed, there is very little knowledge of how aerosol absorption varies with water uptake in the elevated humidity environments often encountered, and thus the variation in absorption with humidity is neglected in atmospheric models. Thus, it is of paramount importance that new techniques are developed that enable observations of absorption for volatile-containing particles.This project develops a new implementation of photoacoustic spectroscopy, referred to as phase shift photoacoustic spectroscopy (PS-PAS), that provides additional phase shift information during measurements on aerosol samples. Through a series of laboratory experiments in combination with modelling of photoacoustic energy transfer processes, we will demonstrate that PS-PAS data enable the correction of measured aerosol absorption coefficients for evaporative effects. In addition to providing robust characterisations of light absorption, the new phase shift data products will allow studies of aerosol particle volatility, a key property needed in climate models to predict the evolving number concentration, size distribution, and chemical composition for atmospheric aerosols. Importantly, these instruments will be readily available for deployment from the FAAM research aircraft, providing the UK with new, world leading observational capability. For the first time, atmospheric scientists will have the critical data sets required for developing new and accurate descriptions of aerosol absorption, as well as volatility, for the next generation of climate models.