Aerosol Mass Measurement
A wide variety of techniques are used to measure aerosol mass concentration (mg/m3), including opacity, reflectance, Laser Induced Incandescence (LII), photoacoustic and gravimetric.
An online reference standard for mass concentration is desireble, to allow both initial calibration of measurement instruments, and subsequent validation in the field.
The CPMA Electrometer Reference Mass Standard
A functional and calibrated online aerosol mass standard may be achieved through combining various aerosol instruments:
The source is an aerosol generator. The nature of the aerosol will depend on the instrument to be calibrated- for many instruments measuring black carbon a soot generator is appropriate.
The Unipolar Diffusion Aerosol Charger uses a controlled corona discharge to highly electrically charge the particles from the source.
Unipolar charging allows much higher levels of charge to be achieved (compared with bipolar devices) and there are no radioactive materials involved, greatly simplifying both licencing and transport.
The Centrifugal Particle Mass Analyzer allows selection of the charged particles from the UDAC and outputs an aerosol which is monodisperse on a mass:charge ratio. That is the particles exiting the CPMA may have a variety of sizes, but they will be of only a single charge to mass ratio.
The electrometer collects the charged particles on a filter, and measures the charge arriving as a current.
Knowing the setpoint of the CPMA in femtograms (1x10-15g) each electron arriving at the electrometer corresponds to a particle of the mass set on the CPMA arriving.
Since double charged particles will only pass through the CPMA if they have double the mass, the calculation of mass concentration from current is strightforward given knowledge of the gas flow rate (measured internally by the flowmeter) and the CPMA mass setpoint, and is automated in software.
The Electrostatic Precipitator removes all charged particles from the airflow when switched on. When switched off, the straight through design ensures low particle losses and the particles pass to the output.
The Condensation Particle Counter measures the concentration of particles downstream of the Electrostatic Precipitator. If very heavy particles are being selected with the CPMA, some very light particles which are not charged in the UDAC may not be removed in the CPMA. Measurements of concentration measured by the CPC with the ESP turned on and then off allows the uncertainty introduced by a zero charge population to given an upper bound. For most mass setpoints, this effect may be ignored.
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