A Novel Optical Module for Detection and Sizing of Particles in a Single Particle Mass Spectrometer - OptiPart-MS

气溶胶粒子是大气污染的关键成分；它们散射和吸收太阳辐射，因此对大气柱的辐射平衡有重大影响；它们既是液态云中云滴形成的场所，也是较冷条件下形成冰的表面，在冰川作用和降水开始方面发挥了关键作用。所有这些过程都有很大的不确定性，其中一些围绕着化学成分在单个颗粒中如何结合。为了解决这些问题，重要的是有快速测量方法，可以观察单个颗粒的化学成分，以便能够确定整个颗粒群体中化学成分的混合情况。单粒子质谱(SPMS)提供了实时分析单个粒子化学成分的为数不多的方法之一。在SPMS中，周围的气溶胶被带入真空区，颗粒被探测到，通常通过光散射来检测，其大小通过它们在相隔已知距离的两个激光之间的空气动力学飞行时间或通过散射强度来测量，这些信息被用来触发高功率激光，在质谱仪的入口处烧蚀颗粒并使其碎片电离。SPMS系统在颗粒检测中不能因尺寸而产生偏差；颗粒大小测定必须准确和稳健。探测效率必须足够高，才能用于解决与大气相关的问题，如冰核形成或在飞机上操作。我们将开发一种用于SPMS系统的新型粒子探测系统，该系统基于将不同、非共振波长的多个激光组合成一束光束，我们的建模工作表明，该系统满足了上述标准。这种方法使用的新技术最初是为电信行业开发的，但直到最近才可用于这一应用所需的高功率激光系统。当使用单一散射角和单一波长的激光时，散射光强度与颗粒大小之间的关系很复杂，这限制了散射光用于颗粒的检测和测量。以前，使用体积较大且不容易安装在消融激光器附近的聚光镜可以克服这一问题，从而导致检测效率降低。我们提出的系统可以检测当前仪器入口覆盖的尺寸范围内的颗粒，并且可以很容易地并入激光烧蚀区域，大大提高了检测效率。使用多波长系统测量颗粒大小有两种可能，一种是使用相隔固定距离的两个激光来测量颗粒的飞行时间；另一种是使用散射强度来测量光学尺寸。第一种方法可以准确测量粒子的大小，没有任何偏差，但这意味着由于粒子束发散，粒子的探测能力可能会降低。第二个系统将极大地提高检测效率，尽管我们需要证明我们的模型模拟正确地预测了散射光强度和颗粒尺寸之间的单调关系。我们的模块化方法将允许使用相同的组成部分对这两种方法进行评估，并为我们的目的对最佳系统进行评估。通过这样做，我们的目标是提供一种SPMS的光学设计，该光学设计可用于与大气科学相关的空气和实验室研究，这些研究需要对数字浓度极低的颗粒物进行化学表征，并提供一种在大气科学和其他领域都有广泛应用的颗粒物的非侵入性光学检测方法。

Aerosol particles are key components of atmospheric pollution; they scatter and absorb solar radiation and so have a major influence on the radiative balance of the atmospheric column; and they act as sites for both cloud droplet formation in liquid clouds and as surfaces for the formation of ice in colder conditions, playing a key role in glaciation and hence the onset of precipitation. There are major uncertainties associated with all of these processes and a number of which surround how the chemical components are combined within single particles.To address such questions it is important that rapid measurement methods are available that can observe the chemical composition of single particles so that the mixing of chemical components across a whole particle population can be determined. Single particle mass spectrometry (SPMS) provides one of the few ways of analysing the chemical composition of single particles in real time. In SPMS, ambient aerosol is entrained into a vacuum region, the particles are detected, usually by light scattering and their size is measured either by their aerodynamic flight time between two lasers separated by a known distance or by the scattering intensity and this information is used to trigger a high powered laser that ablates the particles and ionizes their fragments at the entrance to a mass spectrometer. The SPMS system must not suffer biases in particle detection as a function of size; and particle sizing must be accurate and robust. The detection efficiency must be sufficient for it to be used to address atmospherically relevant problems such as ice nucleation formation or to be operated from an aircraft.We will develop a novel particle detection system for an SPMS system based around the combination of multiple lasers of different, non-resonant wavelengths into a single beam which our modelling work shows addresses the above criteria. This approach uses new technology that was originally developed for the telecoms industry but has only recently become available for use with high powered laser systems required for this application. The relationship between scattered light intensity and particle size when using a single scattering angle and single wavelength laser light is complex and this limits the use of scattered light for detection and sizing of particles. Previously, this has been overcome using light collection mirrors that are bulky and cannot be easily housed close to the ablation laser, leading to reductions in detection efficiency. Our proposed system will detect particles across the size range covered by the inlet of the current instrument, and can be easily incorporated into the laser ablation region, greatly improving the detection efficiency. There are two possibilities for particle sizing with a multi-wavelength system, one involves using two lasers separated by a fixed distance to measure the flight time of the particles; the other is to use the scattering intensity to measure the optical size. The first of these will give an accurate measure of size, free from biases, however, it means that particle detection may reduce since the particle beam is divergent. The second system will greatly improve detection efficiency though we need to demonstrate that our model simulations correctly predict a monotonic relationship between scattered light intensity and particle size. Our modular approach will allow both of these approaches to be assessed using the same components and an assessment made of the optimum system for our purposes. In doing so we aim to deliver an optical design for an SPMS that can be used in airborne and laboratory studies relevant to atmospheric science that require chemical characterisation of particles with very low number concentrations, and provide an approach for non-invasive optical detection of particles that has a wide number of applications both across atmospheric science and in other fields.