Research Edge Working Paper Series, No. 41 p. 1 University of The Bahamas RESEARCH EDGE Working Paper Series Measuring Aerosol Extinction and Aerosol Optical Depth in The Bahamas Using a Camera Laser Remote Sensing System A. Kabir 1 , N. C. Sharma 2 , E. Knowles 1 , C. Bain 1 , S. Gagnon 2 , J. Fagnoni 1 , J. Barnes 3 1 Department of Physical and Earth Sciences, University of The Bahamas 2 Department of Physics and Engineering Physics, Central Connecticut State University, New Britain, CT, USA 3 NOAA/ESRL/Global Monitoring Division, Boulder, CO, USA Department of Physical and Earth Sciences University of The Bahamas, Nassau, The Bahamas Email: firstname.lastname@example.org No. 41 , September 2020
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Research Edge Working Paper Series, No. 41 p. 3 University of The Bahamas Measuring Aerosol Extinction and Aerosol Optical Depth in The Bahamas Using a Camera Laser Remote Sensing System A. Kabir 1 , N. C. Sharma 2 , E. Knowles 1 , C. Bain 1 , S. Gagnon 2 , J. Fagnoni 1 , J. Barnes 3 1 Department of Physical and Earth Sciences, University of The Bahamas 2 Department of Physics and Engineering Physics, Central Connecticut State University, New Britain, CT, USA 3 NOAA/ESRL/Global Monitoring Division, Boulder, CO, USA Department of Physical and Earth Sciences University of The Bahamas, Nassau, The Bahamas Email: firstname.lastname@example.org EXTENDED ABSTRACT Aerosols primarily reside in the atmospheric boundary layer (ABL) and play a key role in climate forcing by scattering and absorbing solar radiation ( Charlson et al. , 1992 ) . Local and regional air quality is also directly affected by aerosols originated from natural and anthropogenic sources. However, the distribution of aerosols and their optical properties vary greatly both over time and location making it challenging for in situ aerosol profiling. Traditional monostatic lidar systems are capable of measuring altitude dependent aerosol optical properties with excellent resolution at higher altitudes but require expensive electronics to measure time of flight of the backscatte red light for altitude calculation and suffer overlap effect at lower altitudes ( MPLnet website: http://mplnet. gsfc.nasa.gov/ ) . An inexpensive CCD camera based bistatic imaging lidar system (Clidar ; Barnes & Sharma, 2012 ) is employed to measure nocturnal aerosol extinctions and aerosol optical depth (AOD) simultaneously. Aerosol extinction is a measure of light extinction per unit length travelling through atmospheric aerosols. AOD represents aerosol extinctions summed thro ugh the entire column ranging from the top of the ABL to the ground. The use of a CCD camera fitted with a wide angle lens allows measurements at all scattering altitudes at once utilizing the geometry of the setup without requiring scanning. High altitude resolution in the near ground regions enables the system to detect air pollution from local sources ( Kabir et al. , 2018 ) . Stars in the image are used to calculate AOD by utilizing star photometry ( Leiterer et al. , 1995 ) . Clidar geometry is demonstrated in Fig ure 1a. A 532 nm CW linearly polarized laser is vertically pointed through a quarter wave plate to transmit circularly polarized light in the atmosphere. A CCD camera fitted with a wide angle lens and 10 nm laser line filter is separated a distance, D away from the laser beam to capture the entire beam from ground to zenith without the need for scanning. Unlike monostatic lidar, the scattering altitudes are determined s imply by Clidar 0 /pixel) of the CCD and the decreasing captured length of the beam in each pixel with falling altitude z, results in excellent Clidar resolution in the near grou nd regions. During several minutes of CCD exposure time, the circularly polarized laser beam mimics unpolarized light for the particle scattering thus
Research Edge Working Paper Series, No. 41 p. 4 University of The Bahamas simplifying the analysis. Fig ure 1b shows a sample CCD image of the laser beam from ground to zenith along the diagonal. The stars are also visible in the image. The pixel intensity of the beam provides the measure of aerosol side scatter at the corresponding altitude. Fig ure 1a Fig ure 1b The experiments were conducted at the sports field of University of T he Bahamas (UB), Nassau , New Providence located about 2 km from the shoreline and at an altitude of a few meters above sea level. The island has an area of 210 km 2 and is situated in the Atlantic Ocean at 25.06 Â° N and 77.35 Â° W. Data were collected on 4 different days in 2018 and 2019 during local times 8:30 pm to 11:30 pm. CCD image of the laser beam contains both molecular and aerosol side scattering. A cloud fr ee image is used to normalize the signal intensity to a model of molecular scattering at an altitude region free of aerosol layer to retrieve a single angle aerosol side scattering. An aerosol phase function (angle dependent scattering) representative of p olluted continental assumed by CALIPSO satellite measurements is used in this analysis to convert the side scatter to aerosol extinction (total scatter plus absorption). Fig ure 2a illustrates Clidar aerosol extinction as a function of altitude in meters above sea level (masl). The larger lens aperture used in experiments Nov ember 5, 2018 and Oct ober 22, 2019 results in smoother data than those of Feb ruary 13, 2018 and Oct ober 18, 2018 where a smaller aperture was employed. The Clidar instrument resolves persistent atmospheric features at low altitudes where traditional monostatic lidar systems may suffer from overlap issues. For example, near 100 m altitude there is a dr op off in extinction that can be seen in the profiles. Clidar instrument demonstrates the potential for long term monitoring and assessment of atmospheric features to help illuminate characteristics of local atmospheric structure. At Nassau , the Clidar sho ws extinction peaks between 150 to 200 m and 400 to 600 m that are visible in all four profiles taken at varying times and seasons. The instrument can also be used to detect the height and thickness of a thin passing cloud or aerosol layers using the extinc tion profile.
Research Edge Working Paper Series, No. 41 p. 5 University of The Bahamas Fig ure 2 a Clidar aerosol extinction as a function of altitude above sea level. (b) CCD image of the star Capella and 2 dimensinnal Gaussian fit of the pixel intensity of the same star. In the measurements of Oct ober 18, 2018 , a thin passing cloud layer is detected near 900 meters above sea level . The instrument offers the potentials for seasonal comparisons as well. For example, in profiles from Oct ober 18, 2018 and Oct ober 22, 2019 extinction drops sharply in a few hundred meters above sea level with increasing altitude and becomes low beyond 1.0 km demonstrating similar altitude dependent extinction profile around the same time in two different years. Aerosol e xtinction is low beyond 1.0 km for all four days indicating the top of the atmosp heric boundary layer in Nassau. The stars imaged at the same location, time, and through the same interference filter as the laser beam are also used in the star photometry me thod to calculate AOD simultaneously with aerosol extinction ( Barnes et al. , 201 6 ) . A t wo star method is employed to calculate AOD which do not require calibration constant ( Leiterer et al. , 1995 ) . Extraterrestrial spectral energy density of the stars (U 0 ) is obtained by Pulkovo spectroscopic catalog ( Alekseeva et al, 1996 ) . Elevation angles (h) are determined by the latitude and longitude of the observer location and the right ascension and declination of the stars. Air masses (F) are calculated using the elevation angles of the stars. dimensional Gaussian fit to the pixel intensity of the CCD image of the stars as shown in Fig ure 2b. The difference in terrestrial brightness at the observer location (m 1 m 2 ) and difference in extraterrestrial brightness (m 01 m 02 ) is then calculated using Equation 3 and Equation 4. Total atmospheric thickness T is determined using Equation 2 and Equation 5. Rayleigh optical thickness for pure gaseous atmosphere (Ray) is obtained using Equation 6 where P is relevant air pressure, P 0 = 1013 hPa, and is 0.532 Âµm. Aerosol optical thic kness T then can be estimated by subtracting Ray from T. Fig ure 2b F= 1 sin h ( 1 ) = m 1 m 2 m 01 m 02 F1 F2 ( 2 ) m 1 m 2 = 2.5 log10 U1U2 ( 3 ) m 01 m 02 = 2.5 log10 U01U02 ( 4 ) T = 1.086 ( 5 ) Ray = PP0*0.00879* 4.09 ( 6 )
Research Edge Working Paper Series, No. 41 p. 6 University of The Bahamas AOD for 5 different pairs are calculated using 5 stars presented in T able 1. The calculated average AOD using the two star method is 0.36 at local time 22:58:57 on Nov ember 5, 2018 at the UB campus, Nassau site. Table 1 Information of the stars to calculate AOD Stars Elevation angle, h [deg] Energy flux density at 532 nm, U 0 (W/m 2 .m) P ixel intensity, U (2D Gaussian fit) Almach (HR603) 72.4098 0.005114 25720 Mirfak (HR1017) 59.1385 0.007125 34447 Caph (HR21) 51.5500 0.00463 20661 Capella(HR1708) 43.9838 0.03487 152781 Elnath (HR1791) 42.0205 0.00853 31772 Note: UB Campus, Nassau. November 5, 2018, 22:58:57 Excellent altitude resolution at low altitudes makes Clidar an efficient tool to profile boundary layer aerosols and detect pollution from local sources. In addition, aerosol optical depth is measured simultaneously along with Clidar aerosol extinction in Nassau. The analysis of the measurements conducted in San Salvador are in progress. In future, Clidar and star phot o metry measurements will be conducted at other islands of The Bahamas to assess aerosols and air pollution in the region. Keywords : Aerosol extinction, aerosol optical depth, bistatic imaging lidar, Clidar, ground level pollution, wide angle lens, star photometer Acknowledgments This work was supported by an internal research grant of the University of The Bahamas. The experiment was conducted at the University of the Bahamas. References Alekseeva, G. A., Arkharov, A. A., Galkin, V. D., Hagen Thorn, E. I., Nikanorova, I. N., Novikov, V. V., Novopashenny, V. B., Pakhomov, V. P., Ruban, E. V. , & Shchegolev, D. E. (1996). The Pulkovo spectrophotometric catalog of bright stars i n the range from 320 to 1080 nm. Baltic Astronomy, 5, 603 838 . http://adsabs.harvard.edu/full/1996BaltA...5..603A Barnes, J., Pipes, R., & Sharma, N. C. (2016). Measuring a erosol o ptical d epth (AOD) and a erosol p rofiles s imultaneously with a c amera l ida r . EPJ Web of Conferences, 119 , 02007 . https://doi.org/10.1051/epjconf/201611902007 Barnes, J. E., & Sharma, N. C. (2012). An inexpensive active optical remote sensing instrument for assessing aerosol distributions. Journal of the Air & Waste Management Association , 62 (2), 198 203. https://doi.org/10.1080/10473289.2011.639927
Research Edge Working Paper Series, No. 41 p. 7 University of The Bahamas Charlson, R. J., Schwartz, S. E., Hales, J. M., Cess, R. D., Coakley, J. J., Hansen, J. E., & Hofmann, D. J. (1992). Climate forcing by anthropogenic aerosols. Science , 255 (5043), 423 430. https://doi.org/ 10.1126/science.255.5043.423 Kabir, A. S., Sharma, N. C., Barnes, J. E., Butt, J., & Bridgewater, M. (2018, May). Using a bistatic camera lidar to profile aerosols influenced by a local source of pollution. In M. D. Turner & G. W. Kamerman, Eds., Laser r adar t echnology and a pplications XXIII (Vol. 10636, p. 19 19 ). International Society for Optics and Photonics. https://doi.org/10.1117/12.2303544 Leiterer, U., Naebert, A., Naebert, T., & Alekseeva, G. (1995). A new star photometer developed for spectral aerosol optical thickness measurements in Lindenberg. Beitrage zur Physik der Atmosphare / Contributions to Atmospheric Physics , 68 (2), 133 142. MPLnet: The NASA micropulse lidar network [Website]. http://mplnet. gsfc.nasa.gov/
Research Edge Working Paper Series, No. 41 p. 8 University of The Bahamas CORRESPONDING RESEARCHER BRIEF BIODATA Dr. Amin Kabir Department of Physical & Earth Sciences Oakes Field C ampus University of The Bahamas Email: email@example.com Dr. Amin Kabir has been working as an Assistant Professor of Physics at University of The Bahamas (UB) since August 2013. He obtained his BSc in Physics from the University of Dhaka, Bangladesh in 2000. He earned his MSc in Physics in 2005 and PhD in Physics in 2010 from the University of Cincinnati. He was awarded the University Graduate Scholarship (UGS) at the University of Cincinnati from 2003 to 2010. He received the Faculty Excellence in Teaching award in 2018 at University of The Bahamas. Dr. Kabir conducted his research work in the area of nonlinear optics and photonics in semiconductor nanostructures. During the PhD program me he had an excellent opportunity to teach undergraduate recitation classes in cooperative learning formats. He was inspired by the effectiveness of these interactive teaching methods and became passionate about teaching Physics. He had been nominated by the Physics Department of the University of Cincinnati for the Excellence in Teaching Award in 2010. After his PhD, Dr. Kabir worked as a postdoctoral researcher in the Terahertz spectroscopy group at the University of Alberta, Canada from 2010 to 2012. He worked extensively on crude oil optical properties using Terahertz time domain spectroscopy in collaboration with the Schlumberger Limite d, an oil company in Alberta. In 2012, he joined the Ultrafast spectroscopy group at McGill University as a postdoctoral fellow and worked on coherent multidimensional spectroscopy in nanostructure samples until he joined UB in 2013. Dr. Kabir has presented his research at many international scientific conferences and has published in high impact scientific journals. He constantly works on the development of the Physics lecture and laboratory classes at UB. In collaboration with Central Connecticut S tate University and NOAA/ESRL/Global monitoring division , he has established an optical atmospheric research project (optical remote sensing) at UB conducting aerosol studies and their dynamics impacting air pollution, precipitation patterns, and climate change regionally and globally. Dr. Kabir has received UB research grants for this project and additional grants for s tudents who work as research assistants under his supervision.