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组成部分;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，在冰川作用中发挥关键作用，从而开始降水。所有这些过程都有很大的不确定性，其中一些是关于化学成分如何在单个颗粒中结合的。为了解决这些问题，重要的是要有快速的测量方法，可以观察单个颗粒的化学成分，以便确定整个颗粒群中化学成分的混合情况。单粒子质谱法（SPMS）提供了为数不多的真实的实时分析单粒子化学成分的方法之一。在SPMS中，环境气溶胶被夹带到真空区域中，通常通过光散射检测颗粒，并且通过它们在由已知距离分开的两个激光器之间的空气动力学飞行时间或通过散射强度来测量它们的尺寸，并且该信息用于触发高功率激光器，该激光器在质谱仪的入口处烧蚀颗粒并电离它们的碎片。SPMS系统在颗粒检测中必须不受作为尺寸的函数的偏差的影响;并且颗粒尺寸测量必须准确且稳健。探测效率必须足够，它被用来解决大气相关的问题，如冰成核的形成或从aircraft.We将开发一种新的粒子探测系统的SPMS系统的基础上结合成一个单一的光束，我们的建模工作表明，不同的，非共振波长的多个激光器，解决上述标准。这种方法使用最初为电信行业开发的新技术，但直到最近才可用于该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激光时，散射光强度和颗粒尺寸之间的关系是复杂的，这限制了散射光用于颗粒的检测和尺寸确定的使用。以前，这已经通过使用体积大并且不能容易地靠近烧蚀激光器容纳的光收集镜来克服，从而导致检测效率的降低。我们提出的系统将检测当前仪器入口所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。用多波长系统测量颗粒尺寸有两种可能性，一种是使用两个相隔固定距离的激光器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。第一种方法将给出准确的尺寸测量，没有偏差，然而，这意味着粒子检测可能会减少，因为粒子束是发散的。第二个系统将大大提高检测效率，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件对这两种方法进行评估，并对最佳系统进行评估。在这样做的过程中，我们的目标是提供一个光学设计的SPMS，可用于空气和实验室研究相关的大气科学，需要化学表征的颗粒具有非常低的数量浓度，并提供了一种方法，非侵入性的光学检测的颗粒，有广泛的数量在大气科学和其他领域的应用。

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.