Aerosol Transport in the California Central Valley Observed by

Sep 22, 2005 - Science Directorate and Systems Engineering Directorate, NASA Langley Research Center, Hampton, Virginia 23681, and Sunlight, Nutrition...
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Environ. Sci. Technol. 2005, 39, 8351-8357

Aerosol Transport in the California Central Valley Observed by Airborne Lidar R U S S E L L J . D E Y O U N G , * ,† W I L L I A M B . G R A N T , †,‡ A N D KURT SEVERANCE§ Science Directorate and Systems Engineering Directorate, NASA Langley Research Center, Hampton, Virginia 23681, and Sunlight, Nutrition and Health Research, 2107 Van Ness Ave., Suite 403B, San Francisco, California 94109

An aerosol lidar system was deployed on the NASA DC-8 and used to measure aerosol vertical profiles in the California Central Valley. The nadir-pointing Nd:YAG lidar operated at 532 and 1064 nm at 20 Hz. The resulting aerosol profiles were plotted in a unique three-dimensional format that allowed the visual observation of the aerosol scattering ratio profiles, the valley topography, and corresponding backward trajectory air masses. The accumulation of aerosols from the Bakersfield area can be seen in the southern end of the valley due to topography and prevailing winds.

1. Introduction The San Joaquin Valley Air Basin occupies the southern twothirds of California’s Central Valley. The valley covers about 25 000 square miles and is flat, with most of the area lying below 120 m in elevation. The valley is formed by the Sierra Nevada Mountains (4300 m) to the east, the Coastal Range (1530 m) to the west, and the Tehachapi Mountains (1200 m) to the south. This unique geography and prevailing northerly winds result in the accumulation and confinement of particulate matter (PM) in the lower San Joaquin Valley. There are a number of moderately sized urban areas along the central axis of the valley, which result in a wide distribution of PM sources. About 9% of California’s population lives in the San Joaquin Valley, and the pollution sources in the valley account for about 14% of the total state pollution emissions (1). Large seasonal variation in PM2.5 and PM10 has been observed. The winter season has the highest concentration of PM dominated by the PM2.5 fraction, whereas during the summer the total PM concentration is much lower and dominated by the PM10 fraction due to agriculture operations. The California Air Resources Board makes annual estimates of PM emission inventories (1). For the summer of 2002, the total PM10 direct emissions for the valley was 420 tons per day (2% nonanthropogenic) and PM2.5 was 170 tons per day (4.4% nonanthropogenic). The dominant PM sources were area-wide. For the PM2.5 fraction, the major sources were waste burning (32%), paved road dust (12%), farming operations (10.7%), and unpaved road dust (9.8%). The PM10 fraction major sources were farming operations * Corresponding author e-mail: [email protected]. † Science Directorate, NASA Langley Research Center. ‡ Sunlight, Nutrition and Health Research, § Systems Engineering Directorate, NASA Langley Research Center. 10.1021/es048740l Not subject to U.S. Copyright. Publ. 2005 Am. Chem. Soc. Published on Web 09/22/2005

(20.4%), unpaved road dust (18.8%), fugitive windblown dust (16%), and paved road dust (15%). While the above describes emission sources that directly emit PM into the atmosphere (primary PM), there are also emissions of gaseous precursors that can be transformed into PM (secondary PM). Such precursors are NOx (vehicle emissions), SO2 (combustion), reactive organic gases (industry/oil production), and ammonia (animal production). PM resulting from these emissions is not inventoried but estimated to be 60% of the ambient PM2.5 and 75% of the ambient PM10 in the San Joaquin Valley (2). Numerous aerosol ground studies of composition (3-7) and winds (8) have been made, but to the authors’ knowledge, no comprehensive study of the atmospheric aerosol vertical distributions has been made in the Central Valley. The only aerosol profile observed was recorded during the Shuttle LITE instrument validation by an aircraft-mounted lidar as it passed over the Valley (9). High concentrations of aerosols were recorded near Bakersfield, CA, increasing toward the Tehachapi Mountains. The winds (10) transport the distributed aerosol sources to the lower valley, where they are trapped by the valley topography and thus increase in concentration. This paper describes the first extensive measurement campaign of aerosol profiles within the California San Joaquin Valley showing the vertical and horizontal extent of the aerosols generated on the surface. These measurements were made with an airborne lidar during the DC-8 Inlet/Instrument Characterization Experiment (DICE) field mission that was conducted from April 31 to June 17, 2003, using NASA’s DC-8 aircraft. The measured lidar profiles were obtained during five flights over the California Central Valley. The resulting vertical lidar profiles were plotted on a topographic relief map of the Central Valley to give a geographic context of the aerosol distributions. Also, air mass back-trajectories were plotted to understand the air parcel temporal history. These data were then displayed on a three-dimensional web-based projection of the Central Valley that could be manipulated to observe different computer-generated projections, giving a complete contextual representation of the aerosol lidar data.

2. LIDAR System Description The aerosol lidar system was built in the Lidar Applications Group (LAG), of the Science Directorate at NASA Langley Research Center. The LAG has an extensive history of airborne lidar measurement programs (11). The lidar system is mounted onto an aluminum frame of dimensions 108 cm length × 53 cm width × 76 cm height. An aluminumhoneycomb breadboard sits atop eight vibration isolation mounts, which are mounted on top of the frame, as shown in Figure 1. The frame sits on seat-track mounts designed to adapt the system into the DC-8 aircraft. The mass of the system is approximately 127 kg. LCD monitors allow the lidar aerosol profile to be observed as a function of altitude and distance. The receiver optics, laser, and laser steering optics sit on top of the aluminum breadboard and are shown schematically in Figure 2. The receiver and laser are mounted in separate light-tight enclosures. The receiver telescope is mounted on the bottom of the breadboard in a nadir looking configuration with a breadboard clearance hole to provide top-side access to the telescope focal plane. A 90° folding prism turns the received telescope beam into the plane of the breadboard. VOL. 39, NO. 21, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Photo of the aerosol lidar system.

FIGURE 2. Schematic of the aerosol lidar optical receiver system and laser transmitter. The laser is a frequency doubled Nd:YAG, 20 Hz, (Big Sky CFR-200) with 1.5 mrad divergence output. The 532-nm (56mJ) and residual 1064-nm (42-mJ) pulses are transmitted collinear into the atmosphere using a dichroic high reflector and a turning prism. The laser electronics/heat exchanger package is mounted in the lower bay of the system frame, and the data acquisition computer is mounted just above it. The aerosol lidar receiver uses a Celestron C-11 (f/10) Schmidt-Cassegrain telescope with 28-cm diameter primary and 2 mrad field-of-view. The received light is collimated and split into 1064-nm and 532-nm channels, with a further split of the 532-nm channel into photon counting (10%) and analogue (90%) signal channels. The 1064-nm signal passes through a 1-nm band-pass interference filter to a focusing aspheric lens that focuses the beam onto a 1.5-mm diameter Perkin-Elmer avalanche photodiode (APD) (C30955E). The 532-nm signal passes through a 0.5-nm band-pass interference filter before the split into high and low signal channels. Both channels are focused using aspheric lenses onto microchannel plate PMTs with a 5-mm diameter photocathode. The analogue signal channel uses the Perkin-Elmer MH-943 module, while the photon-counting channel uses the Perkin-Elmer MP-943 module. Signals from the 1064- and 532-nm analogue channels are filtered by a 1.5-MHz filter and then digitized with a 14bit, 5-MHz waveform digitizer (Gage Applied Inc. #1450). 8352

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The waveforms are averaged for 2 s before being stored on the computer hard drive. This resulted in a 30-m vertical aerosol profile resolution and at the typical aircraft speed of 200 m/s, a horizontal resolution of 400-m. The 532-nm MP943 photon counting channel is sent to a multichannel scaler (Advanced Measurement Technology Inc. # MCS-pci), where typically a 30-s integration (100 ns dwell) time is used prior to storing the file on the computer hard drive. Individual aerosol profiles and continuous aerosol color projections are displayed on a LCD display while in flight.

3. LIDAR Data Presentation Technique To better study the 3-D (three-dimensional) placement of the lidar aerosol profiles with respect to land topography and atmospheric backward trajectories, an interactive computer visualization technique was developed. The DICE aerosol data, underlying geography, and the backward trajectories were all transformed into 3-D graphics representations for simultaneous display within a standard Web browser. Within a 3-D latitude-longitude-altitude coordinate system, the user can interactively view a full-color DICE lidar profile displayed over a 3-D high-resolution land map, and atmospheric backward trajectories can be observed intersecting the lidar profile and passing over the land masses. Data from multiple flights can also be displayed simultaneously.

FIGURE 3. Topographical map of the California Central Valley and the mission lidar flights. One low-level flight through the center of the valley is not shown. All flights departed and returned to Edwards AFB. To produce this 3-D visualization, custom software was developed to encode all the graphics into a standard format known as VRML97, also referred to as VRML2.0, which is a version of the Virtual Reality Modeling Language. A free plugin, such as CosmoPlayer or Cortona, is used within the Web browser to view the result on most desktop PCs. The common technique of texture mapping, by which a raster image is aligned and mapped onto a set of 3-D polygons, is used extensively in this implementation, because it is an accelerated operation available on most graphics cards. Texture mapping is a very efficient approach to visualize very high resolution image-based data applied to true 3-D surfaces, such as lidar profiles and topographical maps. An underlying 3-D topographical map was created by combining two main elements. The first element, resulting in the underlying mountain and valley terrain, was a public domain digital elevation model known as GTOPO30, accessible on the Web at http://edcdaac.usgs.gov/gtopo30/ gtopo30.asp. This database, completed in 1996 by the U.S. Geological Survey’s EROS Data Center, is a global digital elevation model with horizontal grid spacing up to 30 arc seconds (approximately 1 km). Only one-fourth of this resolution, about 120 arc seconds, was necessary for the interactive visualization. The second element of the 3-D map was a large visible image to help identify key cities and land features. The public domain high-resolution Landsat mosaic compiled by the NASA Jet Propulsion Laboratory at http:// mapus.jpl.nasa.gov was the source of this image. This 2-D (two-dimensional) Landsat image was texture-mapped onto a subset of the 3-D GTOPO3D dataset and aligned in latitude and longitude to produce the final VRML97 3-D representation of the U.S. West Coast as a suitable landscape on top of which the DICE data could be visualized. One unit in the X-direction equals 1° of longitude, a unit in the Y-direction equates to 1° of latitude, and a unit in the Z-direction equals 50 000 ft. Labels for major cities or other places of interest were added to the map as well. The first step to overlaying the DICE data on this 3-D map is to create a mesh of polygons representing each flight profile. The top edge of this mesh is the path of the aircraft in 3-D latitude, longitude, and altitude coordinates, and the bottom edge is the projection of this path at sea level. (For display purposes, it is not necessary for every point in the flight to be represented in the mesh, so every 40th point in the flight

path is used.) The set of 3-D points making up this mesh, and the order in which they are connected to finalize the mesh, were then encoded in VRML97 format. The TouchSensor and Route constructs in VRML97 were also used to allow the user to select a 3-D object, namely a flight profile, causing it to be highlighted and labeled accordingly. The second step to visualizing the DICE data is to texturemap a high-resolution image of the lidar profiles onto the respective 3-D flight profile mesh. Since every vertical lidar profile was contained in a separate text file, every file from a flight was parsed to create a single image. A color scale was generated that indicated the intensity of the backscatter coefficient observed. Finally, the alignment of these image textures with the 3-D flight profiles described earlier was automatically encoded in VRML97 to produce the final 3-D DICE visualization. Specific VRML97 constructs utilized to produce this 3-D texture mapping include IndexFaceSet, IndexLineSet, Coordinates, TextureCoordinates, and ImageTexture. The final component added to the visualization was air parcel backward trajectories starting in the lidar profile at 4000, 1000, and 0 m and going backward in time for 60 h. Every 10 h is marked with a dot. After determining which backward trajectories best corresponded with a given flight, each trajectory was converted into a VRML97 representation. First, this conversion required adding the local elevation of the land from the GTOPO30 database, since each raw parcel altitude represented distance above the land surface. Then the parcel altitude was scaled to match the other visualization elements. Finally, all the points within a backward trajectory were connected by lines using the Extrusion and Spine constructs within VRML97.

4. Results Figure 3 shows the topography of the California Central Valley and the lidar flights through the valley. The valley is bounded by the Sierra Nevada Mountains to the east, the Tehachapi Mountains to the south, and the Coast Range to the west. Los Angeles is in the lower right and San Francisco is in the upper left of the figure, and all flights flew near Bakersfield, CA, before landing at Edwards AFB. One flight is not shown, a low level flight down the central portion of the valley, again terminating at Edwards AFB. VOL. 39, NO. 21, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Aerosol lidar profile taken from 3:18 to 3:48 p.m. PST, June 5, 2003. Back-trajectories are shown originating at ground, 1 km, and 4 km, with each dot going backward in time 10 h for a total of 60 h. The color bar scale indicates the aerosol scattering ratio ranging from 0 to 15. The aircraft altitude was 6 km.

FIGURE 5. Lidar profile taken June 5, 2003, from 8:07 (coastline) to 8:20 p.m. (Bakersfield). All aerosol lidar profiles shown used only the 1064-nm laser. This wavelength is less sensitive to Rayleigh molecular scattering and more sensitive to micrometer-sized particles than the 532-nm visible channel and thus gives good signalto -noise characteristics for boundary layer aerosols. Hoff et al. (12) used a similar airborne lidar to measure aerosols in the lower Fraser Valley, BC, Canada, region. They were able to determine the movement of aerosols from more than seven point sources and urban centers. They also were able to identify removal processes of venting through the mountain valleys and drainage flows that are similar to what we observed in the Central Valley. Each 1064-nm lidar profile is a 2-s average (30-m × 400-m resolution) that was converted to aerosol scattering ratio verses altitude. The profile conversion consisted of finding an atmospheric region where molecule backscattering dominated aerosol backscattering. At this altitude, the lidar 8354

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equation constant was calculated for the given molecular backscattering coefficient. This constant was used to derive the total scattering ratio (βA/βM + 1) where βA is the aerosol backscatter coefficient and βM is the molecular or Rayleigh backscattering coefficient. The ratio βA/βM, called the aerosol scattering ratio, is then plotted verses altitude for each 2-s profile, and all profiles are plotted together for a particular aircraft flight. It should be noted that extinction has not been separated from the aerosol scattering ratio, thus this ratio should be considered an attenuated aerosol scattering ratio. The ratio is then scaled to a color chart where white represents a ratio of zero (no scattering from aerosols) and dark red represents strong scattering from aerosols. Figures 4 and 5 are lidar aerosol profiles taken on the same day, June 5, 2003, at 3:15 and 8:15 p.m. PST, respectively, and no clouds were observed in the lidar profiles over the Central Valley. The aerosol scattering ratio is shown in color

FIGURE 6. Aerosol lidar profile on June 3, 2003, at approximately 4:40 p.m. PST.

FIGURE 7. Lidar profile taken June 12, 2003, at 4:20 p.m. near Bakersfield. The Tehachapi Mountain range is below Bakersfield. ranging from 0 to either 15 or 20, and again back-trajectories are shown at ground, 1-km, and 4-km altitude extending backward in time for 60 h. In Figure 4, increased aerosols are shown between Bakersfield and the Tehachapi Mountains and also near the Coastal Range Mountains, whereas in the middle of the valley the ratio is low. The wind pattern at 4 km brings clean air from the Mojave Desert into the valley, but at ground and 1 km the air is a convergence of air from the Pacific and from the north down the Central Valley (other side of profile), thus confining the aerosols to the lower valley region. At about 8:15 p.m., the return flight allowed us to measure the same region 5 h later, as shown in Figure 5. There is a noticeable difference; now the whole valley has a substantial aerosol content and the maximum aerosol scattering ratio is increased to 20. The mixed layer varies from about 1 km to about 2 km near Bakersfield, and aerosols are moving into the Tehachapi Mountains, where they have been lofted into the free troposphere. Figure 6 shows the aerosol profile on June 3 at 4:40 p.m. near Bakersfield. Aerosols have accumulated in the lower valley and penetrated the Tehachapi Mountains. The trajectories at 4 km show air from the ocean, but the surface

and 1-km trajectories are coming from the north down the center of the valley at about 4 m/s, forcing the aerosols into the Tehachapi Mountains, where layers are seen above the mountains. Aerosols can also be seen crossing the mountains, appearing near Edwards AFB. Figure 7 shows an aerosol profile just north of Bakersfield taken on June 12 at about 4:20 p.m. The back-trajectories at 4 km show that clean air has come from the Pacific Ocean, whereas at ground and 1 km all trajectories come from the north down the valley center. Thermals can be seen in the mixed layer extending into the Tehachapi Mountains. Figure 8 is a lidar profile taken down the center of the San Joaquin Valley on June 17, 2003, at about 12:14 p.m. near Bakersfield. At low flight altitudes and over Bakersfield, the lidar was turned off for eye safety reasons. Again, the aerosol scattering ratio increased as we approached the Tehachapi Mountains. Some of the aerosols have penetrated the mountains and then convected into the free troposphere to an altitude of about 4 km. This phenomenon is known as “mountain pumping” and results from the rapid solar heating of the air in mountains, due to its smaller volume, as opposed to the air in the Central Valley. The resulting temperature VOL. 39, NO. 21, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 8. Lidar profile through the Central Valley taken on June 17, 2003, at 12:14 p.m. over Bakersfield landing at Edwards AFB. difference between the mountain and valley drives a heatflow and air mass toward the mountains, transporting valley aerosols into the mountains, where convective cells transport them into the free troposphere. The other figures show similar pumping, although not as pronounced as in Figure 8. Such phenomena have also been observed in the Alpine Mountains (13) and the Lower Fraser valley (14). The source region for the aerosols seems to be located near Bakersfield, with few aerosols being transported from other regions into the valley, at least for the 60-h backtrajectories calculated here. Near Bakersfield, there are at least five oil refineries, major highways, as well as agricultural sources of aerosols. The prevailing winds force these aerosols southward toward the surrounding mountains, where they are largely confined with some venting through the Tehachapi Mountains. Recently Cattrall et al. (15) published lidar (extinctionto-backscatter) ratios of aerosol species from 26 Aerosol Robotic Network sites around the earth and determined that this ratio at 0.55 µm was 71 sr for urban/industrial pollution. It is interesting to use this lidar ratio to derive an extinction for the aerosol backscatter ratios observed in this paper. According to Cattrall, the lidar ratio for our 1064-nm laser would be 71/1.9 or 37 sr. Often we see aerosol scattering ratios of 10 in the 1.5-km mixed layer near Bakersfield, thus the aerosol scattering coefficient would be 10 times the 1064nm molecular scattering calculated for the mixed layer, resulting in 9.3 exp(-7) 1/m-sr. Multiplying by the lidar ratio of 37 sr (1064 nm) and converting to km results in an extinction coefficient of 0.034 1/km. Using Beer’s law, the resulting one-way (1.5-km mixed layer) extinction is 0.05, characteristic of moderately polluted air, as observed from the lidar backscatter profiles. This optical thickness can be compared to the Aerosol Robotic Network sites located at Maricopa and Fresno, CA, which recorded a 1020-nm optical thickness of 0.02 and 0.08, respectively. To summarize, an aerosol lidar system has been developed and deployed over the Central Valley of California and used to measure (1064-nm) aerosol distributions within the valley. Significant aerosol loadings were observed in the boundary layer and seemed to originate and/or accumulate near Bakersfield, CA. These aerosols, due to the prevailing winds as noted from backward trajectories, would be forced against the surrounding mountains, where they were confined. Mountain pumping was observed in the Tehachapi Mountains, pumping aerosols into the free troposphere. 8356

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A 3-D computer projection of these results significantly helped the interpretation of these data in the complex environment of lidar profiles, surrounding topography, and winds. Such projections will be used in the future to aid in the interpretation of other lidar data sets.

Acknowledgments The authors express appreciation for the support of the NASA Dryden DC-8 aircraft team for their valuable and friendly support. The diligent support of Lisa Hawks, Paul McClung, and David Westberg (Science Applications International Corp.) for generation of backward trajectories is also acknowledged. The support of Ed Browell (Lidar Applications Group), Bruce Anderson (DICE Chief Scientist), and NASA Code Y is gratefully acknowledged.

Literature Cited (1) California Air Resources Board. 2003 Estimated Annual Average Emissions. San Joaquin Valley Air Basin; www.arb.ca.gov/app/ emsinv. (2) Air Resources Board Almanac 2003, 155-168; www.arb.ca.gov. (3) Lawless, P. A.; Rodes, C. E.; Evans, G.; Sheldon, L.; Creason, J. Aerosol concentrations during the 1999 Fresno exposure studies as functions of size, season and meteorology. Aerosol Sci. Technol. 2001, 34, 66-74. (4) Chow, J. C.; Watson, J. G.; Lowenthal, D. H.; Hackney, R.; Magliano, K.; Lehrman, D.; Smith, T. Temporal variation of PM2.5, PM10 and gaseous precursors during the 1995 integrated monitoring study in Central California. J. Air Waste Manage. 1999, 49, 16-24. (5) Neuman, J. A.; Nowak, J. B.; Brock, C. A.; Trainer, M.; Fehsenfeld, F. C.; Holloway, J. S.; Hubler, G.; Hudson, P. K.; Murphy, D. M.; Nicks, D. K., Jr.; Orsini, D.; Parrish, D. D.; Ryerson, T. B.; Sueper, D. T.; Sullivan, A.; Weber, R. Variability in ammonium nitrate formation and nitric acid depletion with altitude and location over California. J. Geophys. Res. 2003, 108, 457. (6) Whiteaker, J. R.; Suess, D. T.; Prather, K. A. Effects of meteorological conditions on aerosol composition and mixing state in Bakersfield, CA. Environ. Sci. Technol. 2002, 36, 2345-2353. (7) Blanchard, C. L.; Carr, E. L.; Collins, J. F.; Smith, T. B.; Lehrman, D. E.; Michaels, H. M. Spatial representativeness and scales of transport during the 1995 integrated monitoring study in California’s San Joaquin Valley. Atmos. Environ. 1999, 33, 47754786. (8) Green, M. C.; Flocchini, R. G.; Myrup, L. O. The relationship of the extinction coefficient distribution to wind field patterns in Southern California. Atmos. Environ. 1992, 26A, 827-840. (9) Strawbridge, K. B.; Hoff, R. M. LITE validation experiment along California’s coast: Preliminary results. Geophys. Res. Lett. 1996, 23, 73-76.

(10) Niccum, E. M.; Lehrman, D. E.; Knuth, W. R. The influence of meteorology on the air quality in the San Luis Obispo CountySouthwestern San Joaquin Valley region for 3-6 August 1990. J. Appl. Meteorol. 1995, 34, 1834-1847. (11) Browell, E. V.; Ismail, S.; Grant, W. B. Differential absorption lidar (DIAL) measurements from air and space. Appl. Phys. B 1989, 67, 399-410. (12) Hoff, R. M.; Harwood, M.; Sheppard, A.; Froude, F.; Martin, J. B. Use of airborne lidar to determine aerosol sources and movement in the Lower Fraser Valley, BC. Atmos. Environ. 1997, 31, 2133-2134. (13) Reitebuch, O.; Dadas, A.; Delville, P.; Drobinski, P.; Gantner, L.; Rahm, S.; Weissmann, M. The alpine mountain-plain circulation “Alpine pumping” airborne Doppler lidar observations at 2um and 10.6 um and MM5 simulations. Proceedings of the

International Laser Radar Conference, July 12-17, 2004, Matera, Italy. (14) Mckendry, I. G.; Steyn, D. G.; Lundgren, J.; Hoff, R. M.; Strapp, W.; Anlauf, K.; Froude, F.; Martin, J. B.; Banta, R. M.; Olivier, L. D. Elevated ozone layers and vertical down-mixing over the lower Fraser Valley, BC. Atmos. Environ. 1997, 31, 2135-2146. (15) Cattrall, C.; Reagan, T. K.; Dubovik, O. Variability of aerosol and spectral lidar and backscatter and extinction ratios of key aerosol types derived from selected Aerosol Robotic Network locations. J. Geophys. Res. 2005, 110, D10S11; doi 10.1029/2004JD005124.

Received for review August 12, 2004. Revised manuscript received May 20, 2005. Accepted August 17, 2005. ES048740L

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