Photoacoustic Extinctiometer

The PAX is a sensitive, high-resolution, fast-response instrument for measuring aerosol optical properties relevant for climate change and carbon particle sensing.


Get accurate measurements for both light absorption and scattering in a single instrument. Then correlate the absorption measurement black carbon mass concentration.

Get continuous and autonomous operation, with no filter collection required and no filter data artifacts.

Count on fast response, one-second resolution, and real-time data display on a high-resolution touch screen.

Achieve excellent scattering coefficient sensitivity with reciprocal nephelometry.

Select from four wavelengths (870 nm [standard], optional 405 nm, or 532 nm), and get wide dynamic range suitable for pristine regions to source sampling.

Particle light-scattering and photoacoustic absorption

  • Modulated diode laser simultaneously measures light scattering and absorption.
  • 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 and quantified to determine aerosol light absorption.
  • Wide-angle integrating reciprocal nephelometer measures the light scattering coefficient of all particles.
  • PAX laser wavelength options include:
    • 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

Parameter Specification
Measured Parameters

Absorption coefficient, Babs
Scattering coefficient, Bscat

Auxiliary Parameters

Relative Humidity

Derived Parameters

Black carbon (BC) mass
Extinction coefficient, Bext
Single scattering albedo, SSA
Dew Point

Measurement Range – Absorption and Scattering

< 1 Mm-1 – 10,000 Mm-1 (870 nm, 60 sec. averaging)


870 nm (1.4 W)
Modulation Frequency: 1500 Hz nominal, square wave

Angular Integration for Scattering

6° to 174°

Sample Flow

1 L/min
Flow Control: Critical orifice
Diaphragm Pump

Response Time

< 10 sec; one-second resolution

Data Averaging Time

1, 10 or 60 seconds, user selectable

Zero Check

On demand, or automated at user selectable interval at 5, 15, 20, 30, or 60 minutes. Zero check with high-efficiency filtered air sample.

Front Panel Features
  • Graphical color touch-panel display screen
  • Two USB-A ports
  • Power switch
  • User interface via touch screen or standard keyboard
Rear Panel Connections
  • Sample inlet (compression fitting for ¼” tube)
  • Pump exhaust (compression fitting for ¼” tube)
  • AC and DC power connections
  • Serial RS-232 jack
  • Ethernet port
  • Two analog BNC inputs
  • Four analog BNC outputs
Data Storage Capacity

> 200 days

Communications Output

Ethernet 100/10 Mbps, RS-232 Serial

Power Requirements

90 – 264 V, 47 – 63 Hz (AC Power) or 12 VDC Power Consumption 45 W


18 cm H x 48 cm W x 61 cm D (7″ H x 19″ W x 24″ D); rack mountable


18 kg (40 lb)

Environmental Operating Conditions

Temperature: 0 – 40°C (32 – 104°F)
RH: 0 – 90% RH non-condensing

Computer Requirements for PMC Software (Computer not included)
  • Windows XP, Vista, or Windows 7
  • Minimum 1GB RAM

Hard-shell shipping case suitable for frequent travel

PM 1.0 cyclone inlet (1 liter/min) for particle size selection

PM 2.5 cyclone inlet (1 liter/min) for particle size selection

Droplet offers a three-year service plan to ensure that your PAX continues to deliver high-quality data. This plan locks costs upfront at the time of funding application and covers annual calibration, alignment, cleaning, wear-item replacement, calibration documentation, and shipping. Specific services provided for the PAX include:

  • Shipping to and from Droplet facility for service
  • Incoming instrument evaluation and health check
  • O-ring replacement
  • Hardware replacement
  • Software, firmware, and anti-virus update
  • Ambient temperature sensor verification
  • RH sensor performance evaluation and replacement
  • Optics cleaning
  • Laser alignment
  • Laser heating control adjustment
  • Photodiode current adjustment
  • Filter replacement
  • Thermosiphon replacements
  • Wiring verification
  • 2-day zero filter test
  • 7-day ambient air test
  • High humidity test
  • Software, firmware, and antivirus update
  • 100% unit functionality test
  • Calibration data, calibration report, and Certificate of Calibration provided

Talk to an expert

Speak with one of our scientists or experts at Droplet Measurement Technologies today to discover solutions that can power your research.

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