Aerodyne Compact Time of Flight Aerosol Mass Spectrometer

The Compact Time-of-Flight Aerosol Mass Spectrometer (C-ToF AMS) was developed by Aerodyne Research Inc., Billerica, MA, USA. The instrument measures the volatile and semi-volatile chemical components of aerosol particles as a function of particle size over the size range ~ 60nm to 600nm with 100% efficiency. The efficiency rapidly falls off either side of this window.

The C-ToF-AMS consists of three differentially pumped chambers, constructed within a single aluminium block hereafter referred to as the “AMS body”, to which a number of sub-units or components are attached.

The first chamber in the AMS body houses an aerodynamic lens, which focuses the particles entering it, through a critical orifice, into a collimated beam of less than 1mm diameter. Gas molecules are not focussed and are highly divergent when they exit the lens. Beyond this lens, and still within the first chamber, the focused aerosol pass through a gas skimmer cone which removes the majority of the divergent gas molecules, but allows the aerosol particles (at near 100% efficiency for those 50-600 nm in diameter) to pass into the “Particle Time Of Flight” (P-ToF) chamber.

The P-ToF chamber houses a chopper wheel, which consists of a spinning disc with two radial slits cut into it. This can be positioned to fully block the aerosol beam or to “chop” it by allowing the slits to intercept the collimated aerosol beam as it enters the chamber. The chopper can also be withdrawn from the beam completely to allow all aerosols through. These travel the full length of the P-ToF chamber and into the “detection” chamber.

This third differentially pumped region houses a heated surface onto which the aerosols are directed, a tungsten filament and a time-of-flight mass spectrometer. Particles impacting onto the heated surface undergo flash vaporisation of the volatile and semi-volatile components of the aerosol, and a kinetic gas is formed. This kinetic gas passes the tungsten filament and is ionised separately by standard electron impact ionisation at 70 eV. The charged fragments produced are detected by the time-of-flight mass spectrometer, which samples all mass to charge ratios (Dalton numbers or Atomic Mass Units, AMU) from 1 to 300 AMU. The separation of the ionisation process from the thermal ablation (vaporisation) of the aerosol enables the analysis of component mass fragments to be quantitative.

In addition to the mass spectrometer, a home built SMPS unit is built into the rack covering the size range ~20-350nm at sea level. Although this is a home-built system, the principle of operation is the same as that described in the AMF ‘SMPS’ section.

The system requires a PC and the manufacturer’s software to run the instrument. These will be supplied with the SMPS.

It is possible to connect the PC to the internet and monitor remotely, with packages such as VNC. However, local internet access and firewalls may prevent this. Users must consult with their service provider. Other software for remote connection may be applied to the PC, but this must be agreed with the IS before any installation.

The AMS must be operated in a dry environment and on a level surface within the environmental conditions given in the specifications

When not being used on the aircraft:

• The in-built SMPS unit operates with a sealed radioactive source, known as the neutraliser
• . The source is Sr-90, a beta with an activity of 370MBeq.
• The University of Manchester has a mobile license and can operate it around the country and the research aircraft also has a licence to operate dangerous goods. However, all users who require the use of the AMS must have their own site licence to house this source and have spare capacity on the licence. If this source is to be used away from the host institute, then users must have a mobile licence, with capacity for the source.

The instrument is serviced annually by the manufacturer if the instrument has been used extensively. This includes checking the flow rate, optics alignment and replacing filters and tubing

It should be noted that there is no absolute calibration standard for particle number concentration. The sizing and counting efficiency is checked using PSL spheres and a reference counter.

A basic service check will be performed on the unit before it is shipped to the user.

Costs

• Instrument Insurance
  • The system is insured whilst it is on the research aircraft. The system has been designed to be rugged; however, the end-user must respect the fact that the system is a precision instrument that must be treated with great care. If the AMS is requested for use NOT on the research aircraft, the user must ensure that the instrument is insured to the value of £300K.
• Public Liability Insurance
  • NCAS is not liable for any damage or injury arising from the deployment or operation of this instrument when unattended by the IS.
• Shipping Expenses
  • The user is liable for all costs arising from the shipping of the instrument both to and from the FAAM base of operations at Cranfield.
• IS T&S
  • The user is responsible for covering the travel and subsistence expenses of the IS during mobilisation and demobilisation at the FAAM base of operations at Cranfield. T&S during a campaign are negotiated by FAAM.
• Ionising radiation licence
  • The AMS has a built-in SMPS, which uses a radioactive source for aerosol neutralisation. A license may be required if the AMS is not used on the research aircraft, which can be obtained from the Environment Agency. See ‘Site Requirements’ for information regarding the source.

Shipping

In most circumstances, shipping will be organised by the Instrument Scientist.

Under most circumstances, the Aerodyne Compact Time of Flight Aerosol Mass Spectrometer will be flown on the FAAM research aircraft.

When not being used on the FAAM aircraft

The instrument needs to be housed in a dry environment that does not exceed the environmental conditions as detailed in the specifications. The instrument should be situated on a level surface.

Licenses for Radioactive Materials

The in-built SMPS unit operates with a sealed radioactive source, known as the neutraliser. The source is Sr-90, a beta with an activity of 370MBeq. The University of Manchester has a mobile license and can operate it around the country and the research aircraft also has a licence to operate dangerous goods. However, all users who require the use of the AMS must have their own site licence to house this source and have spare capacity on the licence. If this source is to be used away from the host institute, then users must have a mobile licence, with capacity for the source. Before any loan of the AMF AMS is agreed, the users must supply proof of the licence and the spare capacity to the IS and a copy of the local rules for handling and storage of Radioactive substances for approval by University of Manchester local Radiological Protection Officer.

If the AMS is to be used on a project where the IS will be in attendance all the time, then alternative arrangements can be made. This may incur extra costs, either as T&S or as VAD.

Users are responsible for the transport of these sources and specialist couriers with the appropriate dangerous goods licences may be required. A copy of the courier’s licence will be required before any source is released.

Eye safety

• The AMS has a built-in SMPS, which uses a CPC. The CPC is a class 1 laser-based instrument and is eye-safe for all conditions of use, providing the covers are not removed.

Electric safety

• Under no circumstances should any attempt be made to open the up the main body of the instrument. This will expose the user to high voltages. Only appropriately qualified persons should attempt to fault find these units.

Attended operation

• There is no requirement for the system to be attended during operation from a safety standpoint.

COSHH

• Under most circumstances, the IS will operate the AMS and will use their own COSHH forms.

Manual handling

• The AMS weighs in excess of 200kg. It is advisable to move the AMS with a pallet truck or fork lift.

Ionising Radiation (when not used on the research aircraft)

• The in-built SMPS unit operates with a sealed radioactive source, known as the neutraliser. The source is Sr-90, a beta with an activity of 370MBeq. The University of Manchester has a mobile license and can operate it around the country and the research aircraft also has a licence to operate dangerous goods. However, all users who require the use of the AMS must have their own site licence to house this source and have spare capacity on the licence. If this source is to be used away from the host institute, then users must have a mobile licence, with capacity for the source. Before any loan of the AMF AMS is agreed, the users must supply proof of the licence and the spare capacity to the IS and a copy of the local rules for handling and storage of Radioactive substances for approval by University of Manchester local Radiological Protection Officer.
• If the AMS is to be used on a project where the IS will be in attendance all the time, then alternative arrangements can be made.
• Users are responsible for the transport of these sources and specialist couriers with the appropriate dangerous goods licences may be required. A copy of the courier’s licence will be required before any source is released.

Flow rates: Aerosol 1.3 cc/s (at sea level) determined by pressure gauge
Charger/neutraliser: Bipolar, Sr-85, 10 millicurie
Dimensions: 45.7cm x 41.4cm x 40.6cm
Weight: 220kg (with wheels)
Power requirements: 100 to 240 VAC, 50-60 Hz, 2.1 KW
Environmental Conditions: Indoor use, ambient humidity (0-90% relative humidity non-condensing), temperatures between 10 and 32°C

This instrument measures the size-resolved mass concentration per cubic metre of non-refractory particles in the size range of ~ 60nm to 600nm (at sea level). Non-refractory particles include sulphates, nitrates, organics, and ammonium. Specifically, the AMS will record any substance in the solid or liquid phase with a flash vapourisation temperature of 600°C or less. This excludes most metals, dust and sea salt. These can be presented as time series of mass, mass spectra, and mass size distribution

Field Data

• The instrument produces a range of out files and all are text format.
• The user can download (but not delete) this data from the instrument but it should be noted that this data will not have been quality controlled.

Archive Data

• Data is provided in NetCDF files following the AMOF data standard
• Files contain no more than 24hr of data.
• Instrument name is
  • ncas-aerosol-mass-spec-1
• The data product(s) associated with this instrument:
  • aerosol-no3-so4-nh3-org-concentration
• Example data file
  • How to use example data
  • ncas-aerosol-mass-spec-1_ral_29001225_aerosol-no3-so4-nh3-org-concentration_v0.1.nc