An Airborne Time of Flight Aerosol Mass Spectrometer

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The atmosphere is known to contain large amounts of particulate matter. Numbers can range from several tens of thousands of particles per cubic centimetre of polluted air to mere hundreds in more remote locations. As well as being known to affect human health, they have a significant control on the earth's climate, as they can scatter and absorb solar radiation. They are also thought to indirectly affect climate by dictating the properties of clouds and hence their microphysical and optical properties. However, many of the processes and properties that govern large parts of the lifecycle of aerosols in the atmosphere remain quantitatively uncertain, or in some cases unknown, and this must be addressed to improve predictions of aerosol effects. This is being achieved through laboratory and field measurement programmes and model development, the latter requiring field measurements to test their validity. Part of our lack of understanding and the lack of available model data is due to difficulties in accurately measuring the size and composition of particles in a variety of environments. This is particularly true of the organic fraction, which normally consists of a highly complex mixture of different chemical species. Current models tend to under-predict the amounts of organics in by a large factor, especially in the upper atmosphere, where measurements are scarce. Traditionally methods for measuring particle compositions are offline and so need significant time for sample collection. This is unsuitable for aircraft work, where measurements taken over the course of minutes or less are desired. The UK Facility for Airborne Atmospheric Measurements currently uses an Aerodyne Aerosol Mass Spectrometer (AMS) on many of its activities to measure particle compositions and sizes in real time. While this instrument has proved highly useful in studying aerosol compositions in polluted environments, it lacks the sensitivity to be able to study background particles, especially at higher altitudes within the troposphere or when studying aerosol composition in rapidly changing environments such as pollution plumes and cloud systems. We are proposing to develop this instrument further, using technology proven on the ground, to allow us to probe areas of uncertainty in the aerosol lifecycle such as organic aerosols within the free troposphere; the evolution of aerosols within the atmosphere after emission and aerosol-cloud interaction in greater detail than was possible previously. We will then show its capability and versatility in a series of demonstration experiments that focus on two areas of current scientific uncertainty: the composition of aerosols at many altitudes within the free troposphere, especially their organic content, and the evolution of aerosols within the atmosphere shortly after emission in pollution plumes. The instrument is also capable of probing the chemical nature of the particles that act as sites for cloud droplet formation. We will show its effectiveness at doing this using a size selecting inlet already installed on the FAAM to sample cloud droplets. These experiments will demonstrate the power of the new instrument and, as importantly, will highlight how the new instrument can probe aerosol processes in the atmosphere in the future.