Light Polarization Studies of Sunlight and its Relationship to Aerosols in the Earth’s Atmosphere

1. 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. James Frost Juan Rodriguez, Irving Andino Carla Brathwaite Ilana Lefkovitz Light Polarization Studies of Sunlight and its Relationship to Aerosols in the Earth’s Atmosphere BACKGROUNDAerosols are solid or liquid particles suspended in the atmosphere. They reflect and scatter light, causing some of the radiation from the sun to bounce back into space, generating a cooling effect on the atmosphere. Our goal is to evaluate the role aerosols play in earth’s climate change. Aerosols vary in size, composition, and lifetime. This makes it extremely hard to quantify their cooling effect, which is comparable in magnitude to the warming effect of greenhouse gases. Various remote sensing instruments retrieve information about aerosol properties, which include the size distribution, Aerosol Optical Thickness (AOT) also denoted by τ, and the refractive index of the aerosols. The ongoing project at LaGuardia Community College involves the use of a handheld polarimeter, a CIMEL sunphotometer, and two handheld Microtop sunphotometers to characterize the aerosols in our atmosphere. The studies that we conduct will ultimately help scientists make better computer models which make predictions about future climate change. RESULTS CIMEL AOT Values • The minispectrometer is attached to the polarimeter with a fiber optic cable. It measures the intensity at a much wider range of wavelengths—from 350nm to 1000nm. It contains a diffraction grating to split the light into each of these different wavelengths. It has a resolution of +10/-10 which is better than that of the polarimeter, making measurements more precise. • Data collection is monitored with the Labview computer program. The proper integration time and the number of samples to average must be inserted before running the program. Once the program runs, the maximum and minimum intensities for each viewing angle can be stored. While plots of the intensity versus wavelength (nm) are initially made, further analysis will plot the degree of polarization P, versus the viewing angle. • The AOT value that produces the best fit between color filter / minispectrometer data and model data will be used to retrieve the refractive index. The CIMEL AOT value is used as a guideline for what the right AOT might be. However, the CIMEL AOT is recorded at 500 nm while our polarimeter data analysis is referenced at 550 nm. To account for difference is wavelength, the CIMEL AOT value at 500 nm must be used in conjunction with the angstrom correction equations to find T550 : • α = ln [ (CIMEL AOT500/CIMEL AOT675) – (675/500) ] • T(AOT)550= exp [ ln (CIMEL AOT500) – ln (550/500) α ] Data was collected on July 6th and at three separate times on the 9th, which were both clear optimal days for data retrieval. The CIMEL sunphotometer data is analyzed first. The CIMEL plots the AOT versus time at several different wavelengths. We are interested in the CIMEL AOT at 500 nm because that wavelength most closely corresponds to the 550 nm wavelength referenced by the polarimeter data analysis. We then used this CIMEL value in the angstrom correction equation to obtain an AOT at 550 nm, which corresponds exactly to the wavelength referenced by the polarimeter analysis. Once the AOT value at 550 nm is run through the polarimeter data analysis, the refractive index can be found. The polarimeter refractive index is then compared to the CIMEL refractive index. The refractive indices given by the CIMEL and the polarimeter on the 6th were 1.475 and 1.47 respectively. For the 9th, the values were 1.44 and 1.43 respectively. Next, the CIMEL AOT values at 500 nm are compared to the Microtop II values of the same wavelength (Microtop I does not measure the AOT at 500 nm). The table on the lower right hand corner lists these AOT values from each instrument according to the date and time of day. July 6 July 9 MINISPECTROMETER ATTACHMENT Typical Polarimeter Data Analysis Plot THE INSTRUMENTS HANDHELDPOLARIMETER CONCLUSIONS • DESCRIPTION: • This polarimeter contains a: • Polaroid filter: detects polarized light scattered from aerosols • Infrared filter: blocks infrared radiation • Lens: focuses the light onto the solar cell through either a blue, red, or green filter or the minispectrometer attachment • Solar cell: converts light energy to electrical energy • Multimeter: measures the intensity of the electrical energy • The polarimeter is used to pinpoint the refractive index • METHOD ONE: A tripod protractor assembly aims the polarimeter at a viewing angle, θ, with respect to the sun. The polaroid is then rotated recording the maximum and minimum intensity signals. The degree of polarization P is then calculated using the equation: • P = (Imax – Imin)/ (Imax + Imin) x 100 • This is done for a range of viewing angles in increments of 10, from about 30º to 100º depending on the time of day. This procedure is performed for the red (635 nm), blue (428 nm), and green (527nm) color filters. Plots of the intensity and the degree of polarization versus θ are made. • METHOD TWO:The same procedure is performed with the minispectrometer, instead of the three color filters. Refractive Indices for July 6th & July 9th The refractive indices from the polarimeter and CIMEL instrument matched closely on both July 6th and July 9th, the difference being only .005. This indicates that the CIMEL sunphotometer and the handheld polarimeter are largely in agreement with one another. When comparing the CIMEL AOT values with the Microtop II AOT values, there appeared to be a nice consistency. However, on July 9th at 19:46 there is a noticeable gap in the results. In this case, the CIMEL’s AOT was at .467, and the Microtop II value was at a much higher AOT of .692. The exact reason for this is not known but it is possible that location differences (the CIMEL is located at CCNY and our team is based at LaGuardia Community College, Long Island City) accounted for the discrepancy. Perhaps smoke and exhaust particles from incoming traffic at Long Island City interfered with Microtop II’s results. CIMEL SUNPHOTOMETER DESCRIPTION: This sunphotometer has sensors which align the instrument with direct sunlight. As it absorbs radiation from the direct sunlight, the intensity of the radiation is measured. This radiation is measured at 7 different wavelengths. The CIMEL uses irradiance measurements to record the optical depth and refractive index. METHOD: The optical depth measurements from the CIMEL are obtained from data algorithms located on the Aeronet website. Aerosol Optical Thickness (AOT) At 500 nm HANDHELD SUNPHOTOMETERS: MICROTOPS I & II REFERENCES The Sun Target • Rodriguez, Juan, and Irving Andino. "Handheld Polarimeter Project." LaGuardia/NASA Research.LaGuardia Community College. July2006<http://www.cis.lagcc.cuny.edu/na sa/htm%20files/handheldpolarimeter.htm>. • "What are Aerosols?." http://earthobservatory.nasa.gov.NASA. July2006<earthobservatory.nasa.gov/Librar y/Aerosols/oea.larcnasa.gov/PAIS/Aerosols .html>. • “Aerosol Robotic Network.” http://aeronet.gsfc.nasa.gov/. NASA. July 2007. DESCRIPTION: By measuring the intensity of the direct sunlight the Microtop instruments can calculate the Aerosol Optical Thickness (AOT) at different wavelengths. There are two Microtop sunphotometers: Microtop I measures AOT at 440, 675, 870, 1640, 2100 nm & Microtop II measures AOT at 440, 500, 675, 870, 1020 nm METHOD: The target on the panel is used to focus the direct sunlight onto the instrument and increase precision. The Microtops store all the data and it can be retrieved from the computer with a cable.