Real-time aerosol mass spectrometry with millisecond resolution

The time-of-flight aerosol mass spectrometer (ToF-AMS)determines particle size by measuring velocity after expansion into vacuum and analyzes chemical composition by thermal vaporization and electron ionization mass spectrometry (MS). Monitoring certain dynamic processes requires the ability to track changes in aerosol chemistry and size with sub-second time resolution. We demonstrate a new ToFAMS data acquisition mode capable of collecting high-resolution aerosol mass spectra at rates exceeding 1 kHz. Coupled aerosol size and MS measurements can be made at approximately 20 Hz. These rates are about 1/10 of the physically meaningful limits imposed by the ToF-AMS detection processes. The fundamentals of the time-of-flight MS (TOFMS) data acquisition system are described and characterized with a simple algebraic model. Derived expressions show how improvements in data acquisition and computer hardware will translate into rates approaching the physical limits. Conclusions regarding limits of performance can be extended to other TOFMS that use analog signal detection in a high-speed application outside of aerosol science. The high-speed acquisition mode of the ToF-AMS enables speciated aerosol eddy covariance flux measurements, which demand precise, 10-Hz synchronization of the MS with a sonic anemometer. Flux data acquired over a forest during the BEARPEX-1 campaign are presented as an example of this new technique. For aircraft measurements, faster acquisition translates to higher spatial resolution, which is demonstrated with data from the recent NASA ARCTAS field campaign in Alaska. Finally, the fast acquisition mode is used to measure the rapid fluctuations in particle emissions of a controlled biomass burn during from the FLAME-2 experiment. To our knowledge this is currently the fastest system for acquisition of chemically resolved aerosol data.