Particle Time-of-Flight by Hadamard Transform (ePToF): A new high-duty-cycle approach to size-segregated and total aerosol mass measurements for the Aerodyne Aerosol Mass Spectrometer

Abstract:

Most aerosol mass spectrometers rely on measuring the terminal velocity that the particles acquire in the introduction of the air sample into vacuum to determine the particle size (aerodynamic diameter). This is done by measuring the time-of-flight between two defined points in the particle beam path. In the Aerodyne Aerosol Mass Spectrometer, a chopper with a narrow slit (typically 2% throuput for acceptable size resolution) is used to gate the incoming particle beam and record a velocity distribution as the particles hit the vaporizer/ionization chamber. The low duty cycle of the chopper leads to low signal-to-noise ratio (S/N) for rapidly changing conditions and/or low aerosol concentrations, as well as the need to alternate between a total mass mode (with typically a 60% duty cycle) and size-segregated mode.

A new chopper assembly has been constructed and tested that uses a multi-slit pattern with a particle throughput of 50%. Using an inversion method that has successfully been used previously in ion time-of-flight spectrometry (Hadamard transform), a full particle velocity spectrum can be acquired with both high S/N and very good resolution (equivalent to a traditional 1% chopper wheel).

We have shown that this method retrieves realistic particle size distributions regardless of aerosol volatility and that the S/N improvements are consistent with the increase in duty cycle. These can be improved even further by using noise filtering techniques. We have deployed the new chopper at two field campaigns (SOAS and SEAC4RS) and confirmed the performance of the new technique with ambient aerosol.

The increased duty cycle also means that both total mass and size-segregated measurements can be taken at once within the same (uninterrupted) datastream, greatly increasing the overall data coverage in both modes. Both ambient and chamber data will be presented to demonstrate this new AMS acquisition mode.