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Measurement of Aerosol Optical Thickness Using a Narrow Band Sun Photometer Prepared for ATMS 748 Atmospheric Measurements University of Nevada Reno
David DuBois
September 8, 1998
1. INTRODUCTION This section describes the historical background of measuring the aerosol content of the atmosphere through the use of the concept of turbidity. The section concludes with a brief theoretical basis for this project. 1.1
Historical Introduction
The application of photometry to estimate atmospheric extinction has its origin in 1725 with Pierre Bouguer (Middleton 1961). Bouguer used the moon as the source of radiation to measure the transmissivity of the atmosphere. He used the formula E" / cos I
T e
where T is the transmissivity, E is the optical index of the material in question, O is the thickness of the atmosphere, and I is the angle of the beam of primary illumination measured from the solar zenith. The transmissivity of the atmosphere has a value between 0 and 1, where 0 corresponds to a perfectly opaque atmosphere and 1 corresponds to a perfectly transparent atmosphere. This expression is now named Bouguer's Law. Bouguer compared the transmission of light through the atmosphere to that of light passing through colored glass. To get the transmissivity, he then solved for the quantity EO through the use of two different measurements of I. Angstrom (1929, 1961, 1964, 1970) researched the problem and refined a mathematical framework to characterize the atmospheric optical properties. The product EO is called the "turbidity" and is a dimensionless and positive number. This quantity is also known as the optical depth or optical thickness in the literature. The AOT has been, and continues to be, an important measurement and modeling parameter in the air pollution, atmospheric radiation and climatology fields. "Aerosol optical depth is the principle variable describing aerosol effects in classical radiative transfer calculations," (WMO 1991). A more rigorous definition of turbidity used widely today and bringing out more of the physics involved (Liou 1980) is presented in the next section. 1.2
Theoretical Basis
Incident solar radiation at the top-of-the-atmosphere, FO(f), is attenuated by various molecules and atmospheric aerosols as it propagates down to the earth's surface. This attenuated flux reaching the ground is denoted as FO(0). The ratio of the incident surface flux over the incident top-of-the-atmosphere flux defines the total atmospheric transmissivity or T. The transmissivity can be approximated using Bouguer's Law as:
T
FO (0) FO (f)
e [W
R
( O )W M ( O )]m
Where WR(O) is the Rayleigh optical depth, WM(O) is the aerosol optical depth, m is the air mass or secI (1/cosI) with I being the solar zenith angle. This equation is equivalent to the equation first stated by Bouguer except that the EO is expanded out to better 1
understand the components of the total extinction that was measured by the sun photometer. Furthermore, the aerosol optical depth or AOT can be expressed in terms of the extinction coefficient W (O ) M
f
³E
e
(O , z )dz
0
where Ee(O,z) is the vertical profile of the aerosol extinction coefficient as a function of wavelength. The integral in the AOT equation above integrates from the ground, z = 0, to the top of the atmosphere at z = f. The variable FO(0), at a wavelength of 530 nm, was the measured parameter in this experiment. This expression is the same as the EO in the first equation above. Since the definition of extinction is the sum of scattering and absorption, the expression for Ee(O,z) can be expanded to E e (O , z )
E s ( O , z ) E a (O , z )
where Es(O,z) is the aerosol scattering coefficient and Ea(O,z) is the aerosol absorption coefficient. Considering the wavelength of the measurements and the general aerosol content, the major contributor to the extinction is from aerosol scattering during the measurement period. Particles with sizes that are on the order of the wavelength of visible light contribute the most to light scattering. The parameter E e(O,z) is a complex function of particle composition, size, index of refraction and shape. Efficient light scatterers are typically composed of ammonium nitrate (NH4NO3), ammonium sulfate ((NH4)2SO4) and ammonium bisulfate (NH4HSO4). Efficient light absorbing particles are composed of elemental carbon (black carbon) and organic carbon. Typically, the carbon particles are located near sources of combustion, either classified as anthropogenic or biogenic. Exceptions to this are long-range transport of forest fire effluent or burning fuel smoke. Mineral particles such as fugitive dust can contribute to both solar scattering and absorption depending on the size of the particles and their composition. If their sizes are on the order of the wavelength of light they will contribute to scattering. Some minerals, such as hematite, have relatively high indices of refraction relating to their ability to absorb radiation. The effects of dust on climate have become important recently as uncertainties in the net aerosol radiative forcing in global climate models (GCMs) have attracted attention (Andreae 1996). The particle extinction is also generally dependent on the type of airmass. A moist airmass in the troposphere will tend to have more light scattering aerosols than one that is dry. The magnitude of the turbidity can become very large for optically thick atmospheres. Examples of optically thick conditions are commonly found in stratus and cumulus clouds, smoke plumes and heavy fogs. The exact vertical profile of Ee(O,z) is in general a complex and dynamic parameter that varies from place to place and in time. In-situ measurements of aerosol extinction in arid environments point toward a decrease in aerosol concentration as a function of height (Pinnick et al. 1993). Lidar measurements in desert environments also show a sharp decrease in aerosol concentration as a function of height in the lower troposphere (Spinhire et al. 1980). The conclusion drawn from these measurements is that most of the light extinction can be attributed to aerosols in the lowest portion of the troposphere.
2
The exception to this is when volcanic ash and gases are thrust into the upper troposphere and lower stratosphere. During this measurement, no volcanic activity was present. The turbidity measures the column integrated attenuation due to extinction from aerosols and gaseous species. The term turbidity and aerosol optical thickness (AOT) are synonymous with both being logarithmic indices of atmospheric optical attenuation in a column. This is true only for wavelengths outside of the strong gaseous absorption regions in the infrared. 1.3
Measurement Technology
Turbidity or AOT estimates have evolved from the crude eye measurements to the thermopile and semiconductor detectors used in instruments today. Thermopile detectors are used today in some instruments because of their electrical stability. Modern planar silicon junction photodiode semiconductor sensors have proven reliable as substitutes for the older technology. These sensors have high efficiencies and spectral sensitivities in the wavelength range of 300 to 1100 nm. Filter technology has advanced allowing narrow bandpass values as low as a few tens of nanometers. These narrow bandwidth filters are usually constructed using interference or thin film filter technology. More often than not, these filters incur a substantial cost to the system and may degrade in time due to the environmental exposure. An economical alternative to the filter based photometer systems is the use of a semiconductor diode or photodiode sensitive to visible radiation. The use of a photodiode as the detection mechanism in a sun photometer has been proposed by numerous investigators. Photodiode photometers have been used to measure cloud optical depth (Raschke and Cox 1983). The use of LED in the light sensing configuration has been successfully demonstrated (Mims 1992; Carlson 1997). Instead of operating a LED in the normal way by applying a voltage across the diode to emit light, the light sensing configuration requires the measurement of the current across the diode when sunlight is applied. The primary advantages of using light emitting diode (LED) sensors in photometers are the compact size, inexpensiveness, availablility, reliability and the inherent narrow spectral bandpass. An important application of aerosol optical thickness is to measure the climatalogical aerosol distribution spatially and temporally. A recent measurement study of the ARM CART sites over a 250 x 350 km2 area showed that the annual variations of median AOT, at 500 nm, were found to be 0.30 over the five sites (Nash and Cheng, 1998). This is an important finding since these spatial variations are smaller than the grid spacing of global climate models (GCM). The measurement of AOT during volcanic eruptions has been a valuable tool in mapping the effects of ash plumes globally (Mendonca et al. 1978; Spinhirne and King 1985; Dutton and DeLuisi 1983; Ryznar and Baker 1983; Michalsky and Stokes 1983). These particles may be injected into the stratosphere, giving them long lifetimes and affecting the global balance of radiation. One example is the eruption of Mt. Pinatubo during 1991. During the eruption nearly 30 million tons of aerosols were released into the stratosphere. These aerosols were detected by not only satellite observation platforms but also ground based photometers and spectrometers. Based on satellite observations, three years after the eruption, nearly all of the Mt. Pinatubo 3
aerosols were gone. In the study of global change and its application to global warming, the temperatures of the Earth or any other planet depends mainly on the amount of sunlight received, the amount of sunlight reflected into space and the extent to which the atmosphere retains heat. The purpose of this project was to construct an inexpensive sun photometer using off-theshelf electronic components to measure atmospheric aerosol optical thickness. This project provides an opportunity to apply knowledge in electronics, electro-optical devices and atmospheric physics to solve a long-standing problem in the measurement of aerosol optical thickness. An effort was made to quantify errors, limitations and accuracy of the instrument using an appropriate statistical and theoretical basis. 1. INSTRUMENT DESCRIPTION The measurement of the AOT was accomplished by first measuring the sun's irradiance as attenuated by the atmosphere over a period of several hours. This was accomplished by using an Eppley solar tracker to point the photometer and pyrheliometer at the sun. Of critical importance in designing a measurement system is a basic understanding of the characteristics of the quantities to be measured and a grasp of the equipment's technical requirements to perform the measurement. An instrument meeting the goals for this project was a simple radiometer with a narrow spectral bandpass and narrow field of view. The radiometer needed to be sensitive to radiation in the visible spectrum (400 to 700 nm) and have a narrow spectral width
