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Borrower: ERA

Lending String: *CON,COV,NBU,WYU,OLA

Patron: Michael A. Flagg

Journal Title: need article 'from;Paleoclimatology

and paleometeorology ; modern and past patterns

of global atmospheric transport /

Volume: Issue:

MonthlYear: Pages: 133?165

Article Author: Leinen, Margaret,; Sarnthein, M.;

NATO Advanced Research Workshop on Paleoclimatology and Paleometeo

Article Title: W.G. Nickling and J.A. Gillies,

Emission of fine-grained particulate from desert

soils



# . Il



# . :J Imprint: Dordrecht;, Boston; Kluwer Academic

Pu



### ILL Number:**§~;§.§;O~21**

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### QC981.8.C5 N37 1987

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Ariel:

### Indian Journal of



# . lional Geography. iety of India. 39) 'A study of the

Indian Meteorology

le, Y.R. (1973) 

### 'Summerile convective activity
the country. Indian recasting Manual, part

lnderstorms on 23 

### March
C Meteorology and**~e** 

### of dust haze over In May 13, 1963.' 

### Indian

::s, 20: 56 - 5 7 • EMISSION OF FINE-GRAINED PARTICULATES FROM DESERT SOILS

W. G. NICKLING & 

### J. A. GILLIES Department of Geography University of Guelph Guelph, Ontario
CANADA, N1G 2W1

1. INTRODUCTION

During the past decade, considerable interest has been focused on the
emission, transport, deposition and climatological effects of natural and anthropogenic aerosols (e.g. Gillette, 

### et al., 1982; 1978; Deluisi, 

### et ai., 1977;
Pewe, 1981). Several studies have also considered the effects of mass particle

concentration on light extinction (Pilat and Ensor, 1971)/ climate (Idso and
Brazel, 1974)/ human health hazards (Leathers, 1981), visibility (Patterson

and Gillette, 1977b) and ambient air quality (Hagen and Woodruff, 1973). In
addition, a large volume of literature has been developed on the atmospheric, textural and surface conditions which interact to produce atmospheric aerosols.

Despite this, few studies have attempted to identify and classify (in a quantitative manner) the relative aerosol production potential of natural

and anthropogenic surfaces. As part of a long-range program to investigate

dust emissions in the Southwestern U.S./ an initial detailed field study was
carried out in Arizona to determine the relative aerosol production potential of selected surface types. A total of 13 representative sample sites

were selected for the evaluation of dust emission factors using a newly designed, portable field wind tunnel. Information of this nature is needed

to understand total suspended particulate loadings; especially in areas such

as the U.S. Southwest, Africa, and the Middle East, where dust storm frequencies and total suspended particulate (T.S.P.) loadings are relatively high (Nickling and Brazel, 1984; Brazel and Nickling, 1986). High particulate loadings in these areas have frequently resulted in serious

environmental and human health hazards. For example, in the U.S.

133

M. Leinen and M. Samlhem (eds.), Paleoclimatology and PaleOfneteorology: Modern and Past Patterns o/Global
Atmospheric Transport, 133-165.

© 1989 by Kluwer Academic Publishers.134 Southwest, considerable controversy has arisen over the sources of high background dust concentrations (Nick1ing and Brazel, 1984). Various individuals and groups, including government agencies, have disputed the primary sources of high dust concentrations. Potential sources have included agricultural fields, major construction sites, natural desert surfaces, mine tailings as well as stack and vehicle exhaust emissions.

Despite these ongoing controversies, little quantitative data is presently available to substantiate or refute most arguments. Although a relative wealth of information exists on the horizontal flux of soils, few detailed studies have been carried out on the vertical flux

of fine-grained aerosols from natural surfaces. Chepil and Woodruff (1957) provided the first detailed data on the variation of sediment concentration with height during dust storms in Colorado and Kansas. More recent studies on vertical aerosol fluxes have been carried out in the High Plains of

Texas by Gillette (1977) and in Texas and Nevada by Shinn, et al. (1976). Complementary studies have also been undertaken in Canada by Complin and Piersol (1978) on uranium mill tailings and on undisturbed natural soils by Nick1ing (1978; 1983). The results of Shinn, et al. (1976) are not directly comparable to the present work because their investigation focused on long range transport once sediment was entrained. In contrast, the present study focuses on the emission rate from different surface types. The relative lack of information on the vertical fluxes of aerosols from

natural surfaces in part reflects the logistical problems associated with instrumentation. In almost all the above studies, particulate concentration data were collected during wind erosion events using various types of aerosol collectors and wind instrumentation placed on towers at selected sites. Empirical relationships derived from such long-term field studies have been used with varying degrees of success by other authors to produce emission factors for specific sites. The recent decision by the U.S. Environmental Protection Agency to

modify the current T.S.P. standards to a limitation based on particulates having a mass mean aerodynamic diameter <10 IJ.IIl has necessitated a more detailed understanding of the potential sources and land use activities

which result in high aerosol fluxes so that appropriate environmental standards can be established.

2. FIELD SITES The thirteen sites selected: potential represent a range of su local environmental officials of dust concentrations in Southern basis of their soil textural charac degree of anthropogenic disturb agricu1turallands, abandoned a, construction sites and mine tailJ Figure 1 and a brief description characteristics is given in Table 3. FIELD MEASUREMENT a A fundamental problem ill production potential of surfaces

sediment loss under a wide ran, However, direct field observatil**(~g.** Gillette, 1977; Nick1ing, 197 several serious drawbacks: , 1), they are extremely costly i: logistical support necessaI') 2) field studies are very mud and as a result, one often s weather or surface conditi<

quality and quantity of dab 3) data obtained in such studi the lack of control on the r involved in the wind erosi The Portable Field Wind:

In order to overcome somE 'bs monitoring under natural' tunnel was designed and const! isimilar in design, although con: ;reported by Wooding (1968) anc "12.0 m open-floored working se135

sen over the sources of high

n.d Brazel, 1984). Various

ent agencies, have disputed the

. Potential sources have

ion sites, natural desert

vehicle exhaust emissions.

Llantitative data is presently

unents.

:ion exists on the horizontal

carried out on the vertical flux

$. Chepil and Woodruff (1957)

ltion of sediment concentration

and Kansas. More recent

carried out in the High Plains of

'vada by Shinn, 

### et ai. (1976).

ertaken in Canada by Complin

and on undisturbed natural

: Shinn, 

### et ai. (1976) are not

ause their investigation focused

entrained. In contrast, the**~om** different surface types.

vertical fluxes of aerosols from problems associated with
Lldies, particulate concentration**~nts** using various types of placed on towers at selected

such long-term field studies

:ess by other authors to produce

unental Protection Agency to ,tation based on particulates

<10 IJll1 has necessitated a more

ces and land use activities lppropriate environmental 2. FIELD SITES

The thirteen sites selected for the evaluation of aerosol production potential represent a range of surface types suspected by the authors and

local environmental officials of being major contributors to background

dust concentrations in Southern Arizona. The sites were selected on the

basis of their soil textural characteristics, vegetative cover, land use and degree of anthropogenic disturbance. The surface types include: active agricultural lands, abandoned agricultural fields, natural desert surfaces,

construction sites and mine tailings. The locations of the sites are shown in Figure 1 and a brief description of each site's surface and textural

characteristics is given in Table 1.

3. FIELD MEASUREMENT OF AEROSOL FLUXES

A fundamental problem in identifying and evaluating the aerosol production potential of surfaces is the need for the direct monitoring of

sediment loss under a wide range of atmospheric and surface conditions.

However, direct field observations using specialized monitoring equipment

<e.g. Gillette, 1977; Nickling, 1978; 1983), despite their usefulness, do have

several serious drawbacks:

1) they are extremely costly in terms of the instrumentation and the logistical support necessary to investigate several sites;

2) field studies are very much dependent on the vagaries of the weather,

and as a result, one often spends considerable time waiting for the right

weather or surface conditions that in the end may severely limit the quality and quantity of data obtained; and

3) data obtained in such studies are often extremely complex because of

the lack of control on the many atmospheric and surface variables

involved in the wind erosion process.



### 3.1 The Portable Field Wind Tunnel

In order to overcome some of the limitations associated with direct soil

loss monitoring under natural wind conditions, a portable field wind

tunnel was designed and constructed for 

### in situ testing. The wind tunnel is

similar in design, although considerably larger than portable wind tunnels reported by Wooding (1968) and Gillette (1978). The tunnel has a 0.75 x 1.0 x

12.0 m open-floored working section constructed of fiberglass withIT,l....C1'II-'l:ll0'.oas=­I'DC/leo."<C/l....en:El>:DNoZl>oZr-<wc.n.....<00000:58 8:5.";." ."o-8;11:3/-...t:b.....,,J..­c..C'--....eJifSite'Surface*ndTextUralCharacteristics<'11,."-""\....';"r,SITESITECHARACI'ERISTICSMesa,ArizonaFlat,laserleveledforirrigation;novegetationorsurfacetrash;doubledisked2dayspriorto(agriculaturalland)testing;largeclodsofvaryingsizecoveringthesurfaceMaricopa,ArizonaFlat,laserleveledforirrigation;novegetationorsurfacetrash;secondarytillageoperations(agriculturalland)inpreparationforcottonplanting;largeclodsofvaryingsizecoveredthesurfaceYuma.ArizonaFlat.laserleveledforirrieation:noveeetationorsurfacetrash:surfacewaslooseandfriable'/-­c:l"-,a"::-lr';,,:[',J'--.--­2'o(\.-r.lf'•f"J•I';'!.),•f',m/""V./",('C>I-<,""()j,"r...('»r,r.J'C",,"O',j(,.r--..._,IJ(f:..(IV­<1/1;_,IiJ"......C:I...../,#.,_,'L(''­8..<..,J\,[......A/7(',-'Ic:::::./-...,I'""""''''_''')0#j\';TABLE1.SiteSurfaceandTexturalCharacteristicsSITESITECHARACTERISTICSMesa,ArizonaFlat,laserleveledforirrigation;novegetationorsurfacetrash;doubledisked2dayspriorto(agriculaturalland)testing;largeclodsofvaryingsizecoveringthesurfaceMaricopa,ArizonaFlat,laserleveledforirrigation;novegetationorsurfacetrash;secondarytillageoperations(agriculturalland)inpreparationforcottonplanting;largeclodsofvaryingsizecoveredthesurfaceYuma,ArizonaFlat,laserleveledforirrigation;novegetationorsurfacetrash;surfacewaslooseandfriable(agriculturalland)withfewlargeclodsCasaGrande,ArizonaFlat,smooth,laserleveledatsomeprevioustime;sparsevegetation;primarilyannual(abandonedagriculturalgrasses;weaklycrustedsurface;easilydisturbedland)Glendale,ArizonaFlat,smooth,machine-leveledsurface;novegetation;surfacesoilheavilypulverizedby(constructionsite)earthmovingmachineryandregularlywateredfordustcontrol;easilydisturbedbyvehiculartrafficTucson,ArizonaVeryflat,surface;novegetation;surfacesoilheavilypulverizedby(constructionsite)byearthmoving;fetchlengthsover1kminalldirections;regularlywateredfordustcontrolAjo,ArizonaExtremelyflat,extensivecopperminetailings;novegetation;mainlyunconsolidated(minetailings)sedimentwithsomesparselyintermixedcrustedareas;fetchlengthsgreaterthan1kmHayden,ArizonaExtremelyflatcopperminetailings;novegetation;lightcrustiseasilydisturbed;fetch(minetailings)lengthsgreaterthan1kmAlgodones,CaliforniaRelativelyflat,extensiveoutwashdepositsadjacenttoAlgodonesDunecomplex;sparse(duneflats)vegetationcover;surfaceweaklycrusted;lagoffinepebbles;heavilydisturbedbyoffroadvehicles-..l138 c:: := §

c::

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.... p:::", §- Nt: 'C <", ci.o >0_

..c:"] II)

... 22 .0 .!!3**~", ~~**  11**~~** -2 !'IS v**~E** ; II)**~~** ·C .... s] ·C 8**~Q>** t. .. 'E Q>=**~..c: v~** **~~ ~8**  bO­**~~** : !'IS ....**~=** ... i Q> .,ev ... ::l**.~;** rl :CII)­ .!I.5;S ::l Q>**",~>** §i'"**£~~** ..c:0::t:**~",** 0 tf);S**~1 ~]~**  ........**!'IS2~** E II) Q> ::l .... ", >o:g

.£j 'i i 8 .:::**.~** fa ;.a § .,;:: S to > !'IS 'C £ .... c:: .ll. ]**~~ ~~** **~", ~J**  .2!Qj .... >**~~** 'B .2! t**2'~** ·C N <2 c::" u SjJ! v c::**~~**

o '> Q> g .£j 'i j .!!3 ::l**~rl** §:9 :..c:**s~** t] go ]S t] > ... ·C 2 bO·!!3 c::'"**!'IS~** = '3 '> ",m §..c: >.bO lS:E bOil) ffi5 - Qj c:: !'IS c::**~-5 ~~**  .. ·C :Jl ....**~!**

plexiglass viewing/access windo**t,"!~** dimensional, molded fiberglass i**;:i~$** straightener and a conical fibergl ",I<**~f** is provided by a 95 em centrifugl:**,;~£1';The** fan and engine are transport**~<** pickup truck and connected to tl '\1 flexible hosing. The inlet bell, lo'.l**~i{** on a 8 m flatbed trailer.**;!~,** 3.2 Testing Procedures**~~:j ~;:,-.-:"**  j;:' At each test site the wind tl 'tifundisturbed surface and the wiI1 tJnstalled into the test section. VI &:pitot tubes connected to magneh 'i;';; tubes were positioned above the**~_c"** if Suspended sediment was CI _samplers mounted downwind at**'.:.,~** ," above the surface**(F~g.** 3). The s ..•.. .. •• ,'ti\vacuum pump. Sediment was 0**t~iisokinetically** through preweighi**~~.:(O.1** JlIIl pore diameter) held in (**~.f;'~within** the samplers. Isokinetic:**'~tained** during each test by l**!in~rporated** into each vacuum .. Sediment transported in sal ';(1941) type catcher installed alor '}:)ehind the suspended sediment Following installation of tt ,el was slowly raised until n ers positioned at the plexi ,hold test was completed, a hold was established in the Izzle flow rate set to the cente:

,ear velocity was determined f ,ing the Prandtl equation: u=139

plexiglass viewing/access windows (Fig. 2). The tunnel uses a twodimensional, molded fiberglass inlet 

### bell with a honeycomb flow straightener and a conical fiberglass diffuser. Air flow for the wind tunnel

is provided by a 95 em centrifugal fan powered by a 35 h. p. diesel engine.

The fan and engine are transported and operated from the bed of a one-ton pickup truck and connected to the main working section by 1.0 m diameter

flexible hosing. The inlet 

### bell, working sections and diffuser are transported

on a 8 m flatbed trailer.



### 3.2 Testing Procedures

At each test site the wind tunnel was carefully placed over the

undisturbed surface and the wind velocity sensors and sediment collectors

installed into the test section. Velocity was measured with four N.P.L. type pitot tubes connected to magnehe1ic differential pressure gauges. The pitot

tubes were positioned above the soil surface at heights of 5, 15, 25 and 35 em.

Suspended sediment was collected in two streamlined isokinetic samplers mounted downwind and to either side of the pitot tubes, 50 em above the surface (Fig. 3). The samplers were connected to a high-volume

vacuum pump. Sediment was collected during each run by drawing air isokinetica1ly through preweighed 3.7 em diameter membrane filters
(0.1 J.1ffi pore diameter) held in commercially available sampling cassettes

within the samplers. Isokinetic flow through the 0.64 em sample orifice was
maintained during each test by means of a needle valve and flow meter incorporated into each vacuum line.

Sediment transported in saltation and creep was collected in a Bagnold
(1941) type catcher installed along the center line of the wind tunnel 10 em behind the suspended sediment collectors (Fig. 3).

Following installation of the instrumentation, velocity in the wind

tunnel was slowly raised until movement of particles was noted by

observers positioned at the plexiglass viewing windows. After the

threshold test was completed, a predetermined shear velocity (u

.. ) above

threshold was established in the wind tunnel and the suspended sediment

nozzle flow rate set to the center line velocity at the instrument height.

Shear velocity was determined from the wind velocity profile above the bed using the Prandtl equation:



## u 

## =**~*** In 

### k Zo (1)

## ...

# a5

...­

0171Figure2:Theportablefieldwindtunnelusedintheaerosolemissiontests.Figure3:Suspendedsedimentsamplers(melBagnold-typesaltation/creeptrapinstalledinthewindtunnel.Thesaltation/creeptrapis50eminheight.t• . I

142 where u is the velocity at length z, Zo is roughness length of the surface and

k is Von Karman's constant (0.4). , ,I

The length of an individual test was dependent on the amount of sediment transported and was longer for surfaces with lower flux rates. Duration of individual tests ranged from 10 to 30 minutes. At the completion of the test run, the sediment samples were removed from the samplers and carefully stored for subsequent weighing and grain-size analysis.**~~** <b**m~**

i !

O'lt-.: Net)**\()~** Since the soil surface may become depleted of erodible grains during the test, it was necessary to move the tunnel to a new location for each sample run. Subsequent test locations were normally within 10 m of the original site, with the long axis of the tunnel parallel to the initial orientation. Once the wind tunnel was repositioned, the threshold determination and flux measurements were repeated. In general, five or six runs at different shear velocities above threshold were carried out at each of

the 13 selected sites.**~~ ~~** **~~ ~~**  {/)Q

- IS ,:. - IS ,:.

_ et) t'i et) et)**~(!!;:** C'i(t')1 3.3 Grain-Size Analysis Size analysis of the suspended sediment collected during each run was done using a Quantimet 720 image analyzing computer following the method of Perrie and Peach (1973). In this technique, a small portion of an aqueous dispersion of the sample is placed on a gelatin-coated microscope slide with a pipette. The water is quickly absorbed by the gelatin, leaving the individual grains dispersed and cemented on the slide. The slide is then placed under the optical microscope of the Quantimet 720. By setting class limits, the instrument can be programmed to measure the total particle area larger or smaller than any of the given limits. In using this technique, the

class limits are based on the diameter of a circle with an equivalent area. In practice, accuracies of better than 1 percent can be obtained when the measured area covers more than 5 percent of the viewing area (Peach and Perrie,1974). For each of the suspended sediment samples analyzed, over 2000 grains were counted on each slide. Results of the grain-size analysis for

the specified size classes, <1.0, 1.0-2.5,2.5-10.0 and 10.0 /lIn are shown in Table 2. C'i =

.§ 'U cu {/) '2 1 :r {/) ., . !l S o cu .tt {/)**.~** 15

[ Q OS:; d zu 1""1 oz t:: S IS**~OIaA::1. ~~** **N~** {/)l/e 5 I E;**~S ~.;** **~{/)1""1** • {/) 0 f"4**~~** Q ,..; V

Q_l tTl 0 I o. et) N C

- 10 C ooc • eO I lfnc;**~~~ ~gn**  _Nfl - ]**~~** :r**t·~ ::E~** 

cOJ..,iii'OJ..,.;.:t...OJ,.,......"'0vg-cL':'-S:'"0='n\N"o...."'00 09§;.[...l»::r'n8g.I»p..p..0ali6Efreg.Qqe.gQQ'momp..mI-l.....:i0;:I.(/l9(/l;.go;.(/ls,e;[!.p.![,<as:(/lo£l)!»......,.p..6'-"'0='Qq=isI»p..!nEf;';:1.0e:§...nro::s;.§[Qq!..22:?g90'§I»!::'o:!"=.::!.S'tr.(/l""a.I<:<;:;0.,.,.0$ -rg0,a::s§.0OQ,.,(/l....I»::r<....(/lI»p.."'"p..I-l::s-"i0I»o...(/l•o.....<..,.S're...e.::r0'.0!:,!:'.!.;.0(/loN-\J\J,.,.....Qqn:::...e.e.;:J...(/l§re;.a..0'g.;-Qg.::rI»::np..J,.::r...§='eop..g.O'.Qq::s;.a..,.<.....I»e01»,.,.9nro(/l.....0a.S'80::sre;.g.s·='!»;.I»::sQq"'d.....g....Qq.(/l...p..I»o....g::sI».....><(/lTABLE2.GrainSizeCharacteristicsofSuspendedSedimentsGRAINSIZEDISTRIBUTIONOFMEANSTANDARDSHEARAEROSOLS(%FREQUENCY>SIZEDEVIATIONVELOCITY1.0•2.5-SITE<1.0JlDt2.5JlDt10.0JlDt10.0JlDt(J,lm)(J,lm)(em/sec)Mesa,AZ124.8140.8830.304.013.312.7462.9(agricultural)233.3234.6124.018.053.233.0443.7328.9635.0928.757.203.443.0043.4448.2427.2018.685.882.592.8850.3544.9531.4121.911.732.452.5253.4636.7826.6328.178.423.393.1655.5Glendale,AZ(construction)1242.0032.1631.6837.1321.6123.684.707.cn2.723.152.792.9522.136.7365.5330.651.192.631.281.6949.4417.4821.3342.3818.805.153.3362.6Maricopa,AZ114.2123.3138.7623.715.433.4158.9(agricultural)224.7624.9231.5918.734.553.5366.1319.6827.3038.5414.484.593.28141.1421.4617.6435.6125.285.303.6080.1527.1232.6829.463.783.2172.3Yuma,AZ110.8815.7543.4629.906.193.2727.5(disturbed214.2816.7936.9831.956.013.4938.3desert)39.4921.6252.6916.205.542.9759.649.4213.1145.1532.326.483.1636.6514.3623.8144.0017.835.193.2355.9u.lTABLE2.(Continued)GRAINSIZEDISTRIBUTIONOFMEANSTANDARDSHEARAEROSOLS(%FREQUENCY>SIZEDEVIATIONVELOCITY1.0-2.S.SITE<1.0J.LD1.2.5J.LD1.10.0J.Lm10.0J.LD1.(J.Lm)(J.Lm)(em/sec)Yuma,AZ117.1721.5440.9920.305.213.3729.6(agricultural)214.0327.5944.6913.694.893.1231.9314.1520.4143.4222.015.503.3134.6413.1920.3148.1618.345.443.1657.0510.9118.1445.1224.835.893.2151.7Algodones,CA123.2223.10295524.134.913.6726.7(duneflats)216.1516.7241.6425.495.673.4134.3313.4624.4549.3312.775.043.0450.4420.0432.2437.749.984.193.1149.7527.4232.88255314.173.873.3748.7610.9311.2041.3136566.623.2740.4Yuma,AZ146.771.7429.8421.654.373.91SO.5(scrubdesert)231.0930.1217.1821.604.043.7240.4315.0917.3344.4623.125.623.3241.3423.0021.3340.4115.264.693.3546.7530.9523.1130.9215.034.133.4721.4Tucson,AZ131.7529.1225.3613.763.773.3930.8(SantaCruz226.9924.9933.3014.724.273.4141.7riverterrrace)36.8813.3150.0929.726.532.9734.549.4820.4748.6121.435.763.1018.9510.7816.5147.9024.825.973.1635.1GRAINSIZEDISTRIBUTIONOFMEANSTANDARDSHEARAEROSOLS(%FREQUENCY)SIZEDEVIATIONVELOCITY1.0-2.S.SITE<1.0J.Lm2.5J.LD1.10.0J.LD1.10.0J.LD1.(J.Lm)(J.Lm)(em/sedTucson,AZ(construction)123426.7012.3622.0713.4336.4917.8723.8018.5630.7445.1230.3347.116.0724.6423.8020.903.475.824.945.592.923.263.643.2118.530.930.450.2536.7127.4423.6812.163.493.3549.1Yuma,AZ146.771.7429.8421.654.373.9150.5(scrubdesert)231.0930.1217.1821.604.043.7240.4315.0917.3344.4623.125.623.3241.3423.0021.3340.4115.264.693.3546.7530.9523.1130.9215.034.133.4721.4Tucson,AZ131.7529.1225.3613.763.773.3930.8(SantaCruz226.9924.9933.3014.724.273.4141.7riverterrrace)36.8813.3150.0929.726.532.9734.549.4820.4748.6121.435.763.1018.9510.7816.5147.9024.825.973.1635.1TABLE2.(Continued)GRAINSIZEDISTRIBUTIONOFMEANSTANDARDSHEARAEROSOLS(%FREQUENCY>SIZEDEVIATIONVELOCITY1.0-2.5-SITE<1.0fJJIl2.5fJJIl10.0fJJIl10.0fJJIl(em/seC>Tucson,AZ126.7036.4930.746.073.472.9218.5(construction)212.3617.8745.1224.645.823.2630.9322.0723.8030.3323.804.943.6430.4413.4318.5647.1120.905.593.2150.2536.7127.4423.6812.163.493.3549.1Ajo,AZ122.1523.0235.7619.074.803.4826.8(minetailings)228.6820.0230.4620.844.613.6531.2316.8522.7546.1314.274.963.1725.9431.0527.3323.2218.404.063.6051.4Hayden,AZ115.8522.9947.1014.065.013.1433.2(minetailings)227.9022.3725.8523.884.663.7524.6315.6414.9833.2836.106.153.6027.5414.6615.3336.0333.986.113.5241.7530.0226.7932.7210.473.863.2534.6630.3530.9021.3117.443.893.5441.7Mesa,AZ123.0710.9856.759.204.943.0417.7(saltriver)213.3616.6547.1622.845.753.2322.2330.0526.6722.8920.394.213.6741.749.5918.6242.4729.356.113.2634.2528.1724.8325.7821.224.443.6829.6CasaGrande,AZ125.2543.7327.423.603.142.6736.7(abandoned232.8228.1929.379.633.593.2122.0agricultural)312.1616.6445.6025.605.923.2535.549.3120.1850.6319.885.733.05345VI146 4. TEST RESULTS 4.1 Threshold Shear Velocity 00 (f) It) !J to-: 00 N**~.** (f) .... Threshold shear velocities for the sampled surfaces vary markedly and .. A range from 17.2 to 58.2 cm/s (Table 3). These values are similar to threshold values found by Gillette, et al. (1980) for a variety of undisturbed and Ji oq \C) t"! disturbed desert sites in the Mojave Desert. a ra 'U I- \Q .... 00 ..c .... N Q**~~** fg Also included in Table 3 are the associated 10m wind velocities tj**].~.-** q \C)

required to initiate particle motion and the mean vertical aerosol fluxes (F).

II**-<** 'U -= 6 .... ::1. § The 10m velocities were computed using the Prandtl equation (Eq. 1) and**J!~­** .... .... .... II) the roughness lengths found during the wind tunnel tests. Although there :J (I) cu

is considerable overlap between the threshold .values of the undisturbed c:: .!::I o (f) \C) .. (1)'- and disturbed sites, the disturbed surfaces in general have considerably cu.5 6 g

0\ N 6"::1. .... $ lower threshold wind speeds. This finding is also consistent with the data tbof Gillette, et al. (1980). 'U :x q. o 6 6 6 .... .... .... .... The 10m threshold velocities indicate that wind erosion could be )( )( )( )(**~a** to... o 00 to... initiated at all sites under most normally occurring natural wind cu u N 0) oq 0) to-: .... N**6~** conditions. However, the relatively high threshold values found at the Algodones dune flat site and the three active agricultural sites would suggest that major wind erosion events would be relatively infrequent oS**N~** .... 8 considering the range of naturally occurring wind velocities. o o o o N 0'1**~'2** Chepil (1951) and Gillette, et al. (1980) have shown a general increase in ..,(1) oq It} Lri (f) \C) to... threshold shear velocity with an increase in the modal size of the surface :)!

.... .... .... aggregate size distribution. It should be noted, however, that the empirical relationships shown by these authors are relatively weak and demonstrate 'U -cu 0'1 N**~~** • II) the inherent variability in natural sediments. Results from the wind**~!** d**~~** tunnel tests showed no clear relationship between mean grain size and threshold shear velocity (UIt) for the 13 Arizona sites. This most likely i results from the similarity of modal diameters for the sampled sites and the ] ] ]**N'~** complicating effects of other surface parameters, such as vegetation and ..!!l ..!!l ..!!l < Cl surface crusting, which are not evaluated in a simple bivariate relationship.**jg~jg** !is ala NE .. ENE]J: A significant relationship at the 0.1 confidence level, however, was found to <a B..a <a**~~** exist between threshold shear velocity and the percentage of aggregates "'C S'C ""C OJ: tlSbC'CbC§bCl}Ji tIS tIS _tIS tIS _tq Cl <0.84m (FigA). This best-fit least squares relationship is:**~-~** >o-u ... 0.202 U. t = 20.09 (% aggregates >0.84 mm) (2) r = 0.58 n= 13147

lIed surfaces vary markedly and

;e values are similar to threshold, Lriety of undisturbed and**.~**



## .... C"i



## ....

.... oq

00 It') cx5



## .... o ,....:

N It') 

### o

O'l

00 C"!

It')



## ....

lted 10m wind velocities

mean vertical aerosol fluxes (F).

! Prandtl equation (Eq. 1) and

ld tunnel tests. Although there

ld values of the undisturbed

l general have considerably

ls also consistent with the data o 

# g

.... o



## ....

.... lQ

N



## .... 0\ 

### o



## .... o



## .... o

N 00



## ....

hat wind erosion could be curring natural wind

reshold values found at the



### e agricultural sites would

lId be relatively infrequent

; wind velocities. 

## .... 

# a

o cp

o



## .... )(



## .... cp

o



## .... )( cp

o



## .... )(

t-. O'l

N 

## 6

.... )(

It') -q

It')



## .... 0\ 

### q

o 

## b

.... )(

Cf') oq



## ....

.... q

o 

## ....

.... q

o 

## .... o 

### o cp

o



## .... )(



### ....



## .... q

o cp

o



## .... )(



## .... cp

o



## .... )(

0\ oq

00 cp

o



## .... )(

to..:



### 8



## .... q

o t;-

o



## .... )(

It') -q



## ....

lave shown a general increase in

the modal size of the surface

ed, however, that the empirical .atively weak and demonstrate

,. Results from the wind

tween mean grain size and

lna sites. This most likely

rs for the sampled sites and the

ters, such as vegetation and

a simple bivariate relationship.

ce level, however, was found to

le percentage of aggregates ationship is: Cf') -q

It')



## .... 00 -q

\0 

## ....

.... arl 

## .... C'/

00



## ....

mm) 0.202

r = 0.58 (2)

n= 13l

Chepil (1951) and Gillette, ... CD**"~** ... J:I a: '" CD 2 C "D u 2 8 Iii Ii U) u C**C.~'"** IlIll1a -8a:l'**E~rJa>.!::1ll**

o o o ....

relationships between soil erod agricultural and natural desert tion of large particles or aggreg velocity in at least two ways. F too large to be transported at a**~~'=:.t~~tJ** and absorb a large proportion ( O¢4l<.O. This effect is noticeable even v <J

¢ o

4

t: t;{ I s C • C ._ a:{

. Z**~~ !.~** **a1~** O:{.aC«a - "Dullllll"D I- Ii C"i: E E & « QJ.!! IlI :::J:::J_ U :::El':::E)-)-« o ...J ileO ... <J_ 0 .... E E

-5**.~** Bagnold (1941), from thee observations, found that threst can be defined by:

units is relatively small (ChepiJ tend to shield smaller, more ea Consequently as the concentra threshold shear velocity also it '<:t = '-" 0 /\ c .... u N

(I) w u o •

Cl N 0 'VO S III = CD !5l = III (7) 0 IX)

« (.:J w a: (.:J (.:J «

M (oj • Q) e**~e :g~**  o . .t:: 0

where (J and p are the particle a acceleration due to gravity, Dp empirical coefficient equal to a Reynolds numbers (B) > 3.5 (B of air). [I !**I~I**

,.,

0 N 0 II II .-

... :::J

M .... II C**.t::~** .... (oj Q)bO bOQ) (oj M M bO Q)bO (oj ..... ..... 0 o Q)

Although the Bagnold eql threshold velocity of well sortE " less satisfactory in the predicti , soils (Gillette, et al., 1980; Nick ; no Platter how well sorted, USl , ii' iii iii i. iii 'i i

.... It- ci**~bO** ojg ',tj (oj Q) .... u M**>~**

including clays that cause sigr ',Moreover, non-erodible rough and boulders) decrease the me , particles thereby increasing thl o o .... :J9S/W:J '(1. 0 ) Al13013/\ H'V3HS 010HS3HHl

0 .... Similarly, other surface effects , residues and clay crusts tend tl . (Gillette, et al., 1980; 1982 and 1 ;j bO .... ('149

o ci

o



### ....

o •

II

c 0

& <i:



# • 0



### .... E
E

o:t

0 /\

U)
w



### .... «

c:J

w

ex:: c:J c:J 

### «

*-



### ....

M



### ....
II

C 

## -

....**-~-~ci**

10HS3l:1Hl Chepil (1951) and Gillette, et al. (1980) have also shown similar
relationships between soil erodibility and percentage of aggregates for agricultural and natural desert soils respectively. The relative concentration of large particles or aggregates at the surface affect threshold shear velocity in at least two ways. First, larger grains or aggregates which may be

too large to be transported at a given velocity protrude above the surface

and absorb a large proportion of the shearing stress exerted by the wind.

This effect is noticeable even when the concentration of these non-erodible

units is relatively small (Chepil, 1951). Second, larger stationary particles

tend to shield smaller, more easily entrained particles from the wind shear. Consequently as the concentration of non-erodible units increases, the

threshold shear velocity also increases.

Bagnold (1941), from theoretical considerations and empirical

observations, found that threshold shear velocity for dry, well-sorted sands

can be defined by:

0'

_ P 

## )0.5

U.t=A -p- .g.DP ( (3)

where (J and p are the particle and air densities, respectively, g - the

acceleration due to gravity, Dp

- the mean particle diameter, and A - an empirical coefficient equal to approximately 0.1 for particle friction
Reynolds numbers (B) 

### > 3.5 (B = u

.. 

### Dp / y, where y is the kinematic viscosity

of air)

.

Although the Bagnold equation is quite reliable for predicting the

threshold velocity of well sorted sands >0.1 mm mean diameter, it is much

less satisfactory in the prediction of thresholds for agricultural and natural

soils (Gillette, et al., 1980; Nickling, 1984). Naturally occurring sediments,

no iIlatter how well sorted, usually contain a wide range of grain sizes; including clays that cause significant variation in threshold shear velocity.

Moreover, non-erodible roughness elements (such as vegetation, pebbles

and boulders) decrease the momentum transfer to smaller, shielded soil particles thereby increasing the shear velocity required to initiate motion. Similarly, other surface effects such as moisture, soluble salts, organic

residues and clay crusts tend to stabilize the surface decreasing entrainment
(Gillette, 

### et al., 1980; 1982 and Nick1ing, 1984).150 4.2 Vertical Aerosol Fluxes 10. 6 Direct field observations by several investigators have shown that the concentration of suspended sediment over eroding surfaces decreases as a power function of height, with exponents ranging from approximately -0.25 to -0.35 (e.g. Chepil and Woodruff, 1957; Shinn, et al., 1976; Gillette, 1977 and Nickling, 1978). This relationship can be expressed by: dn=p!!. dz z (4) where n is the dust concentration at height z and P is the average slope of the concentration against height relationship (- -0.3). Gillette, et al. (1972) and Shinn, et al. (1976) suggest that the established power law relationship found in the concentration profiles allows the vertical dust flux (F) to be described by: dn F=K dz (5) where K is the eddy diffusivity. Under neutral stability conditions, K=u.kz (6) where u ... is shear velocity and k is von Karman's Constant (- 0.4). Assuming that the dust concentration follows a power law distribution with height and combining equations 4, 5, and 6, the vertical aerosol flux (F) 10- 9 from a point concentration can be estimated by: F=kPu.n (7) II The suspended sediment concentration (n, J.1g/m 3 ) at the 50 em :1,1 sampling height in the wind tunnel during each test run was calculated by: Ii W ill n=Rt 10. 10 I I:, (8) 10 20 where W is the dry weight (J.1g) of sediment collected on the membrane Shear

filter, R is the flow rate (m 3 / s) through the sampler nozzle and t is the Variation of vertical length of the sampling period (seconds). The mean concentration during velocity (u

... ).

each test was computed by averaging the concentration values measured on

# • • 151**~stigators** have shown that the 'oding surfaces decreases as a nging from approximately -0.25

m, 

### et al., 1976; Gillette, 1977 and

res sed by:

(4)

and P is the average slope of
(- -0.3).

76) suggest that the established

:ration profiles allows the

(5)

ral stability conditions,

(6)

an's Constant (- 0.4).

s a power law distribution

Ld 6, the vertical aerosol flux (F) by:

(7)

(n, Ilg/m3) at the 50 em
ach test run was calculated by:

(8)

collected on the membrane unpler nozzle and t is the

e mean concentration during

lcentration values measured on 10- 6

I

0 LOCATION 

# *
Mesa Ag. Site

• Glendale Const.

o Maricopa Ag. 

## +

A Yuma Dist. Desert 10- 7 

# •

YumaAg l:!.A 

## • A 

# • +

• Algodones**.~** 0 

## + 0 o YumaScrub

u X 

### 0' ott A Santa Cruz River

G) '" 

# • 0 , Tucson Const.

0 N X Ajo E

u I. • Hayden 

# - al 0 

## o Salt River

U.

>cO 

### * Casa Grande



### .= ·0



### i u. 10. 8 0 

# •

0

Gl 

### « 0-;; u 

## .-* iii 

# •

G) 

### .+ > 0



# ,

10. 9

1.889 F = 2.33 x 10- 11 u

..

r = 0.42

0 n = 67

10**.104------,..........--...,.....----r"_...,......----r_-r---.---~** 10 20 30 40 50 60 708090100

Shear Velocity u

.. em/sec

Figure 5: Variation of vertical aerosol flux (F) with increasing shear velocity (u

.. ).,I

10- 6 the two filters installed in the wind tunnel for each test. In almost all cases the differences in the two measured concentrations were less than 10%, indicating the relatively good reproducibility of the sampling technique.

The mean vertical aerosol fluxes (F) measured at the thirteen selected sites are given in Table 3. The measured fluxes range from 1.0 x 10-9 to 6.5 x 'I' 10- 7 g/ em 2 sec and show wide intersite variability. Shear velocities associated with the vertical fluxes range from 17.7 to 80.1 em/sec. As shown in Figure 4, there is a general increase in the vertical flux with increasing shear velocity. Although there is a considerable degree of data scatter, the data can be described by the following least squares regression which is significant at the 0.1 confidence level. F = 2.33 x**10-11u~·889** (9) r = 0.42 n=68 The degree of scatter in the data is not unexpected considering the intersite variability in surface characteristics such as soil texture, the absence or presence of a surface crust, and the type and percentage of vegetative cover. The erosion system is not a simple bivariate relationship between vertical aerosol flux and shear velocity but multivariate, with surface characteristics interacting and influencing the availability of particles for transport and therefore the flux rates. It is noteworthy that the data shown in Figure 4 compares very favorably with that presented by Gillette (1977) in terms of both the range of

vertical fluxes observed and the degree of data scatter. Gillette's data were collected in West Texas during a four-year period over nine agricultural fields with varying textural characteristics. To investigate the scatter in the data set, it was partitioned on the basis of the percentage of silt and clay measured in the surface sediments (Figs. 6 and 7) and also on the similarity of surface morphology and land use. Figure 6 shows the vertical flux against u. for soils having a combined silt and clay content >25%. For these silty loams, a relatively high correlation exists between increasing vertical flux and u.:

'1 I LOCATI * Mesa • Glenc: o Marie {\, i I A Yumi 6 Yum. 10 7 -I I • Algae D Yumc it Santa u. x' ' Tucso ::) X Ajo ..J u. • Haydl O' o Salt R 1 * Casa ( Ljllt

.... W' E lOB

e st . u :'.J' -a. « to' i= ffi >**(~,** 10- 9 F = 6.12 x 10- 15 r = 0.75 n = 21 10 10 , I SHE. Ftgw.e 6: Variation of vertica -F) ;, velocity (u.) for soil153

.r each test. In almost all cases
rations were less than 10%,

r of the sampling technique.

asured at the thirteen selected

:es range from 1.0 x 10-9 to 6.5 x bility. Shear velocities

l 17.7 to 80.1 em/sec.

" increase in the vertical flux**~re** is a considerable degree of :ollowing least squares
fidence level.

(9)

lnexpected considering the
;uch as soil texture, the absence

ld percentage of vegetative

variate relationship between
lultivariate, with surface

availability of particles for

Figure 4 compares very 7) in terms of both the range of

"a scatter. Gillette's data were**~riod** over nine agricultural

it was partitioned on the basis

. the surface sediments (Figs. 6 lorphology and land use

.

.1

... for soils having a combined

lInS, a relatively high

flux and u

... : u..



### x·

:::> -I

u..

-I


### o u 51

a:: N'

W E 10 8**<t~** -I 01
e::t: u

Ia::

w >

10 9

10 10

Figure 6: LOCATION 

# *
Mesa Ag. Site

• Glendale Canst.



### o Maricopa Ag.

... Yuma Dist. Desert

6. YumaAg

• Algodones

o Yuma Scrub

tt Santa Cruz River

, Tucson Canst.

X Ajo

• Hayden 

## o Salt River

* Casa Grande

F = 6.12 x 10- 15 u*4.271

r = 0.75

n = 21

-Iy----,--.-

..... -.

..... ...-TT";------r---r--r---r--

.....................

10 100

SHEAR VELOCITY, U* em/sec

Variation of vertical aerosol flux (F) with increasing shear velocity (u

... ) for soils with silt and clay content> 25%

.I

154 F = 6.12 x 10-15U:·271 (10) r = 0.75 n=21 The variation of vertical flux with increasing u .. for the sandy soils (silt and clay >15%) is shown in Figure 7. The relatively poor relationship, which is evident on this plot, primarily results from the inclusion of the Maricopa agricultural site data. This site, although similar in sand content to the other four sites, was considerably different in terms of surface roughness characteristics. The field on which tests were carried out had been very recently ploughed and was characterized by well-defined ridges 15-20 em in height. In addition, the surface was covered with large clods 8- 10 cm in diameter which resulted from ploughing under somewhat moist conditions. These large clods were not broken down by abrasion during the wind tunnel tests, and thus acted as large non-erodible roughness elements which tend to protect the finer soil fractions from deflation (Chepil and Woodruff, 1963). This results in a much less rapid increase in vertical flux with increasing shear velocity than was observed at the other four sites. If the Maricopa data are omitted from the regression because of the cloddy nature of the soils, the vertical aerosal flux for the sandy soils can be described by: F = 7.79 x 10- 13 u;·027 (11) r = 0.77 n= 19 In comparison to the above results, Gillette (1977) found a fairly uniform trend of increasing vertical particle flux with increasing shear velocity for sites with sandy soils. He attributes this relatively good relationship to the uniformity of the dry aggregate structure of the soils investigated in his study. In contrast, the relationship between vertical flux and shear velocity presented by Gillette (1977) for loamy soils showed great variability, which he attributed to the widely different dry aggregate structures of these soils. This argument is similar to that used to account for the lower fluxes found at the Maricopa agricultural site in this study. Since textural data may not be readily available when estimating

emission factors for various surfaces, an attempt was made to partition the data on the basis of surface morphology and land use. Despite the limited data, five classes were established:

10. 6 .f, 10. 7 - r u.. L x· :::> ..J u.. ..J**H~** u . 0 31.

,L D: N 10 8 5 .- ..J 01 -- ct- i' (J**.~** D: W > I; 10- 9 10 10

LOCATII * MesaJ • Glend o Marice • Yuma A Yuma • Algod C Yuma Santa , Tucsol X Ajo • Hayde ¢ SaltR * Casa c: SHE. Variation of vertica

velocity (u

.. ) for soil Ag. site data are nc155



### (10)

ising u

.. 

### for the sandy soils (silt latively poor relationship,

lts from the inclusion of the hough similar in sand content

rent in terms of surface

l 

### tests were carried out had

erized by well-defined ridges

,as covered with large clods 8- 

### ghing under somewhat moist

n down by abrasion during the

In-erodible roughness elements

from deflation (Chepil and rapid increase in vertical flux

rved at the other four sites.

Le regression because of the

flux for the sandy soils can be

7 (11)**~tte** 

### (1977) found a fairly

flux with increasing shear

tes this relatively good 'egate structure of the soils ationship between vertical flux ) for loamy soils showed great

different dry aggregate

nilar to that used to account ;ricultural site in this study.

vailable when estimating

npt was made to partition the

land use. Despite the limited 10- 6

10- 7

u..



### x-

::J

...J

u..

...J


### o u en Q) 

### o '"

cc:

-

N
UJ E 10 8**«~** ...J Ol 

### «

u

lcc:

UJ 

### >

10- 9



### Figure 7: 

# ,

LOCATION 

# * Mesa Ag. Site

• Glendale Const.



### o Maricopa Ag.

... Yuma Dist. Desert

6 YumaAg

• Algodoneso YumaScrub



# , o 0 tl Santa Cruz River

, Tucson Const. 

### o

X Ajo

• Hayden 

## o Salt River o



### * casa Grande



# , o

F = 7.79 x 10- 13 u* 3.027

r = 0.77

n = 19

10 100

SHEAR VELOCITY, U* COl/sec



### Variation of vertical aerosol flux (F) with increasing shear velocity (u

.. ) 

### for soils with silt and clay content < 15%. Maricopa Ag. site data are not included in the regression.1

156 1) Natural and Disturbed Desert Sites, 4) Mine Tailings 2) Sites Developed or Modified by Fluvial Processes, F= r = 3) Construction Sites, n= 4) Mine Tailings, and The wide data scatter and 5) Agricultural Sites. for the agricultural sites is mos1 parameters which are not acc01 Significant regressions were derived. for all classes except agricultural .This is borne out by the fact the sites. In general, the regressions are typified by relatively high correlation individual sites except Maricop coefficients and may prove useful for estimating emission rates from surfaces when textural data are lacking. A great deal of cautipn must be Several authors have sugg exercised, however, since the criteria on which the classes were based. is related to the amount of materi somewhat arbitrary. dlaracterize this relationship, t: j]ux (F) to horizontal soil flux, q The regression relationships derived for the morphological land use The horizontal soil flux (q') is c classes are: transport rate q (g/ em sec) by tl (Gillette, 1977). 1) Natural and Disturbed Desert Sites . Despite the data scatter a s fe1ationship can be described by F = 7.99 x 10- 13 u;·99 (12) r = 0.76 (F/q'): n = 10 r =0. 2) Fluvial Sites n=**6~** F = 1.59 X**10-13u~·32** .. A somewhat clearer pictur (13) die basis of soil texture. The ml r = 0.61**~o~** a significant increase in f] n = 16 (F/q'): 3) Construction Sites r = 0, F = 5.82 X**1O-15u~·24** n=21 (14) r = 0.81 : This result is very similar n= 10 found that the flux ratio increa: ,pproximately 4.0. .. A similar, but somewhat" Sandy textured soils (silt + clay 'Vi157

, Fluvial Processes,

all classes except agricultural by relatively high correlation Lting emission rates from**~eat** deal of caution must be

:h the classes were based is

r the morphological land use

99 (12)

32 (13)

Z4 (14) 4) Mine Tailings

F 

## = 1.59 x 1O- 12 u;,93 (15)

r 

## = 0.76

n= 10

The wide data scatter and the lack of a significant correlation coefficient

for the agricultural sites is most likely related to textural and surface

parameters which are not accounted for in a simple bivariate relationship.

This is borne out by the fact that significant relationships are present for all

individual sites except Maricopa.

Several authors have suggested that the vertical emission of aerosols is

related to the amount of material transported in saltation and creep. To

characterize this relationship, the dimensionless ratio of vertical aerosol

flux (F) to horizontal soil flux, q' (g/ cm 2 sec) has been plotted in Figure 8.

The horizontal soil flux (q') is calculated by dividing the horizontal

transport rate q (g/ em sec) by the height of the saltation/ creep sediment trap (Gillette, 1977).

Despite the data scatter a statistically significant trend is apparent. This relationship can be described by:

fF/q'}= 3.14x 1O- 10 u;·85 (16)

r 

## = 0.54

n=68

A somewhat**clear~r** picture is obtained if the data set is partitioned on the basis of soil texture. The more loamy textured soils (silt 

### + clay> 25%)**sho~** a significant increase in flux ratio with increasing shear velocity.

fF/q'l = 5.01 x**1O-12u~·01** (17)

r 

## = 0.63

n=21

This result is very similar to that presented by Gillette (1977) who also

found that the flux ratio increased with shear velocity with an exponent of approximately 4.0.

A similar, but somewhat weaker, relationship was also found for the sandy textured soils (silt + clay <15%).••

158 10- 3 10-4 i l ,

II**10-5~** l J
u.. I**,~ ~I**  10-6 -I III 1",1,'1 !

l :(1 111 II J**~l'** 10- 7 I 10. 8 ...,.../-- 10 Figure 8:

0 • • tr

l:> l:> • l:> • ., • 70 0 • ,

trtr • •

0 I

LOCATION x

- ... X ,. 0 ox. 0 "'Ie • Mesa Ag. Site • Glendale Const. o Maricopa Ag. ... Yuma Dist. Desert Xf- • ...

• l:> YumaAg 0 • -. • Algodones l:> ° Yuma Scrub 0 tr Santa Cruz River , Tucson Const. - X Ajo / • Hayden • o Salt River , • Casa Grande • , F/q' = 3.14 x 10. 10 u. 2 . 85 tr r=O.54

(> n=68 ---r- I

-- - I I I I I I 20 30 40 50 60 708090100 Shear Velocity. u. em/sac Variation at the ratio of vertical aerosol flux (F) to horizontal soil flux (q') in saltation and creep with increasing shear velocity,'

(F/q'): r = O. n =**1~ IJ.·~**  ,. The above relationships dl the emission of fine particulate produced more fine dust per u

textured soils. The increase of , Je"centage of silt and clay also particulates at a much higher rc 4.3 Aerosol Grain-Size Chara£**~.;** Frequency size distributiol tUnnel tests are shown in Table of

' both the aerosols and the ass In general, the aerosols art distributions with weak to moe grains). The most striking feat similarity. Almost all the dish 15-10.0 J.1ID size range. The on agricultural site, which has strc . classes reflecting the relatively parent soil (Table 3)

.**~~,** , . ,. The mean size of the aero: l'emarkable uniformity of meat**~th** standard deviations rangi being from 3.25 to 3.55 J.1ID. n

been reported by Gillette, et al. Nickling (1983). It is suggestee .fine-grained aerosols 0-20 J.1ffi . t:elocities of these particles

. .-: Gillette (1974) suggests th i.mall and that their sedimenta**~ost** all eroding winds and'**~ce** all particles «20 J.1ffi) the remain in suspension, the SizE (i.e. poorly sorted) as a result c velocity or textural characteris159



### I

LOCATION

t Mesa Ag. Site

• Glendale Const.

J Maricopa Ag.

.. Yuma Dist. Desert

YumaAg

• Algodones

] Yuma Scrub

:t Santa Cruz River

• Tucson Const. ( Ajo

Hayden

> Salt River

, Casa Grande

.85

I
00

aerosol flux (F) to horizontal**~p** with increasing shear velocity. (F/q'l 

## = 5.37 x**10-9u~·10** (18)

r 

## = 0.51

n= 19

The above relationships demonstrate the importance of soil texture in

the emission of fine particulates. In general, soils with finer textures produced more fine dust Per unit horizontal soil flux than the coarser textured soils. The increase of the exponent value of u

.. with increasing

percentage of silt and clay also indicates that the fine, textured soils produce particulates at a much higher rate as shear velocity increases.



### 4.3 Aerosol Grain-Size Characteristics

Frequency size distributions of the aerosols collected during the wind

tunnel tests are shown in Table 2. The mean sizes and standard deviations

of both the aerosols and the associated surface sediments are also included.

In general, the aerosols are characterized by unimodal size

distributions with weak to moderate positive skewness (i.e. tail of coarser grains). The most striking feature of the size distributions is their similarity. Almost all the distributions have modal diameters in the

2.5-10.0 Jllll size range. The one major exception to this is the Mesa agricultural site, which has strong modes in the <1.0 Jlm and 1.0-2.5 Jllll size

classes reflecting the relatively high silt and clay content (18.6%) of the

parent soil (Table 3).

The mean size of the aerosols range from 1.28 to 6.62Jllll, with a
remarkable uniformity of mean sizes. The aerosols are rather poorly sorted,

with standard deviations ranging from 1.69 to 3.65 Jllll, with the majority being from 3.25 to 3.55 Jllll. The relatively poor sorting of aerosols has also

been reported by Gillette, 

### et al. (1972), Patterson and Gillette (1977b), and Nickling (1983). It is suggested that the rather poor sorting associated with fine-grained aerosols (1-20 Jllll) results from the low sedimentation

velocities of these particles.

Gillette (1974) suggests that particles less than 20 Jllll are sufficiently

small and that their sedimentation velocities are usually less than 0.1 u

.. for

almost all eroding winds and remain in suspension for great distances.

Since all particles «20 Jllll) that are ejected into the air stream tend to

remain in suspension, the size distributions tend to become more uniform
(Le. poorly sorted) as a result of turbulent mixing regardless of the shear velocity or textural characteristics of the parent soil.1

I .

160 As previously indicated, there is a great similarity in the mean sizes of the suspended sediment despite a considerable difference in the textural characteristics of the surface sediments from which the aerosols were derived. Moreover, no consistent relationship is evident between the mean size and the shear velocity at which the suspended sediments were transported. The lack of consistent relationships between the mean size of aerosols in the 1-10 J.1IIl size range with shear velocity and textural characteristics of the parent soils has been noted by several authors. Patterson and Gillette (1977a), from their study of aerosols over eroding surfaces, suggest that the distributions are characterized by three distinct

modes which may not all be present under a given set of conditions. The authors have labelled the characteristic modes A, B and C. Mode A, which contains a majority of particles having diameters between 2 and 20 J.1IIl, is characteristic of soil-derived aerosols but does not appear to be related to the size distribution of the parent soil. Mode B is centered between 20 and 200**~m.** It is characteristic of the particle size distribution of the parent soil. This mode is present only under conditions of heavy to moderate dust loading. Mode C is centered between 0.04 and 0.1 J.1IIl. In general, it is not

related to the other modes in composition or origin but is characteristic of a background aerosol concentration related to the transient nuclei mode of Willeke and Whitby (1975). Gillette and Goodwin (1974) suggest that mode B is characteristic of particles derived from loose soil aggregates, while mode A results from the breakup of aggregates by the saltation process (sandblasting) and the subsequent injection of disaggregated material into the atmosphere. As the 1'.,1: wind speed increases over the threshold for erosion, the first particles to be 1'1 1 Ai set in motion are those with diameters between 40 and 100 J.1IIl. The initial ii' movement is primarily due to saltation in which the particles bounce along 111' close to the surface. These particles collide with other particles on the II, surface, dislodging and disaggregating smaller particles which are injected !I into the atmosphere and produce mode A. The larger particles which form mode B (20-200 J.1IIl) quickly settle out because of their high sedimentation velocities. As a result, a relatively narrow range of particle sizes is kept aloft Ii in suspension by the vertical velocity fluctuations. :1", This selective transport process, related to the turbulent nature of most eroding winds, accounts for the narrow range of mean sizes and poorly

sorted distributions associated with samples collected during the wind tunnel tests.

DISCUSSION The collected data reveale .**~cluding** the Algodones dune agricultural sites, have higher

particulate fluxes than the othe: . iPSQciated with several importe resulting from the presence of I these soils from erosion for all lD\ount of vegetative cover pIa aerosol flux because of momeI1 effects. Precipitation is also an**~ticulate** emissions from des. emissions by the simple wet tin short lived because of the higt Of greater importance is the in, \Yegetation pattern and surface During periods of**increa~** \when temperatures and evapol annual grasses may become est if these grasses do not survive stubble can persist for several ,emission (Brazel and Nickling, The frequency and intensi fdevelopment and maintenanCE and redistributes clay particles sediment out during infiltratic

rainfall events may break up S' effectiveness; thereby increasir (Gillette, et al., 1982).

, A final consideration for tusruptions of the surface. Lo) deflation of erodible material c effectively armours the surfacl The stability of these soils can such as off-road vehicles or 0' erodible particles available for ,agricultural soils is related to t which protect the finer soil fre threshold shear velocity.161

t similarity in the mean sizes of

Jle difference in the textural

which the aerosols were lip is evident between the mean lended sediments were hips between the mean size of

r velocity and textural

lted by several authors.

r study of aerosols over eroding

aracterized by three distinct

. given set of conditions. The**~s** A, B and C. Mode A, which

eters between 2 and 20 JlI11, is

s not appear to be related to the

:; centered between 20 and

distribution of the parent soil.

:>f heavy to moderate dust
\d 0.1 JlI11. In general, it is not origin but is characteristic of a the transient nuclei mode of

t mode B is characteristic of

while mode A results from the

i (sandblasting) and the



### at into the atmosphere. As the
!rosion, the firs t particles to be**~n** 40 and 100 JlI11. The initial

hich the particles bounce along

rith other particles on the

!r particles which are injected

'he larger particles which form

ie of their high sedimentation \ge of particle sizes is kept aloft

tions.

to the turbulent nature of most

! of mean sizes and poorly

:ollected during the wind 5. DISCUSSION

The collected data revealed that in general the natural desert surfaces, including the Algodones dune complex outwash area and the tested agricultural sites, have higher average threshold shear velocities and lower particulate fluxes than the other tested sites. The stability of these areas is

associated with several important factors. The crusting of desert soils, resulting from the presence of clay and natural salts, frequently protects

these soils from erosion for all but extremely high winds. In addition, the

amount of vegetative cover plays a significant role in the reduction of

aerosol flux because of momentum partitioning and surface roughness

effects. Precipitation is also an extremely important factor in decreasing particulate emissions from desert soils. Although precipitation can decrease

emissions by the simple wetting of the surface, this control is relatively

short lived because of the high evaporation rates in desert environments.

Of greater importance is the indirect effect precipitation has on the vegetation pattern and surface crusting.

During periods of increased rainfall, particularly in the winter months

when temperatures and evaporation rates are low, vegetation such as annual grasses may become established thereby stabilizing the surface. Even

if these grasses do not survive subsequent summer droughts, the dry

stubble can persist for several years acting as an effective control on aerosol

emission (Brazel and Nickling, 1986).

The frequency and intensity of precipitation can also affect the development and maintenance of surface crusts. Surface water mobilizes

and redistributes clay particles and allows them to seal the surface as they

sediment out during infiltration and evaporation. In contrast, heavy

rainfall events may break up surface crusts, reducing the bonding
effectiveness; thereby increasing the susceptibility of the soil to erosion
(Gillette, et al., 1982).

A final consideration for the desert soils is the time interval between disruptions of the surface. Long disturbance-free periods will allow

deflation of erodible material and the development of a lag deposit which effectively armours the surface; thereby reducing the emission potential.

The stability of these soils can be seriously affected by anthropogenic effects,

such as off-road vehicles or overgrazing, which rework the surface making

erodible particles available for transport. In contrast, the stability of the agricultural soils is related to their ridged surface and cloddy soil structure

which protect the finer soil fractions from deflation and increase the

threshold shear velocity.162 An important consideration in the determination of aerosol fluxes which is frequently not addressed is the decrease in flux through time as sediment is removed, and a lag of non-erodible grains develops at the

surface. The fluxes in this study were measured in the order of minutes and during many of the test runs the deflation of the fine-grained particulates occurred relatively quickly. The surface then became stable as a lag of nonerodible soil units developed (>0.84 mm) which effectively sheltered the erodible soil particles. Consequently, the erosion system involves an emission decay function. The surface will not produce a given aerosol flux for a given shear velocity for an indefinite period of time. This relationship has not been defined, and further research is necessary to examine the decay function for various type surfaces, soil textures and for the degree of disturbance at the surface. 6. CONCLUSIONS This research has shed more light on the question of the relative importance of different surfaces in the production of atmospheric aerosols in southern Arizona. Of the variety of sites tested, it was apparent that those which experience periodic disturbance such as the river channels and mine tailing sites have the greatest potential for increased aerosol production because of the greater sediment supply and lower threshold shear velocities. The natural desert sites, controlled construction sites, and the agricultural sites have reduced emission potential because of their higher threshold shear velocities and lower flux values because sediment supply is limited by factors such as progressive armouring, surface crusting roughness, and vegetation.

This study also clearly**indi~tes** the usefulness of in situ testing using the portable wind tunnel. Previous to this research, the most detailed data on aerosol fluxes from natural surfaces was that presented by Gillette (1977). His data collection took place over a four-year period, with a limited range

of surface conditions. In contrast, data collection for the present study was accomplished in approximately eight weeks for a wide range of velocity and surface conditions with comparable results. REFERENCES Bagnold, R.A., 1941: The Physics of Blown Sand and Desert Dunes, Morrow, New York, 265 p.

Il'azel, A.J., and Nickling, W.G., dust storm generation in A

'Chepil, W.S., 1951: Properties 0 State of dry aggregate strucl Chepil, W.S., and Woodruff, N.F storms: IT, Visibility and dl Chepil, W.S., and Woodruff, N.F control, Advances in**Agr~** Complin, P.G., and Piersol, P., 1 emissions, Proceedings Thi Measurement and Control, Gillette, D.A., 1977: Fine partict ASA, 80: 890-897. Gillette, D.A., 1978: A wind tun of soil texture, sandblasting production, Atmos. Envir01 tGillette, D.A., and Goodwin, P.A ., soil aggregates eroded by \\ ':Gillette, D.A., Blifford, I.H., Jr., a aerosol on land subject to , tGillette, D.A., Adams, J., Endo, .. velocities for input of soil I Res., 85: 5621-5630.

Gillette, D.A., Adams, J., Muhs, , velocities and rupture mod ; soil particles into air, J. Gee {'Leathers, C.R., 1981: Plant com] significance for man, In: Pl characteristics and effects on Special Paper 186, p. 191.163



### termination of aerosol fluxes

:rease in flux through time as
lible grains develops at the

. ured in the order of minutes 

## anil

)f the fine-grained particulates

on became stable as a lag of non-, rhich effectively sheltered the "

rosion system involves an lot produce a given aerosol flux, period of time. This relationship:;

s necessary to examine the decay

ll'es and for the degree of

he question of the relative

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e such as the river channels and

I 

### for increased aerosol supply and lower threshold

mtrolled construction sites, and

L 

### potential because of their

flux values because sediment

:ive armouring, surface crusting

efulness of in situ 

### testing using
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.