Two portable aerosol time-of-flight mass spectrometers
(ATOFMS) of identical design are described. These
instruments are powerful new tools for providing temporal
and spatial information on the origin, reactivity, and
fate
of atmospheric aerosols. Each is capable of analyzing
the
size and composition of individual particles from a polydisperse aerosol in real-time. Particles are
introduced
into the instrument through a particle beam interface,
sized by measuring the delay time between two scattering
lasers, and compositionally analyzed using a dual-polarity
laser desorption/ionization time-of-flight mass spectrometer. These are the first dual-ion TOFMS instruments
to
utilize a dual reflectron design. The instruments
measure
72 in. long × 28 in. wide × 60 in. high and weigh
∼500
lb. Pneumatic tires allow them to be transported
through
standard doorways, elevators, and handicap ramps, granting access to virtually any location. Furthermore,
because
of rugged construction they will be able to operate during
transport by automobile, boat, or aircraft.
The heterogeneous replacement of chloride by nitrate in individual sea-salt particles was monitored continuously over time in the troposphere with the use of aerosol time-of-flight mass spectrometry. Modeling calculations show that the observed chloride displacement process is consistent with a heterogeneous chemical reaction between sea-salt particles and gas-phase nitric acid, leading to sodium nitrate production in the particle phase accompanied by liberation of gaseous HCl from the particles. Such single-particle measurements, combined with a single-particle model, make it possible to monitor and explain heterogeneous gas/particle chemistry as it occurs in the atmosphere.
Aerosol time-of-flight mass spectrometers (ATOFMS) measure the size and chemical composition of single aerosol particles. To date, these instruments have provided qualitative descriptions of aerosols, in part because the fraction of particles actually present in the atmosphere that is detected by these instruments has not been known. In this work, the particle detection efficiencies of three ATOFMS instruments are determined under ambient sampling conditions from the results of colocated sampling with more conventional reference samplers at three locations in southern California. ATOFMS particle detection efficiencies display a power law dependence on particle aerodynamic diameter (D a ) over a calibration range of 0.32 < D a < 1.8 microns. Detection efficiencies are determined by comparison of ATOFMS data with inertial impactor data and are compared to detection efficiencies determined independently by the use of laser optical particle counters. Detection efficiencies are highest for the largest particles and decline by approximately 2 orders of magnitude for the smallest particles, depending on the ATOFMS design. Calibration functions are developed here and applied to scale ATOFMS data to yield continuous aerosol mass concentrations as a function of particle size over an extended period of time.
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