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Vertical variability of aerosol backscatter froman airborne-focused continuous-waveCO2 lidar at 9.1-mm wavelength

Maurice A. Jarzembski, Vandana Srivastava, and Jeffry Rothermel

Atmospheric aerosol backscatter measurements taken with a continuous-wave focused Doppler lidar at9.1-mm wavelength were obtained over western North America and the Pacific Ocean from 13 to 26September 1995 as part of a NASA airborne mission. Backscatter variability was measured for ;52flight hours, covering an equivalent horizontal distance of ;30,000 km in the troposphere. Somequasi-vertical backscatter profiles were also obtained during various ascents and descents at altitudesthat ranged from ;0.1 to 12 km. Similarities and differences for aerosol loading over land and oceanwere observed. A midtropospheric aerosol backscatter background mode near 3 3 10211 to 1 3 10210

m21 sr21 was obtained, which is consistent with those of previous airborne and ground-based data sets.OCIS codes: 010.3640, 280.3640, 290.1350, 280.0280, 280.1100, 290.0290.

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1. Introduction

Atmospheric aerosol backscatter variability gives adirect indication of aerosol loading, which is impor-tant for studies of pollution, hydrological processes,and climate.1–5 Inasmuch as aerosol variability isgoverned by regional sources and sinks as well asbeing affected by transport owing to meteorologicalconditions, it is important to characterize this loadingat different locations and times. Both pulsed6–12

and continuous-wave13–19 ~cw! lidars are sensitive in-truments that can effectively provide high-esolution, large-scale sampling of the atmosphereemotely by measuring aerosol backscatter, therebyapturing detailed temporal and spatial variability oferosol loading. Although vertical backscatter pro-les are usually obtained by pulsed lidars, airborne-ocused cw lidars, with high sensitivity and shortime integration, can provide higher-resolution ver-ical sampling, thereby revealing detailed structuref aerosol layers.15–19 During the 1990 National

Aeronautics and Space Administration ~NASA!Global Backscatter Experiment20 ~GLOBE!, excellent

All the authors are located at the Global Hydrology and ClimateCenter, 977 Explorer Boulevard, Huntsville, Alabama 35806.M. A. Jarzembski and J. Rothermel are with the NASA MarshallSpace Flight Center. V. Srivastava is with the Universities SpaceResearch Association. The e-mail address for M. A. Jarzembskiis maurice.jarzembski@msfc.nasa.gov.

Received 31 July 1998.

908 APPLIED OPTICS y Vol. 38, No. 6 y 20 February 1999

agreement was found between a vertical backscatterprofile from the Jet Propulsion Laboratory ~JPL!pulsed lidar at 9.25-mm wavelength and a subse-quent quasi-vertical backscatter profile from theNASA Marshall Space Flight Center ~MSFC! cw lidart 9.1-mm wavelength during aircraft ascents andescents.9 This comparison demonstrated that,

even though a cw lidar obtains quasi-vertical back-scatter profiles of the atmosphere during aircraft as-cents and descents, these profiles can accuratelydetermine vertical aerosol structures.

In this paper we present a cw lidar backscatterdata set obtained during the test flights of a pulsedDoppler lidar system21 during the 1995 NASA Mul-ticenter Airborne Coherent Atmospheric Wind Sen-sor ~MACAWS! mission. As part of this mission, aNASA MSFC airborne-focused cw CO2 Doppler lidar,operating at 9.1-mm wavelength, obtained high-resolution in situ aerosol backscatter measurementsto characterize aerosol variability. Flights wereconducted over a 2-week period in mid-Septembernear the West Coast of the United States, over bothland and ocean. Approximately 52 flight hours ofaerosol backscatter data at 9.1-mm wavelength wereobtained at various altitudes over a combined hori-zontal distance of more than 30,000 km. Samplingwas also performed during numerous ascents anddescents at altitudes from ;0.1 to 12 km. Our ob-ectives in this paper are to show the observed vari-bility in backscatter at 9.1-mm wavelength with

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altitude as well as to compare some of the quasi-vertical profiles with pulsed lidar profiles.

2. Flight Mission and cw Lidar Description

The MACAWS mission consisted of nine flights ~de-oted F1, . . . , F9! on the NASA DC8 aircraft from 13o 26 September 1995. All the flights originated andoncluded at the NASA Ames Research Center, Mof-ett Field, California. Regions of overflights in-luded the North American West Coast fromouthern California to northern Oregon, the San Joa-uin and Sacramento valleys in California, the Sierraevada and Coastal Range mountains, and the Pa-

ific Ocean off the coast of Oregon, California, and theaja Peninsula of Mexico. The flights supported thealidation of the MACAWS instrument21, therefore

many of the flights were conducted over repeatedtracks for intercomparison and to simulate samplingpatterns for a future space-based lidar. Neverthe-less, data were collected over wide coastal marineregions, urban areas, and inland over California.The data over land and ocean constituted 44.8% and55.2%, respectively, of the total data.

The NASA MSFC cw 9.1-mm Doppler lidar beamwas focused at ;54 m from the aircraft ahead andbeyond the DC8 right wing tip, into the atmosphere,to measure the Doppler-shifted backscattered signalfrom aerosols in the lidar sample volume. Measure-ments were obtained with a 3-s integration time, be-gan within 10 min after DC8 takeoff, and continuedfor the flight duration to within an ;0.1-km altitudejust before landing. The measured signal-to-noiseratio was converted into an absolute backscatter co-efficient b ~m21 sr21! based on a calibration techniquethat employs well-characterized aerosols in the lab-oratory22 and a hard target as a transfer standard inhe field during operations. Details of calibration,nstrument design, signal processing, and perfor-

ance are provided elsewhere.22,23 The resolutionof the lidar depended on the DC8’s speed, which gen-erally varied with altitude. From low to high alti-tudes, the DC8’s speed generally ranged from 0.12 to0.24 km s21, which is equivalent to a resolution of;0.36–0.72 km, for a 3-s integration. The corre-sponding integrated lidar sample volume rangedfrom approximately 200 to 400 m3. Absolute uncer-tainty in the b measurements was ;20% for high tomoderate signal conditions and ;34% for low signalconditions, whereas relative uncertainty between ad-jacent b values was less than 1% and was due mostlyto subtle laser power fluctuations. The lidar b sen-sitivity was ;8 3 10212 m21 sr21.

3. Backscatter Profile Measurements

Figure 1 shows locations where selected quasi-vertical b profiles were obtained during various air-craft ascents and descents. The profiles, located byvarious symbols and profile numbers, are categorizedinto four sets: profiles over coastal ocean regions~open and filled circles and pluses!, profiles over aoastal urban region ~asterisk!, profiles over inlandural regions ~open squares and filled triangles!, and

rofiles showing midtropospheric aerosol layers overarine, urban, and rural regions ~labeled P1–P9 for9 and by a filled diamond for F5!. The b profilesver ocean ~Subsection 3.A! and over land ~Subsec-ions 3.B and 3.C! show similarities as well as differ-nces in b for these regions and clearly indicate theomplexity of the variation of aerosol layers in theower troposphere. However, from this 2-week sam-ling general trends in b with altitude can be dis-erned. In addition, b profiles during two flightshowed a significant elevated aerosol layer above thelanetary boundary layer over both ocean and landSubsection 3.D!, giving an indication of occasionalarge-scale aerosol loading in the middle troposphere.or all the profiles, whenever b was below ;10210

m21 sr21 the scatter in the data increased because ofan intermittent encounter with sparsely distributedlarger particles ~approximately 0.5–1 mm in diame-er! that contribute significantly to b in the infrared.

The altitude for the profiles was derived by pressureand not by radar. Typically, a 1-km change in alti-tude corresponded to roughly 10-km horizontal dis-tance. The time ~in UTC! for some of the profilesrepresents the approximate midpoint of the ascent ordescent. Also, wind speeds and wind direction wereobtained from the DC8 wind data. A description ofthe main features that contribute to b variability aswell as a comparison of these features with one an-other and with other pulsed lidar data sets taken oversimilar terrain is also presented.

A. Profiles over the Western U.S. Coastal Marine Region

The first set of b profiles over the western U.S. ma-rine coast is shown in Fig. 2. This region is affected

Fig. 1. Locations where quasi-vertical b profiles, identified by thevarious symbols and labeled P1–P9, were obtained with the NASAMSFC 9.1-mm cw lidar during ascents and descents of the NASADC8 aircraft from 13 to 26 September 1995. Asterisk representsMoffett Field just south of San Francisco Bay where the flightsoriginated and concluded and also where profiles were obtained.

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by marine as well as land air masses because of theirclose proximity to the coast. Profiles over the PacificOcean near the western U.S. coast were made duringF3, F4, and F7. Profiles for F4 ~filled circles! and F7~open circles! were made near the Oregon coast; pro-files for F3 ~pluses! were located ;170 km off thecentral California coast.

The height of the coastal marine boundary layer~MBL! aerosol found during F4 @Fig. 2~a!# was not

eep, going only up to ;1.5-km altitude, after whichdecreased by ;2 orders of magnitude. There also

appeared to be a slight elevated layer near 3-km al-titude. Throughout this profile the wind direction~;10 m s21! was northerly to easterly, from the uppero the lower troposphere. The two profiles were ob-ained ;3.3 h and ;150 km apart and showed quiteood agreement for the altitude regime covered.

Fig. 2. b profiles over the ocean ~a!, ~b! off the coast of Oregon~locations shown in Fig. 1 as open and filled circles, respectively!and ~c! off the coast of central California ~pluses in Fig. 1!. Forcomparison, the dotted curve in ~a! is a geometric mean aerosol bprofile retrieved by JPL’s airborne pulsed lidar7 at 9.25-mm wave-ength over the northern Pacific Ocean between latitudes of 20 °Nnd 60 °N for the fall 1989 and spring 1990 GLOBE missions.

10 APPLIED OPTICS y Vol. 38, No. 6 y 20 February 1999

his result indicates horizontal and temporal unifor-ity of the relatively clean large-scale air mass en-

ountered in this region. Figure 2~b! shows tworofiles taken three days later during F7, separatedy ;2.5 h but located in the same region as F4.hese profiles for F7 display similar b trends; how-ver, one reveals weaker aerosol loading. The highthat is due to MBL aerosols extended to ;1.5-km

ltitude, as in F4; however, there was an aerosolayer ;1-km wide just above it with quite high b.his layer may be due to the advection of surface-erived aerosols by an ;8-m s21 southeasterly wind

at the time of sampling at this altitude. Near;5.5-km altitude during F7, where the winds weremainly westerly, the relative minimum in b was oneof the lowest levels encountered at this altitudethroughout the 2-week period. In comparison,strong aerosol loading up to ;2.5-km altitude in thelower troposphere is shown for F3 @Fig. 2~c!#, which isunlike the shallower aerosol layer in the MBL duringF4. Also, several distinct aerosol layers at altitudesfrom ;3 to 9 km were observed. The winds werefrom the north throughout the whole profile; there-fore much of this loading was probably due to land-derived aerosols advected over the coastal regions.In the middle troposphere, b values generally rangedfrom a few times 10211 to a few times 10210 m21 sr21;n the upper troposphere b decreased with altitudeor F4 @Fig. 2~a!#, whereas a slight increase was de-ected for F7 and F3 @Figs. 2~b! and 2~c!, respectively#.

For comparison, Fig. 2~a! includes a geometricean aerosol b profile retrieved by the airborne JPL

ulsed lidar7 at a 9.25-mm wavelength over the north-rn Pacific Ocean between latitudes 20 °N and 60 °Nor the fall 1989 and spring 1990 GLOBE mission.his mean profile over water, averaged over a spatialxtent of several hundred kilometers in the mid-acific Ocean, shows good overall agreement with theoastal profiles shown in Fig. 2 and even more withhe shallower aerosol layer in the MBL shown in Fig.~a!. The major difference is in values of the MBLerosol b, which for the mean profile from JPL’sulsed lidar is generally lower than that observeduring the MACAWS mission for which a possibleontinental land mass effect off the coast may con-ribute to the increase by approximately an order ofagnitude. This agreement shows that the cw lidar

rofiles, which are quasi-vertical during ascents andescents, can be compared favorably with a pulsedidar profile. It also shows that profiles obtainedver clean remote marine regions are not signifi-antly dissimilar from those observed near coastalegions, except when there are land-derived aerosolsoading the MBL. These types of comparison areeeded, for it is harder to obtain data over remotearine regions than over coastal regions; hence, un-

erstanding the similarities and dissimilarities be-ween the remote and the coastal marine regions forifferent times and locations can facilitate under-tanding of global aerosol distribution.

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B. Profiles over the San Francisco Coastal Urban Region

A second set of b profiles, shown in Fig. 3, was takenover the coastal urban region of the south San Fran-cisco Bay area, which could also reflect air massesfrom both ocean and land, including urban pollution.These profiles were made during the DC8 takeoff andlanding, and their location is shown by the asterisk inFig. 1 ~the descents into the lower troposphere for allthe profiles occurred within the San Jose metropoli-tan area as the DC8 landed at Moffett Field from thesouth!. These profiles show several layers andchanging features with considerable variability, es-pecially in the lower troposphere. High b values,enerally greater than 1027 m21 sr21, were found in

the planetary boundary layer ~PBL!, above which bdecreased sharply to midtropospheric values. Theheight of the PBL aerosols also varied from as low as1.3 km in Fig. 3~a! to nearly 3 km in Fig. 3~b!. Thesedistinct differences in aerosol loading could be due, inpart, to the direction of the wind that was transport-ing different air masses. Figure 3~a! shows a com-posite of profiles during F1, F7, and F8 when the

Fig. 3. b profiles obtained during takeoff and landing near theouth San Francisco Bay area ~location shown in Fig. 1 by ansterisk! for ~a! F1, F7, and F8 with winds in the lower troposphererom the west and ~b! F2, F3, and F8 with winds in the lower

troposphere from the south or north. For comparison, the heavysolid curve in ~b! shows the geometric mean b climatology profileobtained by JPL’s ground-based pulsed lidar at 9.25-mm wave-length ~1984–1992! over Pasadena, California.8

winds in the upper troposphere were from the north-west and in the lower troposphere were mainly fromthe west, transporting cleaner marine air into thesouth Bay area with a lesser influence of land aero-sols. In fact, Fig. 3~a! shows a remarkably similarstructure to that found off the coast of Oregon @Fig.2~a!# with a similar shallow MBL. Figure 3~b! showsprofiles taken during F2, F3, and F5 for which windsin the upper troposphere were mainly from the westor north and in the lower troposphere were mainlyfrom the south or northeast, transporting more land-derived aerosols and showing high aerosol b in thelower troposphere, extending to ;3 km. Despite thedramatic differences in b values in the lower tropo-sphere, b levels in the middle and upper tropospheredo not show significant differences and also exhibitvalues similar to profiles shown in Fig. 2.

For comparison, the b climatology obtained by theJPL ground-based 9.25-mm pulsed lidar is shown as ageometric mean b profile over Pasadena, California~1984–1992! in the Fig. 3~b! profile.8 Agreement isquite good ~within a factor of 2! between climatolog-ical observations over this ;8.5-year period in theLos Angeles coastal urban area and the 2-week pe-riod of observations over the coastal urban area of thesouth San Francisco Bay region. The cw lidar withhigh resolution and high sensitivity exhibits detailedvariations in b in the cleaner middle and upper tro-posphere.

C. Profiles over the Inland California Rural Region

Figure 4 shows a composite of several profiles duringF1, F2, F3, and F8 further inland over the SierraNevada Mountains ~solid curve! and the San JoaquinValley ~dotted curve!. The locations of these profilesn Fig. 1 are shown as filled triangles and openquares, respectively. These sets of profiles tend tohow more effect of land processes and less effect ofarine air mass. In this rural region, which com-

rises both forested and agricultural areas, the PBLerosol height associated with high b was found to bepproximately 3–4.5 km. This vertical increase in

Fig. 4. b profiles inland over the Sierra Nevada Mountains andover the San Joaquin Valley ~locations shown in Fig. 1 as filledriangles and open squares, respectively!.

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aerosol loading in the lower troposphere was encoun-tered over the mountains and the valley. Profileswith the deeper PBL b values were associated with

inds from either the north or the south ~the twoprofiles with the most extensive loading in the PBLwere taken when winds were from the north!. Onhe other hand, the two profiles that had weaker balues in the lower troposphere were associated withesterly winds, with some influence of marine airass.Despite the dramatic increase in the vertical extent

f high b values reaching ;4.5 km, there are fewifferences among all the profiles mentioned thus farbove 4.5 km at the midtropospheric levels. Thereas a cloud layer encountered in one profile; never-

heless, the b above the PBL decreased by 2 orders ofagnitude, to lie roughly between 10211 and 10210

m21 sr21. This range of b values in the middle toupper troposphere was found consistently in thecoastal marine, urban, and rural profiles.

D. Midtropospheric Aerosol Layers in Marine, Urban, andRural Regions

A deep aerosol layer above the PBL was encounteredat altitudes from 3 to 7 km in all the profiles obtainedduring F9 ~Fig. 5!. This flight spanned a spatial

istance of ;1500 km off the North American Coasto the Sacramento Valley, California, and south ofan Francisco Bay ~Fig. 1!. Evidence of this deep

ayer is shown over the Pacific Ocean in P2–P6 @Figs.~a! and 5~b!#, over the Sacramento Valley in P7 and8 @Fig. 5~c!#, and over the San Francisco Bay area in1 and P9 @Fig. 5~d!#. The b levels in the middle

roposphere were more than an order of magnitudeigher than those observed for other profiles shown

n Figs. 2–4 over both ocean and land. Winds dur-ng F9 were westerly at 10–20 m s21, confirmed by

500-mb weather map data. This finding suggeststhat the aerosol loading in the middle tropospherecould be due to an Asian continental dust plumeevent being advected by the jet stream over the Pa-cific Ocean. Because several profiles were obtainedat widely different regions over both ocean and land,with an enhanced b aerosol layer encountered at allthe locations, it shows the widespread nature of thisdeep midtropospheric plume. In the profiles overthe Pacific Ocean ~P2–P6!, just below the layer near2 and 3 km, b dropped to very low values ~b ; 3 310211 m21 sr21!, approaching the detection thresholdf the lidar. Low b values so close to the top of the

MBL aerosol were not observed at this altitude in anyof the other profiles. However, b did not drop sosharply over the land ~P7–P9! just below the layer,reaching a minimum slightly greater than 1 3 10210

m21 sr21. The b in the MBL was highly variableompared with that in any other sampling in theoundary layer. Profiles over the ocean, P2–P6,how shallow MBL aerosol heights of approximately–1.5 km, whereas profiles farther inland ~P7 and P8!xhibit higher MBL, reaching 3 km @Fig. 5~c!#, similaro those in Fig. 3~b!. The boundary layer aerosoleight over the coastal San Francisco Bay area @Fig.

12 APPLIED OPTICS y Vol. 38, No. 6 y 20 February 1999

~d!# was also found to be quite shallow, similar tohose in Figs. 3~a!, 5~a!, and 5~b!, possibly because ofhe influence of the sea breeze, as the winds wererom the northwest.

As another example ~Fig. 6!, during a long transitight, F5, to Hurricane Juliette24 over the Pacific

Ocean, a profile was taken ;90 km off the west coastof the Baja Peninsula of Mexico ~shown as a filleddiamond in Fig. 1!. Clean upper tropospheric aero-sol conditions were encountered with low b above 6

Fig. 5. b profiles obtained during F9 ~flight path shown in Fig. 1!showing elevated aerosol layers above the planetary boundarylayer ~a!, ~b! off the coast of California ~P2–P6!, ~c! over the Sac-ramento Valley ~P7, P8!, and ~d! over the San Francisco Bay region~P1, P9!. For comparison, the dotted curve in ~b! is a profile fromJPL’s airborne pulsed lidar9 at 9.25-mm wavelength during thesecond GLOBE mission off the east coast of China on 31 May 1990.

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km. Below 6-km altitude there was a significantincrease in b that extended down to ;3 km. At;10-km altitude, wind direction was westerly; how-ever, below ;6-km altitude, wind direction was east-erly. At the time of the profile there was aconcurrent high near the Mexican west coast witheasterly winds over the country, as observed in the500-mb daily weather map. Hence this deep aerosollayer aloft over the ocean could be a signature ofaerosols that were being advected from Mexico outover the ocean.

As is well known, when aerosol layers get lofted tomidtropospheric heights they can extend over largedistances. Evidence of such large-scale elevatedaerosol layers, displaying similar characteristics tothose shown in Figs. 5 and 6, was also observed atother times by pulsed lidars. During the secondGLOBE mission a layer extending over altitudes of4.5–7 km was observed off the east coast of China on31 May 1990 by use of the JPL airborne pulsed lidarat 9.25-mm wavelength and is shown in Fig. 5~b! forcomparison.9 The layer consisted of an aged, well-mixed Asian continental aerosol that was trans-ported over the Pacific Ocean and diluted with cleanmarine tropospheric air. Clean b conditions werefound below this elevated layer, similar to that of Fig.5. Also, with the same pulsed lidar during the samemission, an aerosol layer ~which most likely was dueto island volcanic influence! was observed at 4- to6.5-km altitude south of the Hawaiian Islands7 on 20May 1990. This aerosol layer weakened farthersouth, away from the Hawaiian Islands, but evidenceof its extent ranged as far as ;900 km to the south.Both of these layers in the middle troposphereshowed evidence of land surface origin over theocean.

4. Vertical Backscatter Variation Statistics

Figure 7 summarizes the frequency of occurrence of bs a function of altitude for the entire 9.1-mm cw lidarata obtained during the MACAWS mission, includ-

Fig. 6. b profile showing an elevated aerosol layer above theplanetary boundary layer over the ocean off the west coast ofMexico ~shown in Fig. 1 as a filled diamond!.

ing both quasi-vertical b profiles and horizontal tran-sits. This figure represents a total of 52,982measurement opportunities, which have been classi-fied into b distribution over the Pacific Ocean ~29,266

easurements! in Fig. 7~a! and b distribution overand ~23,716 measurements! in Fig. 7~b!. Figure 7~c!hows the combined data from Figs. 7~a! and 7~b!.he statistical analysis and data visualizationethod are the same as those used in Ref. 25. Es-

imates of b are sorted into 0.5-km altitude bins andne-third-decade b bins, resulting in color-coded his-ograms representing the frequency of occurrence of balues as a function of altitude. The high b en-ancement in the upper-right-hand portion of eachgure is due to upper-level clouds. Cloud b withinurricane Juliette, which lasted ;3 h, is not includ-

d.24 It is noteworthy that the quasi-vertical pro-

Fig. 7. Histogram of frequency of occurrence for aerosol b as aunction of altitude for all nine flights. Frequencies of occurrenceithin each altitude interval sum to unity across all b intervals.he column at the left shows the frequency distribution of mea-urement opportunities at each altitude interval, which sum tonity over all altitudes.

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files shown in Fig. 3~b!, obtained during takeoff andanding, are similar to the total b distribution shownn Fig. 7~c!, except for the enhancement from 5 to 7m, which was due to the midtropospheric layer from9 and F5.The MBL aerosol feature @Fig. 7~a!# is generally

shallow, with b decreasing sharply above ;1.5-kmaltitude, whereas the height of the boundary layeraerosol over land @Fig. 7~b!# can extend to ;3 kmbefore the distinct decline to midtropospheric b lev-els. In the distributions over ocean and land, theenhancement observed from 5- to 7.5-km altitude wasfound during F9 @Figs. 5~a!–5~d!# and F5 ~Fig. 6!,where a large-scale, midtropospheric aerosol layerwas encountered, suggesting the advection of anaerosol plume over long distances. During theGLOBE mission in 1990 a similar aerosol layer wasencountered up to ;8-km altitude in a flight fromJapan to Hawaii.9,13,14

Above ;7.5 km over both ocean and land, b de-reased to quasi-stable aerosol background valuesanging from 3 3 10211 to 1 3 10210 m21 sr21. Thiserosol background mode for the middle and upperroposphere was also found in earlier data sets.14,25,26

It is a feature that appears consistently in most large-scale data sets, indicating that the middle tropo-sphere aerosol population reaches a quasi-stablestate when aerosol production and removal processesare in equilibrium.27

Figure 7~a! is more representative of a b distribu-ion over the ocean near land masses and is not solean as that observed in the remote clean marineonditions encountered during the 1990 GLOBE mis-ion over the southern Pacific Ocean @Fig. 1~b!; Ref.5#. However, in Fig. 7~a! the vertical b structureompares well with the structure of the b climatologybtained with the United Kingdom’s Royal Signalsnd Radar Establishment airborne 10.6-mm lidarystem @Fig. 1~a!; Ref. 26# over the United Kingdomuring 1981–1984. Their data comprise soundingsver coastal and marine areas. The shallower aero-ol layer above the surface and the consistent aerosolackground mode are evident in both data sets.In the present study the b distribution over land

hows that b decreases more slowly with altituderom boundary layer values to upper troposphericalues than the decrease found over the ocean. Sim-lar behavior was found in the Wave Propagationaboratory of the National Oceanic and Atmosphericdministration b climatology over Boulder, Colorado,

rom 1981 to 1984 @Fig. 1~b!; Ref. 26#. The deeperoundary layer over land is attributed to character-stically stronger convection than for the shallower

BL over the Pacific Ocean. Again, the aerosolackground mode in the middle troposphere is evi-ent in Fig. 7~b!, showing its existence over land withpproximately the same values as over the ocean.The total distribution shown in Fig. 7~c! comparesell with the 1990 GLOBE data set @Fig. 1~a!; Ref. 25#

or the Northern Hemisphere. The backgroundode value given in Refs. 25 and 26 in terms of bixing ratio ~b divided by air density! was measured

14 APPLIED OPTICS y Vol. 38, No. 6 y 20 February 1999

o be ;1 3 10 m kg sr for Wave PropagationLaboratory and Royal Signals and Radar Establish-ment data and ;2 3 10210 m2 kg21 sr21 for only theNorthern Hemisphere in GLOBE data. Figure 7~c!hows a b mode value ranging from ;3 3 10211 to 1 30210 m21 sr21, which, when converted to a b mixing

ratio for an altitude of 7–10 km ~air density, ;0.53 kg23 at ;8 km! would range approximately from 6 3

10211 to 2 3 10210 m2 kg21 sr21, comparing very wellwith the Wave Propagation Laboratory, Royal Sig-nals and Radar Establishment, and GLOBE results.

5. Conclusion

High-resolution, quasi-vertical profiles of b obtainedwith an airborne-focused cw CO2 Doppler lidar havebeen presented. These measurements depict vari-ous loading conditions over the western North Amer-ican coast. A comparison of the quasi-vertical cwlidar profiles with mean profiles obtained from pulsedlidars showed remarkably good agreement for gener-ally the same types of regions. This suggests that,barring any rapid fluctuations ~e.g., frontal move-ments and storm systems! in the atmospheric airmass, there is a characteristic pattern in to whichaerosol loading settles with respect to altitude andlocation. The constant presence of the aerosol back-ground mode in the data set further emphasizes thissuggestion. In general, aerosol loading over theocean was weaker than over land, with a stronger fallfrom surface MBL values to midtropospheric levelsthan for the boundary layer over land. As is to beexpected, aerosol loading depends on wind directionand location of aerosol sources. The cw lidar, be-cause of its high resolution and sensitivity, could pro-vide detailed structure of the aerosol layers both inthe boundary layer and in the layers above it. Theupper-level aerosol layers that seemed to have beenadvected long distances over the ocean significantlyaltered the midtroposphere structure of aerosolsabove the PBL. These high-b midtropospheric aero-sol layers over long distances can also provide passivetracers for wind detection from space-based lidars28,29

in remote clean marine regions. Thus, using air-borne lidars, one can study the transport of aerosollayers into cleaner atmospheric regions along withgeneral vertical aerosol profiles. Because aerosolloading and removal of aerosols are regional phenom-ena, routine monitoring of them is required on largescales for an understanding of their global variationand effects.

This research was partially supported under NASAsubcontract 95-193. The authors gratefully thankR. Kakar, Office of Earth Science Enterprise, NASAHeadquarters, for funding the NASA MSFC cw lidaron the MACAWS mission and the staff and crew ofthe NASA DC8 and D. Chambers for mission support.

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