Particles - Aethlabs microAeth MA200 Operating Manual

4.6. GPS
The microAeth has access to the Global Positioning System with a built-in antenna. The GPS is used for
precise, automatic time synchronization and for optional location tracking.
4.7. Flow
Two separate mass flowmeters each make a flow measurement of the sample air stream every Timebase.
These sensors are always recorded. In the data output, these values are recorded in the column labeled "Flow
total (mL/min)" and "Flow1 (mL/min)." The accuracy of the flow measurements is typically ± 5% FS (25 °C
characteristic).
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5. Configuration and Operation
5.1. Overview
The microAeth MA Series instruments are highly sensitive, portable, and miniature five-wavelength
instruments designed for measuring the light absorbing carbon ('LAC') particles. The instruments have an
880 nm optical channel which is primarily interpreted as Black Carbon ('BC'). The instruments also measure
Ultraviolet Particulate Matter ('UVPM') and makes measurements at three other wavelengths which can be
used to calculate the angstrom exponent for source apportionment or other investigations into the optical
properties of light absorbing particles in the atmosphere. The instruments are based on the well-established
Aethalometer® measurement principle used for over 30 years in laboratory-sized analyzers. The microAeth
draws an air sample at a flow rate of 50, 100, or 150 ml/min through a 3 mm diameter portion of the filter
media. Optical transmission through the 'Sensing' spot is illuminated by stabilized 880 nm (IR), 625 nm
(Red), 528 nm (Green), 470 nm (Blue), and 375 nm (UV) LED light sources and measured by a detector. The
optical attenuation (ATN) due to absorbance of particles collected on the spot is measured relative to an
adjacent 'Reference' portion of the filter where no particles are accumulated. This change in ATN is derived
using a starting measurement and an additional measurement at the end of the timebase period. The gradual
accumulation of optically-absorbing particles leads to an increase in ATN from one period to the next.
The air flow rate through the spot is measured by one or more mass flow sensor(s) which are also used to
stabilize the pump. The electronics and microprocessor measure and store the data each timebase period to
determine the ATN increment during each timebase. This is then converted to a mass concentration of BC
expressed in nanograms per cubic meter (ng/m³) using the known optical absorbance per unit mass of Black
Carbon material. The instrument's operating parameters are set up by an external software application or by
the on-board interface. Operation is completely automatic after the instrument is turned on and sampling is
started. During operation, the microprocessor performs the optical measurements, measures and stabilizes
the air flow, calculates the BC mass concentration and records data to internal nonvolatile memory. The data
may be downloaded at a later time by the same external software package or over serial. The microAeth
derives its power from an internal rechargeable lithium-ion battery.
5.2. Recommendations for Best Use Practices
The small size and light weight of the microAeth® allow it to be used to gather data in a wide range of
operational scenarios, not always possible using larger instruments. Optimization of performance across the
breadth of applications requires an understanding of scientific objectives, operational settings, their impact
on instrumental sensitivity and trade-offs, as well as proper maintenance of the instrument. The following
recommendations provide general guidelines.
5.2.1. Instrument Settings: Measurement Timebase and Flow Rate
In order to get the best data from the microAeth for a sampling campaign, we highly recommend that the
instrument warm up for approximately 30 minutes so that it can equilibrate. The microAeth can acquire
data on six timebase settings: 1, 5, 10, 30, 60, or 300 seconds. The 1 second timebase should only be
used under special circumstances where a decreased signal-to-noise ratio is acceptable. At this setting,
instrumental noise is larger and typically requires post-processing. The microAeth pump can operate at
three sampling flow rate settings: 50, 100, and 150 ml/min. The choice of these parameters affects the
operation and data. On a 1 second timebase, the instrument will acquire about 25 megabytes of data per
day, which may be more challenging to handle and take longer to download.
AethLabs AethLabs microAeth MA Series MA200, MA300, MA350 Operating Manual
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