Artifical Chemical Ageing of Ambient Atmospheric Aerosol

大气气溶胶或悬浮的凝聚相物质是一种污染物，对健康和生态系统有害，为气相物质提供反应介质，并通过与大气辐射的直接相互作用和为水蒸气的凝聚提供成核位置而影响能见度和气候。这些影响的程度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，如成分和吸湿性）。气溶胶颗粒物是作为一次排放物产生的，来自一系列自然过程（如灰尘、海盐、生物质燃烧）和人为活动（主要与城市地区的矿物燃料燃烧和再悬浮有关）。还有一个重要的次要来源，来自无机物质和半挥发性有机化合物的冷凝。气溶胶粒子的组成在大气中随时间而变化;低挥发性蒸气冷凝形成新的或增加原有粒子，特别是有机化合物的二次有机气溶胶。颗粒成分也可以经历化学处理，既可以在内部进行，也可以由外部试剂、气相氧化剂（如OH自由基和臭氧）驱动。这种氧化或化学老化可能会改变粒子的物理特性（大小、挥发性）和化学成分;这反过来又可能影响粒子的寿命、异质反应性、与太阳辐射的相互作用以及作为云凝结核的可能性。了解周围颗粒物可能发生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，在其中我们将使环境气溶胶样品受到大大提高的氧化剂水平，如OH，在几分钟内达到相当于大气中几天的暴露量。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和颗粒相内特定化合物的变化。在这个应用程序中，我们的目标是演示的应用程序的化学老化反应器耦合到ATOFMS调查周围大气气溶胶的老化，并开始，以评估气溶胶性质的变化范围的结果。该技术的未来应用将使我们能够开始解决有关大气气溶胶的命运和演变的一系列问题，从一般问题，如由给定来源或在给定环境中排放的颗粒的可能最终性质，到有关颗粒相中特定化合物的命运的更具体问题-例如，多环芳烃（PAH）如苯并（a）芘是众所周知的致癌污染物，但从大气中去除它们的方法还不很清楚-因为它们强烈地分配到冷凝相，从大气系统中去除可能是由气相氧化驱动的，受解吸限制，或通过在凝聚相中处理;由于PAH既被吸附到颗粒上又被掺入颗粒内的液相中的可能性，阻碍了对此类系统的先前实验室研究的解释：使用真实的环境颗粒评估氧化速率将解决该问题。

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.