26
microAeth
®
MA Series MA200, MA300, MA350
Operating Manual
microAeth
®
MA Series MA200, MA300, MA350
Operating Manual
27
AethLabs
AethLabs
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 and incorporate
the patented DualSpot® loading compensation method. 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. Due to the flow split between
Spot 1 and Spot 2 the measurement sensitivity in DualSpot® mode will be lower than in SingleSpot™
mode for the same total flow rate. DualSpot mode may not be compatible with 50 ml/min flow rate unless
measuring higher mass concentrations.
5.2.2. Battery Runtime on Single Charge
Battery Runtime on Single Charge: Affected by flow rate and timebase settings.
NOTE: Battery life will gradually diminish after many cycles (~ 1 year of use). Runtimes vary based on
individual microAeth instruments and specific environments.
5.2.3. Effects of Contamination
Effects of Contamination, Vibration, and Impact: Primarily affected by timebase setting.
1 second
5 seconds
10 seconds
30 seconds
60 seconds
300 seconds
very large
large
large
moderate
low
least
5.2.4. Recommended Settings for Different Scenarios
Different Black Carbon measurement scenarios require different operational settings for optimum
performance. The 1 second timebase setting is a ‘Data Acquisition Mode’ intended for subsequent
processing, and should NOT be used for routine monitoring. 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. Data collected on a 1 second timebase should always be smoothed or averaged over longer
periods, in order to optimize the signal-to-noise ratio at the desired time resolution.
5.2.5. Contamination, Maintenance, and Cleaning of Sample Chamber
If a loose particle of contamination enters the sample chamber of the microAeth or the instrument
experiences vibration or impact, the data may be degraded. Shaking or tapping a “dirty” instrument may
create data excursions that are far larger than those of a “clean” unit. These effects are amplified greatly
at the shorter timebase settings. Our recommendations for cleaning are based upon the likelihood of
contamination and the nature of use.
5.2.7. Contamination Probability for Various Use Scenarios
Sampling Scenario without use of microCyclone™
Contamination Probability
Dry, dusty environment
High
Occupational settings with combustion exhaust
High
Exposure to “oily” smokes such as biomass-burning plumes,
2-cycle engine exhaust
High
Presence of suspended fluff, fibers, pollen
High
Immediate vicinity of traffic and roadways
High
Outdoor urban environments
High
Outdoor rural environments (without dust, fluff, pollen)
High
Residential indoor environments
High
