Single-particle measurements of bouncing particles and in situ collection efficiency from an airborne aerosol mass spectrometer (AMS) with light-scattering detection

摘要

A light-scattering module was coupled to an airborne, compacttime-of-flight aerosol mass spectrometer (LS-AMS) to investigate collectionefficiency (CE) while obtaining nonrefractory aerosol chemical compositionmeasurements during the Southeast Nexus (SENEX) campaign. In this instrument,particles scatter light from an internal laser beam and trigger savingindividual particle mass spectra. Nearly all of the single-particle data withmass spectra that were triggered by scattered light signals were fromparticles larger than ∼ 280 nm in vacuum aerodynamic diameter.Over 33 000 particles are characterized as either prompt (27 %), delayed(15 %), or null (58 %), according to the time and intensity of theirtotal mass spectral signals. The particle mass from single-particle spectrais proportional to that derived from the light-scattering diameter() but not to that from the particle time-of-flight (PToF)diameter () from the time of the maximum mass spectralsignal. The total mass spectral signal from delayed particles was about80 % of that from prompt ones for the same . Both fieldand laboratory data indicate that the relative intensities of various ions inthe prompt spectra show more fragmentation compared to the delayed spectra.The particles with a delayed mass spectral signal likely bounced off thevaporizer and vaporized later on another surface within the confines of theionization source. Because delayed particles are detected by the massspectrometer later than expected from their size, they canaffect the interpretation of particle size (PToF) mass distributions,especially at larger sizes. The CE, measured by theaverage number or mass fractions of particles optically detected that hadmeasurable mass spectra, varied significantly (0.2–0.9) in different airmasses. The measured CE agreed well with a previous parameterization whenCE  0.5 for acidic particles but was sometimes lower than the minimumparameterized CE of 0.5.