Artifical Chemical Ageing of Ambient Atmospheric Aerosol

大气气溶胶或悬浮的凝聚态物质是一种对健康和生态系统有害的污染物，它为气相物质提供了反应介质，并通过与大气辐射的直接相互作用和为水蒸气的凝结提供成核场所来影响能见度和气候。这些影响的大小取决于气溶胶颗粒的性质（物理特性，如数量、浓度、大小和挥发性；化学特性，如组成和吸湿性）。气溶胶颗粒作为主要排放物产生，来自一系列自然过程（如粉尘、海盐、生物质燃烧）和人为活动（主要与城市地区的化石燃料燃烧和再悬浮有关）。大量的二次来源也存在，来自无机物质和半挥发性有机化合物的缩合。气溶胶粒子的组成在大气中随时间变化；低挥发性蒸汽凝结形成新的或增加原有的颗粒，特别是有机化合物的二次有机气溶胶（SOA）。颗粒成分也可能在内部和外部试剂、气相氧化剂（如OH自由基和臭氧）的驱动下进行化学处理。这种氧化或化学老化可能改变颗粒的物理特性（尺寸、挥发性）和化学成分；这反过来又可能影响粒子寿命、非均相反应性、与太阳辐射的相互作用以及充当云凝结核的潜力。了解环境粒子可能经历的潜在变化，对于了解它们在大气中的作用至关重要。我们的目标是通过人工老化环境大气气溶胶来研究这些变化——我们将建造一个小型反应堆，在这个反应堆中，我们将使环境气溶胶样本受到高浓度的氧化剂（如OH）的影响，在几分钟内达到相当于在大气中暴露几天的水平。然后，我们将使用一系列仪器测量气溶胶特性的变化，特别是气溶胶飞行时间质谱仪，这将使我们能够确定特定颗粒类型的变化，以及颗粒相内特定化合物的变化。在本应用中，我们的目标是演示将化学老化反应器与ATOFMS相结合的应用，以研究环境大气气溶胶的老化，并开始评估由此导致的气溶胶性质变化的范围。该技术的未来应用将使我们能够开始解决一系列关于大气气溶胶的命运和演变的问题，从一般问题，如给定来源或给定环境中排放的颗粒的最终特性是什么，到更具体的问题，如颗粒相中特定化合物的命运-例如，多环芳烃（PAH），如苯并(a)芘是众所周知的致癌污染物。但是，将它们从大气中移除的过程还不是很清楚——由于它们强烈地分裂到凝聚相，从大气系统中移除可能是由气相氧化驱动的，受解吸限制，或者在凝聚相中处理；由于多环芳烃既可能被吸附在颗粒上，又可能被纳入颗粒内的液相，对以前对这类系统的实验室研究的解释受到阻碍：使用真实环境颗粒评估氧化速率将解决这一问题。

Atmospheric aerosol, or suspended condensed phase material, is a pollutant which is harmful to health and ecosystems, provides a reaction medium for gas phase species, and affects visibility and climate, both through direct interaction with atmospheric radiation, and by providing a nucleation site for the condensation of water vapour. The magnitude of these effects depends upon the nature (physical characteristics such as number concentration, size and volatility; chemical characteristics such as composition and hygroscopicity) of the aerosol particles. Aerosol particles are produced both as primary emissions, from a range of natural processes (e.g. dust, sea salt, biomass burning) and anthropogenic activities (primarily related to fossil-fuel combustion and resuspension in urban areas). A substantial secondary source also exists, from the condensation of inorganic species and semi-volatile organic compounds. The composition of aerosol particles changes with time in the atmosphere; low-volatility vapours condense to form new or augment pre-existing particles, notably secondary organic aerosol (SOA) in the case of organic compounds. Particle constituents may also undergo chemical processing, both internally and driven by external reagents, gas-phase oxidants such as OH radicals and ozone. Such oxidation or chemical ageing may change particle physical characteristics (size, volatility) and chemical composition; this in turn may affect particle lifetime, heterogeneous reactivity, interaction with solar radiation and potential to act as cloud condensation nuclei. An understanding of the potential changes ambient particles may undergo is essential to understand their atmospheric role. We aim to investigate these changes by artificially ageing ambient atmospheric aerosol - we will construct a small reactor, in which we will subject ambient aerosol samples to greatly elevated levels of oxidants such as OH, achieving exposures equivalent to several days in the atmosphere, in a matter of minutes. We will then measure the changes in aerosol characteristics using a range of instruments, notably an Aerosol Time-of-Flight Mass Spectrometer, which will permit us to determine changes in specific particle types, and in specific compounds within the particle phase. In this application we aim to demonstrate the application of a chemical ageing reactor coupled to an ATOFMS to investigate the ageing of ambient atmospheric aerosol, and to begin to assess the range of changes in aerosol nature which result. Future application of the technique will allow us to begin to address a range of questions over the fate and evolution of atmospheric aerosol, from general issues such as what are the likely final properties of the particles emitted by a given source or in a given environment, to more specific questions over the fate of particular compounds in the particulate phase - for example, polycyclic aromatic hydrocarbons (PAH) such as benzo(a)pyrene are well-known carcinogenic pollutants, but the processes removing them from the atmosphere are not well understood - as they are strongly partitioned to the condensed phase, removal from the atmospheric system may be driven by gas-phase oxidation, limited by desorption, or by processing in the condensed phase; interpretation of previous laboratory studies of such systems is hindered by the possibility of the PAH being both adsorbed onto particles and being incorporated into the liquid phase within particles: assessment of the oxidation rate using real ambient particles would resolve this issue.