Droplet Measurement Technologies (DMT)

Model PAX - Photoacoustic Extinctiometer



This is a sensitive, high-resolution, fast-response instrument for measuring optical aerosol properties relevant for climate radiative forcing and carbon particle sensing. A sensitive, high-resolution, fast-response instrument for measuring aerosol optical properties relevant for climate change and carbon particle sensing.


  • Direct in-situ measurement of light absorption and scattering in a single instrument
  • Absorption measurement correlates to black carbon mass concentration 
  • Reciprocal nephelometry provides excellent scattering coefficient sensitivity 
  • Choice of wavelengths: 870 nm (standard); optional 405 nm or 532 nm
  • Fast response, one-second resolution, real-time data display
  • Wide dynamic range suitable for pristine regions to source sampling
  • Continuous and autonomous operation
  • High-resolution touch screen display for real-time data and instrument status
  • No filter collection required -- no filter data artifacts


  • Air quality and visibility
  • Atmosphere and climate
  • Health effects
  • Combustion source emissions
  • Biomass burning

Suitable for fixed site, mobile or airborne sampling        

Photos:The research station at Rocky Mountain National Park (near right); the PAX inside the research station (far right). Photos by Gavin McMeeking.


The PAX is controlled by fully-integrated software that presents an intuitive user interface via either the front-panel touch-screen or a web browser on any networked computer. This interface provides real-time display of measured and calculated data and instrument status, and allows the configuration of the instrument's operating parameters. Access to this interface by web-browser is password-protected and allows full instrument operation and configuration from remote locations. Data is written to an easy-to-read ASCII file. An external program which can be run on a network-connected computer can be used to download, archive, and display these data files. This program can also upload calibration information to the PAX and perform other maintenance functions.

How It Works

The PAX uses a modulated diode laser to simultaneously measure light scattering and absorption. The standard 870-nm wavelength is especially sensitive to black carbon particles, since there is relatively little absorption from gases and non-BC aerosol species at this wavelength. 

PAX laser wavelength options:

  • Red (870 nm)– absorption is highly specific for black carbon (soot) particles; scattering best for large particles
  • Green (532 nm)– measures in the visual range, typically what the human eye observes
  • Blue (405 nm)– absorption correlates to the organic, or brown carbon content; efficient scattering for fine and ultrafine particles

A 1 L/min aerosol sample flow is drawn into the PAX using an internal vacuum pump controlled by two critical orifices. The flow is split between the nephelometer and photoacoustic resonator for simultaneous measurement of light scattering and absorption.

The absorption measurement uses in-situ photoacoustic technology. A laser beam directed through the aerosol stream is modulated at the resonant frequency of the acoustic chamber. Absorbing particles heat up and quickly transfer heat to the surrounding air. The periodic heating produces pressure waves that can be detected with a sensitive microphone. The system then determines the resonator quality factor and resonance frequency, which are needed to quantitatively determine aerosol light absorption. Phase-sensitive detection is used for all sensors.

The PAX uses a wide-angle integrating reciprocal nephelometer to measure the light scattering coefficient. The scattering measurement responds to all particle types regardless of chemical makeup, mixing state, or morphology. 

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