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

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.

Having an instrument that can measure VOCs with the capability of the PTR-ToF-MS will make a valuable addition to the science and, therefore, impact associated with facilities that are important for UK national capability (i.e. FAAM aircraft, RVG Air-Sea-Ice Chamber, Weybourne Atmospheric Observatory), as well as add to the UK's ability to mount comprehensive field campaigns. The PTR-ToF-MS instrument will be among the first to be incorporated into a research aircraft and will help to keep the FAAM aircraft at the forefront of modern technology. The instrument will increase the capability and attractiveness of the Air-Sea-Ice chamber which is already attracting a high number of national and international collaborations. Weybourne is part of NCAS's contribution to National Capability for observations. Impact will be tracked through usage of data in publications, citations and evidence provided to policy makers, e.g. through Defra's Air Quality Expert Group.

VOC emissions are part of the EU National Emissions Ceiling Directive that feeds into the UNECE Convention on Long-range Transport of Air Pollution. Current projections suggest the UK will not meet the 2030 targets. VOCs contribute to the formation of PM2.5 and research commissioned by Public Health England (PHE) found that the health and social care costs of PM2.5 in England were £76M in 2017. The Department of Health and Social Care's (DHSC) advisory Committee on the Medical Effects of Air Pollutants estimate that long-term exposure to air pollution (PM2.5 and NO2) in the UK has an annual impact equivalent to ~30,000 deaths. The UK Government's Clean Air Strategy 2019 aims to reduce the proportion of the population exposed to more than the WHO PM2.5 guideline. This will require policies based on evidence of the contributing sources, including VOCs. The PTR-ToF-MS will provide crucial information on VOCs including, critically, some not measured by Defra's UK monitoring network.

The mix of VOCs in the atmosphere has changed dramatically over the last few decades as emission sources (e.g. petrol, diesel, solvents) have changed such that many of the VOCs that once dominated have declined and others have gained importance. Several of the VOCs with greatest emissions are not measured by the Defra monitoring network, including several OVOCs measured by the PTR-ToF-MS (e.g. methanol, acetone, butanone, formaldehyde). Defra is therefore a likely beneficiary. Running the PTR-ToF-MS at Weybourne for extended periods will provide information on the background concentrations of many VOCs, how they change seasonally and from year to year, and how the UK is impacted by pollution imported from continental Europe. Deploying the instrument to an urban supersite will provide insight into the urban VOC mix and will contribute better understanding of secondary organic aerosol (SOA) formation. Similarly, deployment on the aircraft will improve quantification of the UK (and global) sources of VOCs and their role in SOA formation. The PTR-TOF-MS could also be used to provide insight into VOCs in the indoor environment, which is highly relevant given that domestic products are now a major source of VOCs, and people spend much of their time indoors. Other potential beneficiaries include PHE and DHSC.

The results produced by studies using the RvG-ASIC facility and from potential future studies in polar regions will be of interest to the many international working groups assessing global changes in the atmosphere and marine ecosystems, in order to support policymaking. These include AMAP (Arctic Monitoring and Assessment Program), SOLAS (Surface Ocean Lower Atmosphere Study), BEPSII (Biogeochemical Exchange Processes at the Sea-Ice Interfaces), the Cryosphere and Atmospheric Chemistry working group (CATCH), etc.