Sponsors: National Aeronautics and Space Administration (NASA) NASA Goddard Space Flight Center (GSFC) NASA Goddard Institute for Space Studies (GISS) NASA New York City Research Initiative (NYCRI) Contributors: Dr. Barbara Carlson (PI) Dr. Brian Cairns (PI) Dr. James Frost (PI) Lisa Meirowitch (High School Teacher) Christopher Bussetti (Undergraduate Student) Arianna Moshary (High School Student) An Introduction to Aerosols Key Concepts Optical Depth (τ): Some of the intensity of light is lost to scattering and absorption, thus the atmosphere is not perfectly transparent. Optical depth is a measure of the transparency of air along a certain path. It can thus also indicate the concentration of aerosols in the air. Aerosols are studied because they have effects not only on climate but also on public health. High aerosol concentrations have been connected to respiratory problems such as asthma. They also directly affect climate by absorbing and reflecting solar radiation. Aerosols are harder to study than greenhouse gases and are less well understood, so they need to be further investigated. Aerosols are tiny liquid or solid particles (0.001 to 100 µm) suspended in the air. Aerosols are always liquid or solid particles. Some occur naturally, originating from volcanoes, dust storms, forest and grassland fires, living vegetation, and sea spray. Human activities, such as the burning of fossil fuels and the alteration of natural surface cover through farming, also generate aerosols. These humans activities are responsible for roughly 10% of all aerosols. Polarized Light: Polarized light is light made up of waves vibrating in the same direction. As sunlight (unpolarized) enters the atmosphere, it is scattered and absorbed, which partially polarizes the light. The polarization of light is dependent on the size, concentration, and composition of particles in the atmosphere. To build and test a solar cell polarimeter. To build and test an improved polarimeter. To analyze aerosol composition using data collected from the polarimeters and other sources. Project Goals Equipment CIMEL Sun Photometer: This is an automatic sky scanning radiometer which measures direct solar irradiance and the sky radiance at different wavelengths. These measurements can be used to calculate aerosol optical depth, particle size, and refractive index. Aerosol Robotic Network (AERONET) has CIMELs set up at a variety of locations (such as CCNY). Unfortunately, there was no data available for use in our calculations. This instrument measures direct solar radiation at five different wavelengths, which it uses to calculate aerosol optical depth using angle of the sun, intensity of solar radiation, and known constants according to the formula: I=I o e- τm and by finding the slope of ln(I) = ln(I o ) – τm, where ln(I) is a function of m. Microtops II Handheld Sun Photometer Rayleigh Scattering: Rayleigh scattering is the reason why the sky appears blue. As solar radiation enters the atmosphere, it is in a variety of wavelengths. When these different wavelengths hit the gas molecules in the atmosphere, they are scattered according in a ratio of 1/ 4. Thus, in the visible spectrum, the shorter wavelengths are scattered the most, and the light we see in the sky is blue. Microtops II Data The graphs to the right represent measurements taken by two different handheld sun photometers of the aerosol optical thicknesses at various wavelengths. The flatter parts of the graphs represent the most accurate data, and the fluctuations most likely reflect human error that accompanies the use of handheld instruments. These graphs compare the two different instruments at the wavelengths 440nm, 675nm, and 870nm. The average percent difference between the sun photometers was approximately 3.55%. This very small difference allowed us to use the data that we collected by the Microtops II Handheld Sun photometers in our analysis of the polarimeter data. We then used this data to calculate what the AOT would be at 550nm using the angstrom exponent (α) as follows: and found that the AOT for 550nm is approximately , which we then later used in our polarization calculations. Solar Cell Polarimeter Polarimeter Data Abstract This polarimeter detects polarized light that has been scattered by particles in the air. To do this, it uses a polaroid to detect the amount of light in each direction by rotation. After passing through a series of filters, the light is focused by a lens onto a solar cell, which detects the amount of light and converts it into electricity. The degree of polarization of light at each angle can then be calculated using the minimum and maximum values according to the formula: Analysis and Conclusion Future Work Using the data we collected, the optical depth value from the Microtops II Sun Photometer, and the zenith angle calculated by a GPS, we ran a program which generated the best fit graphs below. This program considers all of these factors in addition to the effects of Rayleigh scattering and Mie scattering caused by aerosol particles. The below graphs show polarization curves mapping degree of polarization against scattering angle (similar to the above graph). However, these graphs also have a number of curves representing possible different indices of refraction. Our data points, represented by the +’s on the graph, most closely match up to an index of refraction between 1.40 and 1.45 for the aerosol particles in the atmosphere during this measurement. We would have liked to compare our results to the CIMEL results; however, no CIMEL information was available for this date. Our plan to improve the polarimeter is to use a Wollaston prism. The Wollaston prism is made from two orthogonal calcite prisms cemented together at their bases. It splits incident rays into two perpendicularly polarized rays called extraordinary and ordinary rays. These rays split because the index of refraction in the Wollaston prism is polarization dependent. This graph represents the data that we collected using the solar cell polarimeter that we built. Understanding Aerosols Through Polarization Arianna Moshary, Christopher Bussetti, Lisa Meirowitch This project studies the different ways to gather information about the variety of aerosols in the atmosphere. This project also focuses on trying to improve the current methods of collecting such information. Instruments such as the Microtops II Handheld Sun Photometer can be used to measure the aerosol optical thickness of the atmosphere; however, it does not tell us the refractive indices of the particles. Furthermore, these instruments cannot tell us the degree of polarization of sunlight at different scattering angles. In this study, we focused on using the polarization of light in the atmosphere to try to gain insight into what was causing that polarization of previously unpolarized natural light. To do this, we built a solar cell polarimeter which detects and measures the intensity of light at different angles, polarizations, and wavelengths. We collected data using this polarimeter and used it to further understand the aerosol particles in the air. Another aspect of this project was improving our solar cell polarimeter so that it works more efficiently and increases the validity of our data. To improve our polarimeter, we began to design a Wollaston Prism polarimeter which will collect data more accurately and efficiently. The Wollaston Prism polarimeter is still in the process of being built, but early analysis shows promise for the device. Our Design: We plan to use the prism to split the light into the maximum and minimum rays. We can then measure the intensity of the two separate light beams using a dual spectrometer, which will make a spectrum showing the amount of light at each wavelength. Thus, this improved polarimeter will be able to simultaneously measure the minimum and maximum amounts of light at each wavelength. This will decrease the amount of time required for the measurements at least six fold, as well as decreasing the amount of error associated with rotation of the polaroid.