Proton-Transfer-Reaction Time-of-Flight Mass Spectrometer (PTR-ToF-MS) for aircraft and ground-based applications

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Volatile organic compounds (VOCs) are trace gases that play an important role in many atmospheric and biogeochemical processes. They are a major component of air pollution, being emitted directly to the atmosphere by natural and anthropogenic sources, e.g. transport, industrial processes, biomass burning, solvent use, etc., and also formed as secondary products by chemical reactions in the atmosphere. VOCs contribute to the formation of ozone and particulate matter in the lower atmosphere. Ozone is a respiratory irritant, a greenhouse gas and can decrease crop yields, leading to substantial economic losses. Fine particulate matter, such as PM2.5 (particles less than 2.5 um in diameter), is linked to numerous human health conditions (e.g. asthma, heart disease) and affects the radiation balance at the Earth's surface (links to climate). VOCs also play a key role in determining the oxidising capacity of the atmosphere (the Earth's ability to cleanse pollutants from the atmosphere), and it is becoming increasingly apparent that VOCs contribute significantly to indoor air pollution. In polar regions, biological and photochemical production of VOCs occurs at the snow-ice interface and in the surface ocean. However, the biogeochemical cycles of VOC formation in Arctic atmosphere remain poorly described, with unknown feedback responses to Arctic sea ice decline. As our understanding of atmospheric processes increases and computer models become more sophisticated, there is a requirement for ever-better measurements, in terms of analytical sensitivity (measuring smaller amounts), chemical speciation (identifying and measuring a larger range of compounds) and speed (faster response times to enable us to study, e.g., from a moving aircraft or to measure fast fluxes). This will help us to understand the fundamentals which are controlling these often-complex atmospheric interactions. For VOC measurements the Proton Transfer Reaction Time of Flight Mass Spectrometer (PTR-ToF-MS) will be of great benefit in this respect and will be deployable across a broad range of measurement platforms (aircraft, lab/chamber studies, observatories) and topical research areas (global atmospheric composition, indoor/outdoor air quality, mechanistic studies, fluxes).The PTR-QMS currently used on the FAAM aircraft was bought 17 years ago. It is based on a quadrupole detection system which limits the capability of the powerful PTR technique. These deficiencies include low mass resolution (inability to resolve compounds of equal mass), the necessity to pre-select a limited number of compounds (typically ~10), low sensitivity, particularly at higher masses (>100 Da). We therefore propose to replace the current PTRMS with a state-of-the-art PTR-ToF instrument which will help keep the FAAM aircraft at the cutting edge of global atmospheric research. Advantages of the PTR-ToF include: (1) Large increase in the number of compounds measured (ToF records all masses over a wide mass range, whereas the quadrupole only records a small number of pre-selected compounds); (2) Improved mass range (1-1000 Da) without loss of sensitivity at masses >100 Da; (3) Improved sensitivity (lower limit of detection); (4) Higher mass resolution (e.g. quadrupole cannot distinguish between isoprene (mass 68.117), an important biogenic compound, and furan (mass 68.075), a product of biomass burning; (5) Faster scanning; (6) Selective reagent ionisation (the use of different ion source reagents allows for improved specificity of the instrument, e.g. separation of aldehydes and ketones).The deployment of the PTR-ToF-MS in the unique RVG-Air-Sea-Ice chamber will allow the study of VOC production processes in a controlled environment, enhancing our understanding of the key parameters of VOC cycles in sea ice areas. Beyond the RvG-ASIC, the PTR-ToF-MS will open new avenues for field campaigns in polar seas, e.g. the impact of increasing shipping activity on air quality in the Arctic.