Measurement of aerosol optical properties using pulsed laser

Measurement of aerosol optical properties using pulsed laser

Measurement of aerosol optical properties using pulsed laser cavity
ring-down spectroscopy (CRDS).
ABSTRACT
Accurate measurements of optical extinction coefficients and scattering parameters for
atmospheric aerosols are needed in order to quantify the effects aerosols have on climate
change. Although this field was once dominated by Fluorescence FAGE and long-path
absorption spectroscopy DOAS, cavity ring-down spectroscopy (CRDS) has emerged as a
cost-effective, accurate alternative technique that has numerous advantages. CRDS is an
optical absorption technique based on Beers Law that allows for the characterization of
extremely low concentration target samples by using two highly reflective mirrors to create
an optical cavity with a pathlength thousands of times longer than its meter long
laboratory footprint. System ring-down information will be monitored and collected using
LabView, a program capable of quantifying sensory data and associated ring-down
measurements. Well-characterized laboratory generated aerosols of various sizes and
compositions will be used to characterize the instrument.

Logan Bevill, Julio Castillo, and Jay Pittman, and Kristin S. Dooley
Chemistry Department, The University of Central Arkansas, Conway, AR 72035

INSTRUMENTATION
The basic schematic for a typical CRD experiment is
shown to the right. CRD spectroscopy is a direct
absorption technique based on the measurement of the rate
of absorption rather than the magnitude of absorption, so
fluctuations in light source intensity do not have an effect
on overall detection limits. The CRD spectrometer
consists of a gas sample cell with highly reflective mirrors
at each end. The mirrors, with reflectivities greater than
99.99%, act as entrance and exit windows for the sample
cell.

BACKGROUND

Pulsed Cavity Ring-Down Spectroscopy allows for the characterization of aerosol samples
through the calculation of the rates of light decay and absorption observed during
experimental procedure.

The Beer-Lamberts Law is expressed by the following equation:

The pulsed laser beam is directed into the sample cell through the back of
one of the mirrors. The small amount of light that is trapped inside the
optical cavity is reflected repeatedly inside the cavity resulting in an
extremely large pathlength. Pathlengths of several kilometers are normal.
The small amount of light that escapes the cell through the mirror is
detected using a photodetector suited to the wavelength and speed required
by the system. The intensity of light leaving the cavity is recorded as a
function of time. In an empty cavity, the intensity decay rate is a single
exponential function. After a sample is introduced, the change in the decay
rate is due to absorption and scattering by the target species.

Cavity ring-down spectroscopy is an extension of an original technique
introduced by J.M. Herbelin in the early 1980s to test the relative reflectivity of
high-finesse mirrors in a closed vacuum system. Growing demands for precise
atmospheric data led to the development of this technique in its modern form.
CRDS has also been used in numerous instances to look at the scattering and
absorption by aerosols, and to measure concentrations of aerosols in field
campaigns. Below is a portrait of global aerosols that was produced by a GEOS-5
simulation at a 10-kilometer resolution. Dust (red) is lifted from the surface, sea
salt (blue) swirls inside cyclones, smoke (green) rises from fires, and sulfate
particles (white) stream from volcanoes and fossil fuel emissions. (NASA)

THEORETICAL TREATMENT

CURRENT PROGRESS

In this equation, refers to the absorption coefficient (cm-1) of the sample, c is the speed of
light, and RL is the ratio of the cavity length, L, to the length of the cavity where the aerosol
is present. 1/0 and 1/ are the first order decay rate constants, where 1/0 refers to the decay
rate constant from the cavity without a sample and 1/ is the decay rate constant of the cavity
with the sample.
Decay rate constants are affected by outside variables that do not pertain to the absorption
factor of the target sample and contribute to the loss of light. Therefore, accounting for mirror
transmission, light scattering (Rayleigh and Mie) and absorption by species other than the
target species allows for an accurate calculation of decay rate and absorption coefficient.
Assuming that the cavity is filled with a pure sample, initial decay rate can be found in
respect to these variables through the following equation:

Here, T represents mirror transmission, Rayleigh and Mie accounts for scattering, and i are the
Diffusion Drier

Aerosol
Generator
Atomizer Aerosol Generator

Natural sources of aerosols include sandstorms, volcanoes, and wildfires while
human sources of aerosols mainly consist of the burning of fossil fuels, and
changes in land use.
Aerosols play an important role in our earths climate. They can be transported
from many miles around the globe and are one of the most important factors in
climate forcing.

Particle
Neutralizer

The Atomizer Aerosol Generator (shown to the left attached
to a diffusion drier) will be used as the chief method of
generating aerosols for cavity ring-down spectroscopy
calibration. The generator produces aerosols by using a builtin compressor to force air through a high-efficiency filter. The
contaminant free air then made to create a jet by expanding
through the atomizer nozzle. This jet of air then atomizes a
solution or suspension into droplets which are suspended in
the air flow.
We will use NIST traceable polystyrene spheres in the
generator as a method of calibration and instrument
characterization.

Aerosols are important to characterize and understand for many reasons including
their influence on climate change. A main focus of this research is to better
characterize the optical properties of aerosols of various compositions and sizes in
order to help understand the warming or cooling effect aerosols have on earths
radiation energy budget.

We are also working on the Labview interface that will control and
record all data collection from the CRDS spectrometer.

The amount of radiative forcing due to atmospheric aerosols depends on the type
of aerosol and also the characteristics of the earths surface below the aerosols.

Although it has taken time to progress with the Labview
programming language, we are now making progress in this area.

Aerosol and surface combinations that have
the largest influence on climate change

We use a NI BNC-2120 for a convenient method of connecting all
of our analog and digital signals to an NI PCIe-6361 housed in the
computer chassis.

Absorbing
aerosol

Low Surface Albedo

Scattering
aerosol

Our program design will record raw data from the spectrometer as
well as pressure and temperature readings, and concentration
analysis information. All averaged and analyzed data will also be
saved and recorded using the program. A portion of the program
is shown to the left. The VI shown collects triggered analog signal
and converts it to a form that is able to be saved as both a binary
file and an excel spreadsheet.
High Surface Albedo

To CRDS
Inlet

The Sioutas Multistage impactor (exploded
image shown below) will be used to collect
particles in the exhaust of the CRDS to aid in
discerning the concentration of particles in
the CRDS system.
The Impactor is able to separate the collected
particles into rough size ranges. Stage A
collects particles greater than 2.5 m, Stage B m, Stage B
collects particles 2.5 1.0 m, Stage B m, Stage C collects
1.0- 0.50 m, Stage B m particles, and Stage D collects
0.50 0.25 m, Stage B m.

absorption coefficients for any species other than the target sample.
Taking the limit of the equation,

allows for the calculation of the minimum detectable absorption coefficient, min. The
minimum absorption coefficient accounts for the integration time necessary to attain certain
sensitivity. This calculation is shown in the equation below:

FUTURE WORK
Current work is centered around setting up the instrumentation and designing a
Labview interface that will control the instrumentation.
Begin testing and adjusting the aerosol generation setup using polystyrene spheres as
perfectly scattering aerosols. Polystyrene spheres are able to be modeled using Mie
scattering, and will allow us to calibrate and characterize the instrumentation.
Begin experiments involving indoor air quality by looking at aerosols produced by
candles and other indoor aerosol producers. In a recent study conducted by Li, cavity
ring-down spectroscopy was used to calculate ring-down times for smoke, incense
smoke, natural gas flame, propane flame, and candle flame emissions. In a published
work, Nriagu and coworkers used CRDS to quantify the aerosol characteristics of heavy
metals as emitted from metal-core wicks.

ACKNOWLEDGEMENTS
Dr. Simon W. North, Texas A&M
The UCA Department of Chemistry
The UCA College of Natural Science and Mathematics

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