1. Presented by: Robyn D. Williams EAS 6410 April 19, 2004
Assessing Optical, Thermodynamic and Physical Properties of Aerosols By Simulating LIDAR Response
Presented by:
Robyn D. Williams
EAS 6410
April 19, 2004

2. Objectives
To explore how thermodynamic and physical aerosol properties affect optical properties
To simulate LIDAR measurements by coupling thermodynamic and light scattering models
To determine if LIDAR measurements can be used to relate aerosol properties

3. Background

4. Aerosols Particulate Matter Sources Large Variability Natural
Anthropogenic
Large Variability
Small spatial and temporal scales
An aerosol is defined as particulate matter suspended in a fluid medium that typically range in sizes up to approximately 10nm
They have direct and indirect affect on climate
Sources
Natural: Dust, sea spray, volcanic action, biomass burning
Anthropogenic: fuel combustion, transportation sources, industrial processes
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Due to the large array or sources, various gas to particle conversion processes, and relative short residence times, tropospheric aerosols vary widely in size and composition over small spatial and temporal scales.

5. Estimated Aerosol Properties
Ground Based Measurement
Dynamic Atmosphere
Continuous Measurement
LIDAR
Signal Transmission
Particle/ Signal Interaction
Signal Backscatter
Backscattered Signal Analysis
ground based measurements are ideal to aid in characterizing tropospheric aerosols because their continuous measurements can relate how aerosol properties change with the dynamic atmospheric conditions
Light and Detection and ranging – is a ground based instrument that sends out a signal to a particle and then analyzes the signal that is scattered back from the target.
Changes in the properties of the signal can tell a lot about the properties of the target

6. Aerosol Optical Properties
Factors
Particle size
Wavelength
Refractive Index/ Composition
Particle Hygroscopity
Figure shows four of the major interactions of light and particles
Scattering and absorption of light by a particle is governed by particle size, wavelengh, and composition
The refractive index is a function of composition - The ratio of the speed of light in vacuum to the speed of light in the material.
Particle Hygroscopity or ability to take up water affects the particle size and composition

7. Mie Theory Relates light scattering governing factors
Assumes homogenous spherical geometry
Valid domain:
It is valid in regions where the particle is approx. equal to the incident wavelength

8. Phase Function Angular Distribution of the Intensity
Ratio of scattered intensity at a particular angle to total the scattered intensity at all angles
LIDAR backscattering, =180º
Describes the scattering intensity as a function of angle
Since the LIDAR is a ground bases measurement – we consider back scattering at an 180 degree angle

9. Thermodynamics of Aerosols
Water affects both the size and composition of aerosol
Water Uptake = f(Relative Humidity)
Deliquesence Relative Humidity
Spontaneous particle growth
Inorganic salts
Chemical Equilibrium
As previously stated – a particles ability to uptake water affects the particle size and composition
The amount of water uptake depends on the relative humidity
As more of the inorganic salts begin to dissolve and water uptake increases the concentration of the aerosol is diluted and the composition and refractive index is affected
The water uptake process is governed by chemical equilibrium

10. Thermodynamics of Aerosols
This diagram shows how once the deliquesence relative humidity is reached – the particle size grows exponentially
Source: Pandis et al (1995)

11. LIDAR Simulation

12. Simulation Schematic
A thermodyamic model, ISORROPIA developed by Thanos Nenes et al and a light scattering interface MIETAB developed by Augustus Miller

13. Aerosol Type
Two aerosol types: urban and rural were used to consider lidar’s backscatter’s dependence on aerosol type/composition

14. Size Distributions
The size distribution for rural and urban particles are tri modal
The respective number distributions for each aerosol types were divided into three modes: fine, accumulation, and course
Each mode was then separated into bins

15. Simulation Schematic
Next, the median diameters from each bin along with the typical aerosol compostion was inputed into the thermodynamic model, ISORROPIA
The thermodynamic model then calculated the water uptake, corresponding wet diameter, and refractive index as a function of relative humidity
Three relative humidities, 30%,60%,90% representing dry, semi-wet, and wet were isolated for each diameter
The corresponding wet diameter and refractive index were then entered into MieTab along with a constant wavelength of 0.55 microns
0.55 microns represents the median/peak value of the sun’s concentrated radiation region
Lastly, Mietab provided the phase function as a for angles from 0 to 180 for each input combination

16. Results & Discussion

17. Backscatter vs. Dry Particle Size
In the essence of time only a subset of the results will be shown
This graph represents the trend seen by all modes – with an exception of the mode three- the size parameters were large in this mode which pushes the scattering into the geometric scattering regime which exhibits lots of scatter
As expected the results show that as a particle gets larger the lower the backscatter signal. This happens because as a particle scatters more forward light as its size increases
As the relative humidity increases the greater the magnitude of backscatter decrease, because the particles uptake more water as the relative humidity increases

18. Backscatter vs. Relative Humidity
As previously stated the backscatter decreases as the relative humidity increases

19. Backscatter vs. Aerosol Type
The backscatter of urban mode II and rural mode II were compared in attempt to have relative constant particle size.
The log-normal distribution mean diameter is approximately the same for both of these modes
As you can see the backscatter curves lie on top of each other at each relative humidity
This occurs because the composition differences in the urban and rural particles are not distinct enough to have a impact on the backscatter

20. Conclusions Good Simulation
Relative Humidity/Particle Hygroscopity affects:
Physical Properties/Composition
Optical Properties
Inconclusive aerosol type/composition effect
The coupled thermodynamic and light scattering appears to provides a good representation of LIDAR measurements
From this simulation it can be seen that the relative humidity had the most prominent effect
– the simulation showed its effect on physical properties greatly affects the optical properties
The composition effect could not be determined from the simulation because of the similar nature of the aerosols considered

21. Conclusions
Backscatter vs. Dry Aerosol size and Backscatter vs. Relative Humidity Trends can possibly be correlated by a power or exponential model
Based on shapes of curve there may be a possibility that quantification of the relationship between backscatter and other properties may be achieved by using some type of numerical method such as Gauss-Newton Method –
However, this was beyond scope of project

22. Alternate Simulations
Aerosol Size
Vary Wavelength
Aerosol Type/Composition
Same Diameters
Vary Composition
Distinct Aerosol Types
Vary Modal Composition
Determine Backscatter
To improve the simulation,
Since the relative size between wavelength and particle size is so important
The wavelength should be varied and this could possibly provide a method of simulating how two lidars emitting different wavelength signals can estimate particle size
Aerosol type and composition can also be better analyzed by choosing the same diameter sizes and entering compositions of more distinct aerosols – such as marine aerosol which contains more sodium chloride
A better estimate would also be obtained if the composition were varied from mode to mode – because different species are different sizes therefore existing in different modes
Than he backscatter can be determined as more of a function of compostion

23. Questions??