> For the complete documentation index, see [llms.txt](https://docs.aethlabs.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.aethlabs.com/microaethalometer-tm-ma600/manual/measurement-principles.md).

# 5. Measurement Principles

Aethalometer method, five-wavelength optical analysis, DualSpot loading compensation, source apportionment, environmental sensors, and flow control for the microAethalometer™ MA600.

## 5.1 Aethalometer Method

The microAethalometer MA600 measures aerosol light absorption using the Aethalometer method. Aerosol-laden air is drawn through the instrument at a controlled flow rate and deposited onto a filter tape. The instrument measures the optical transmission through the filter at five wavelengths simultaneously, reporting the rate of change as aerosol accumulates.

**Black Carbon (BC):** Measurement at 880 nm (infrared) is interpreted as the concentration of Black Carbon. BC is a strong light absorber across all visible and near-infrared wavelengths and is the dominant light-absorbing aerosol at this wavelength in most ambient environments.

**UVPM (Ultraviolet Particulate Matter):** Measurement at 375 nm (ultraviolet) is sensitive to light-absorbing organic carbon from sources such as woodsmoke, tobacco smoke, and biomass burning. This channel is reported as UVPM.

**BCc (Loading-Compensated BC):** The raw BC measurement is subject to a loading artifact — as the filter accumulates aerosol, the apparent absorption per unit mass changes. BCc is the loading-compensated value produced by the DualSpot® algorithm (§5.3). BCc is the recommended output for most applications.

## 5.2 Five-Wavelength Optical Analysis

The MA600 measures at five wavelengths simultaneously:

| Channel | Wavelength | Primary sensitivity                          |
| ------- | ---------- | -------------------------------------------- |
| IR      | 880 nm     | Black Carbon (BC)                            |
| Red     | 625 nm     | Mixed source discrimination                  |
| Green   | 528 nm     | Mixed source discrimination                  |
| Blue    | 470 nm     | Mixed source discrimination, biomass burning |
| UV      | 375 nm     | UVPM, brown carbon, biomass burning          |

Each wavelength produces independent BC1, BC2, and BCc values (BC2 and BCc for DualSpot® only). The DualSpot® mode and IR and Blue wavelengths output enable source apportionment calculations (§5.4).

## 5.3 DualSpot® Loading Compensation

DualSpot® is a real-time loading compensation system that corrects the filter-loading artifact: the progressive nonlinearity in apparent absorption as aerosol accumulates on the filter spot. The artifact grows with filter attenuation (ATN), which accumulates on a given spot over time until the tape advances — independent of ambient aerosol concentration — so the correction matters most at high ATN.

![DualSpot flow and optical schematic: sampled air enters the inlet and splits across the filter cassette (reference spot, Spot 2, Spot 1), which is illuminated by the five-wavelength LED bank and read by the light sensors; the Spot 1 path runs through the spot-1 mass flowmeter and the Spot 2 path through its valve and orifice, both merging into the total mass flowmeter, total orifice, and pump before the outlet.](https://272274374-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F1fOQ2MnLWcq0ntHW3LLY%2Fuploads%2Fgit-blob-8f68a1d089ab4cdca256ab8d49680f2c7d7adf46%2Fmax-dualspot-schematic.svg?alt=media)

**How it works:** The instrument draws aerosol-laden air through two filter spots simultaneously, Spot 1 and Spot 2, at different flow rates. The total flow rate setpoint is split between the two spots: Spot 1 (the primary spot) receives 75% of the total flow and Spot 2 (the secondary spot) receives 25%. At a 150 ml/min total setpoint, this is approximately 112 ml/min through Spot 1 and 38 ml/min through Spot 2. Because Spot 2 loads at a lower rate, it serves as a reference for the loading state of the filter.

At each wavelength, the compensation factor K is determined so that the loading-compensated black carbon from the two spots converges. The compensated value BCc is then calculated for each wavelength from BC1, K, and ATN1.

**When DualSpot is active:** Status code bit 64 is set. Both spots are sampling and the K calculation is active.

**DualSpot® vs SingleSpot™:** SingleSpot samples on a single spot at the full flow rate. It does not apply loading compensation. Use SingleSpot only for low-loading environments or when DualSpot flow constraints cannot be met. See [Chapter 6: Instrument Settings](https://docs.aethlabs.com/microaethalometer-tm-ma600/manual/pages/Wdh3XfSYR8hfsnx78Z6A#id-6.4-dualspot-r-vs.-singlespot-tm) §6.4 for the decision guide.

## 5.4 Source Apportionment

When DualSpot® is enabled and the required wavelengths are active (IR + Blue, at minimum), the instrument calculates real-time Source Apportionment outputs using the Absorption Angstrom Exponent (AAE) method.

**Angstrom Exponent:** The wavelength dependence of light absorption differs between BC source types. Fossil fuel combustion (FF) has an AAE near 1.0. Biomass burning (BB) has a higher AAE, typically 1.5–2.5. By comparing absorption at the IR and Blue wavelengths (at minimum), the instrument estimates the fractional contribution of each source type.

**Source Apportionment output columns in the data file:**

* `Cref`: Multiple scattering coefficient. An empirical constant that corrects the measured aerosol absorption coefficient for the filter multiple-scattering artifact (default 1.3 for microAethalometer PTFE)
* `AAE biomass`: Angstrom exponent for biomass burning used in the calculation
* `AAE fossil fuel`: Angstrom exponent for fossil fuel used in the calculation
* `Biomass BCc`: Estimated biomass burning contribution (ng/m³)
* `Fossil fuel BCc`: Estimated fossil fuel contribution (ng/m³)
* `AAE`: Calculated Absorption Angstrom Exponent from the data
* `BB%`: Estimated percent contribution from biomass burning
* `Delta-C`: Difference between UV-derived and IR-derived BC concentrations

**Source Apportionment requires:**

* DualSpot® mode enabled
* **IR (880 nm) and Blue (470 nm)** wavelengths enabled for the Sandradewi aethalometer model, a two-wavelength source-apportionment method
* **IR (880 nm) and UV (375 nm)** wavelengths enabled for the Delta-C model
* Status code 4096 (S.A. disabled) must NOT be active

If Source Apportionment is disabled, check instrument settings and verify that all required settings are enabled.

## 5.5 Environmental Sensors

The MA600 includes environmental sensors that record alongside the aerosol measurements:

| Sensor            | Output                                        | Notes                                        |
| ----------------- | --------------------------------------------- | -------------------------------------------- |
| Temperature       | Sample stream temp (°C), barometric temp (°C) | Two sensors: sample stream and internal case |
| Relative Humidity | Sample stream RH (%)                          | Indicates filter loading conditions          |
| Pressure          | Internal case pressure (Pa)                   | Barometer                                    |
| Dewpoint          | Calculated from temp and RH                   | Derived field                                |
| Accelerometer     | X, Y, Z axes (integer counts)                 | Useful for motion artifact detection         |

The MA600 does not include GPS or WiFi; position and network fields present on other MA Series models are not recorded.

## 5.6 Flow Measurement and Control

The MA600 uses internal mass flowmeters with closed-loop control to maintain the configured flow rate setpoint. Flow is drawn by a brushless DC (BLDC) pump.

**Flow setpoint:** The total flow drawn through the instrument. In DualSpot® mode, total flow is split between Spot 1 / sense1 (75%) and Spot 2 / sense2 (25%).

**Flow ranges:** 100–250 ml/min. The recommended setpoint is 150 ml/min. Not all flow rates are compatible with every DualSpot® configuration — see [Chapter 6: Instrument Settings](https://docs.aethlabs.com/microaethalometer-tm-ma600/manual/pages/Wdh3XfSYR8hfsnx78Z6A#id-6.3-flow-rate) §6.3.

**Mass flow correction:** Internal flowmeters measure mass flow and convert to volumetric flow using a calibration table that accounts for temperature and pressure at the time of calibration.

**Flow calibration:** Required periodically using the MA Series Flow Calibration Kit (see [Chapter 10: Calibration & Maintenance](https://docs.aethlabs.com/microaethalometer-tm-ma600/manual/pages/rKTSjRcxhGXvsZsNR9rj#id-10.3-flow-calibration) §10.3). Do not attempt flow calibration with third-party flowmeters — the calibration process requires the proprietary AethLabs kit.