Scanning lidar measurements of marine aerosol fields at a coastal site in Hawaii Shiv K. Sharma*, Barry R. Lienert and John N. Porter
University ofHawaii, Hawaii Institute ofGeophysics and Planetology, School ofOcean and Earth Science and Technology, 2525 Correa Road, HIG, Honolulu, HI — 96822. USA ABSTRACT On the windward side of Oahu, a multi-wavelength Mie-Rayleigh (MR) scanning lidar is used on a regular basis for measuring aerosol attenuation in the marine boundaiy layer. The lidar data are being used for investigathg dynamic effects of marine aerosol fields on electro-optical (EO) properties. The lidar has been operated mostly at 532 and 1064 nm, and recently at 355 nm. We have observed that the vertical aerosol distribution can be very non-uniform. Under certain atmospheric conditions, ascending and descending streaks of aerosols with high extinction (2x 1O per meter) have been observed, indicating that both the surface and cloud drizzle effects are important. Horizontal lidar scans at 6 meters above the
sea surface indicate that aerosol is fairly uniform on a large scale but can exhibit significant variability on small scales particularly close to protruding reefs and shorelines. Above the reefthe enhanced aerosol fields have been observed to rise as high as 100 meters. As expected there is a strong correlation between wind speed and sea salt extinction values. The temporal and spatial distribution ofthese aerosol fields and their dependence meteorological parameters and wave height are discussed. Keywords: Marine aerosol, scanning lidar, marine boundary layer, electro-optical properties, extinction coefficient, Hawaii
1. INTRODUCTION In the lower marine boundary layer (MBL), light extinction by scattering and absorption is dominated by atmospheric aerosol. In unpolluted regions close to the ocean's surface, a large portion of this aerosol consists of sea salt and organics generated by wave/wind interaction. Kite studies1'2 have shown that salt aerosol concentration decreases by an order of magnitude in the lowest 30 meters ofthe marine atmosphere. Above the lowest 30 meters, Blanchard et a11 found the aerosol concentration decreases slowly with height up to cloud base. Entering the cloud layer, the aerosol salt concentration again drops sharply. It is commonly observed that the boundary layer can be separated into three separate aerosol layers. On a day to day basis, there is significant variability at any level2. The structure and intensity of this aerosol vertical variability is probably related to atmospheric dynamic and thermal stability. Efforts to model aerosol EO properties must take into account the aerosol size distribution, composition, local meteorological dispersion and advection (e. g., ref.4). The aerosol size distribution in the coastal marine boundary layer is typically bimodal (on a mass basis) with an accumulation mode (often sulfate) near 0.25 micrometer Qim) diameter and a coarse mode at or
above 2.0 tm (often sea salt). Both the sulfate and sea salt are hygroscopic which changes their diameters and optical properties due to the uptake of water at different relative humidity (RH)58. For visible to near JR wavelengths, EO attenuation can be controlled by either the accumulation mode or the coarse mode aerosol. In open marine regions, the sulfate accumulation mode concentration is typically near 0.5 mg/m3 and the sea salt is 7 mg/m3 (assuming a wind speed of 7 rn/s)8 . Based on these aerosol concentrations and the aerosol size distribution models of Porter and Clarke8, it is found that 7O% of the light scatter is due to sea salt with the remainder coming from the accumulation mode of sulfate aerosol. On the other hand, in more polluted regions, the sulfate concentrations are frequently much larger so that sulfate aerosol becomes the
dominant factor controlling EO propagation. This is often the case over much of the Northern Hemisphere, where anthropogenic emissions are now proposed to have a significant influence on global radiative transfer9.
It is well known that in the open ocean, wind speeds over about 7 m/s cause increased white capping (due to drag forces) and hence increased sea salt aerosol production4"°. In a coastal environment, aerosol production is more complex because of the interaction ofprocesses that do not play a role in the open ocean. For example, complex atmospheric circulation in the coastal region could have significant influence on aerosol fields. Remote sensing is the logical method of studying marine aerosol optical properties in the MBL. Preferably, the remote sensing measurement should obtain the aerosol spatial structure and the *Coffespondence: E-mail:
[email protected]; www: http://soest.hawaii.edu/lidar; telephone: 808 956 8476; Fax: 808 956 3188 Lidar Remote Sensing for Industry and Environment Monitoring, Upendra N. Singh, Toshikazu Itabe, Nobuo Sugimoto, Editors, Proceedings of SPIE Vol. 4153 (2001) © 2001 SPIE · 0277-786X/01/$15.00
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aerosol multi-wavelength optical effects. Lidar (e. g., refs. 1 1, 12) is an ideal tool for these measurements, as lidar data can be acquired very rapidly allowing imaging of volumes as large as 100 km in minutes13. For the past two years, we have been collecting scanning lidar data at two sites on the northeast side of the island of Oahu, Hawaii. From land based site, we are able to observe relatively unpolluted marine air because the predominant trade wind direction is from the northeast. We have also been able to study the behavior ofplumes of salt-water spray generated by near shore islands and reefs'4"5.
2. DATA ACQUISITION AND ANALYSIS A schematic diagram of our scanning lidar is shown in Fig. 1. The laser is a 20 Hz Continuum Surelite S20 with a maximum energy of4SO mJ/pulse at 1064 nm. Collinear doubling and tripling crystals are used to simultaneously obtain wavelengths
UV-VisibIe-NIR 20 Hz Nd:YAG Laser System
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Fig. 1 Schematic diagram ofthe SOEST scanning lidar.
of 532 and 355 nm. Laser pulse energies are monitored continuously at all three wavelengths using an integrating sphere coupled with optical fibers to filters and high speed photodiodes. The photodiode signals are digitized using a 500 MHz oscilloscope. The outgoing laser beam is expanded to a diameter of 5 cm and reflected off an electronically adjustable turning
mirror mounted on top of a 30 cm diameter Schmidt-Cassegrain telescope. It is then reflected off two electronically controlled 30 cm wide scanner mirrors, which are used to scan the multi-wavelength laser beam both at azimuth and elevation angle. A scan speed of 1 deg/sec is normally used for both azimuthal and elevation scans. The primary laser beam is shielded from the return optics using 5 cm diameter tubes extending out to the second scanner mirror. The telescope output is collimated and focussed on two photomultiplier tubes through 532 and 355 nm laser line filters, and on an avalanche photodiode through a 1064 urn filter. All these signals are digitized at 60MHz using three Gage 6012 A/D cards mounted in a PC. Pretrigger data is used to calculate a baseline before each laser shot. This baseline is then subtracted from the posttrigger data. A custom program'3 was written using C programming language to control the hardware and process the data in real time. The program is capable of displaying two-dimensional scattering cross-sections during data acquisition as well as allowing time-varying features such as plumes of aerosol to be tracked in real time. Real time analysis and display makes it much easier to isolate and correct problems in alignment, detector saturation, etc., as they•occur during field measurements.
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The stacked lidar detector data at each wavelength are corrected for the telescope solid angle, outgoing pulse energy, detector
sensitivity and distance increment to give backscaUer (in m1srT1) between successive pairs of points. An iterative procedure1'1 is used to successively correct each calculated backscatter value for transmission losses along the total path length. These transmission losses are estimated by assuming a constant value of the backscatter to extinction ratio and thereby calculating the cumulative extinction out to each point. Calibration factors for the lidar are also determined empirically by examining the far field response for horizontal profiles, where it is assumed that over long scales the aerosol scatter over the open ocean is horizontally homogeneous. Comparisons have been made between sun photometer and nephelometer and we find the calibrations agree to within 15% using different measurements. For our site, we have found that the empirical lidar calibration method is more useful in practice than hard target calibration. This is because the optics, particularly the outer scanner mirror, rapidly become coated with salt spray, significantly changing the system's calibration
constant. It is more useful to have a calibration technique that can be implemented using data acquisition so that the calibration factor can be regularly adjusted. The interactive nature of our program makes it possible to adjust the calibration factor while the data is being acquired as well as during later reanalysis ofthe raw data.
For investigating the vertical atmospheric structure above the Bellows beach site during the ONR supported Shoreline Environment Aerosol Study (SEAS) project held during April 2000, we released a Vaisala digital radio sonde, attached to a 2-meter helium filled balloon. The radio sonde was released from a small boat 2 km upwind ofthe lidar site while performing vertical lidar scans. The sonde instrument converts pressure, temperature, and humidity sensor data to digital form, and modulates a radio transmitter with the digital data that are captured and stored in the ground-based computer. This data transmission is repeated every two seconds, providing high-resolution temperature and humidity profiles as a function of pressure.
3. RESULTS AND DISCUSSION Our scanning lidar system is presently located at Bellows Air Force Station on the SE side of Oahu. This site is in the direct trade wind flow and has access to the air coming from the open ocean east of Oahu. The lidar is located on the beach 4x iO ni') are due to high concentrations of salt spray aerosol caused by the wave activity on the outer reef. On this occasion, the prevailing winds were 5.4 rn/s and the waves were 1 rn high. Figure 3 is typicalofwhat we see in the lidardata as the salt spray forms atthe
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site of the breaking waves with the plume decreasing with distance from the reef. The decrease is due to gravitational settling of the largest salt spray aerosols and dispersion/evaporation of the smaller ones. 10
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Fig. 2. Vertical aerosol extinction coefficient, cy (in m') profiles at 1056 and 532 nm and the ratio of g 's calculated from lidar data
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Fig. 3. Typical horizontal variation of aerosol scattering coefficients derived from horizontal lidar measurements at 532 nm on June 10, 1999.
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Fig. 4. A vertical lidar scan performed at 532 nm wavelength a few minutes after the sonde was released on April 24, 2000.
In order to investigate the vertical structure of aerosol fields above the Bellows beach site during SEAS experiments, we measured atmospheric profiles of pressure, temperature and humidity with a radiosonde. As discussed in Section 2, the radiosonde was released from a small boat 2 km upwind of the lidar site a few minutes before the vertical lidar scan at 532 nn wavelength on April 24, 2000 around 1600 hr. The sonde temperature and relative humidity profiles are shown in Figs. 5 and 6, respectively. Virtual potential temperature18 profile calculated for the sonde data in Figs. S and 6 is displayed in Fig. 7. The aerosol layering seen in the vertical lidar scan in Fig. 4 indicates that the atmospheric aerosol fields were highly stratified and thin layers of clouds were also present.
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Figure 6 shows a common surface relative humidity (75%), a layer with elevated humidity values robably in and out of cloud), and the expected dry layer (RH
