DUST PARTICLE FORMATION AND CHARACTERISTICS A. Buekens Department of Chemical Engineering
CHIS 2, Vrije Vrije Universiteit Brussel, Belgium
Keywords : Aerosols, Elutriation, Cascade Impactor, Particle Size Dis tribution, Sieving, Source apportionment, Speciation. Contents
1. Survey . Sources !. P"ysical c"aracteristics #. C"emical c"aracteristics $. Source apportionment %. Dust Sample Analysis &. Particle Evaluation '. E((luent Dust Analyzers ). Dust Sampling Procedure 1*. Conclusions +elated C"apters lossary -ibliograp"y -iograp"ical Setc"
Su!ry
Dust is a very comple/ sub0ect. Its origin may be natural, or a conseuence o( "uman activity. Primary particles are as t"ey 2ere (ormed, secondary particles originate in atmosp"eric conversion processes. Aiten particles are t"e largest in number, but represent little in mass units. 3"ey are important in cloud (ormation. Aerosols create "aze and are relatively stable. 3otal Suspended Particles represent t"e bul o( atmosp"eric dust and mass is still t"e basis o( dust emission codes. 4rom a "ealt" point o( vie2, "o2ever, "o2ever, it is important to no2 2"et"er dust is in"alable and a nd "o2 deeply it penetrates into t"e lungs. 3oday 3oday,, t"ere is a s"i(ting concern, (rom particulates smaller t"an 1* 5m 6P7 1*8 to P7.$ and even muc" smaller. 3"e diameter o( a particle may be de(ined in numerous 2ays, depending on t"e property considered. Sieving addresses cross9sectional area, sedimentation, centri(ugation and elutriation an aerodynamic diameter. Particles can be e/amined using a microscope, Coulter counter, laser di((raction, and cascade inertial impaction. Sampling and dust monitoring met"ods are discussed. ". Sur#ey
Particulate matter 6P78 is very variable in origin, nature, concentration, and size distribution. atural particulate arises (rom erosion, sand storms, and sur(, (rom (orest (ires and volcanic activity. -io9aerosols compre"end pollen, a seasonal allergen, as 2ell as trans(ormation products o( natural semi9volatiles, suc" as terpenes and isoprene.
Ant"ropogenic particles originate (rom combustion, metallurgy, metallurgy, bul and (ilter dust "andling, and are a carrier o( adsorbed c"emicals, bio9contaminants or condensed gases, 2"ic" can trigger various "ealt" e((ects. Primary particles are introduced into t"e air in solid or liuid (orm, 2"ile secondary particles are (ormed in t"e air by gas9to9particle conversion o( o/idation products o( emitted precursors. Atmosp"eric particles are evaluated very di((erently, depending on 2"et"er mass or number is "andled as a main criterion. 3"e amount o( 3otal Suspended Particles 63SP8 in t"e atmosp"ere is e/presses in mass units. Strong 2inds 2ill en"ance 3SP, 3SP, by remobilizing settled dust. In"alable particles, presented as P71*, are considered more important (rom a "ealt" vie2point. Size strongly in(luences in 2"ic" part o( t"e respiratory tract t"e particles are deposited. ;arger particles are deposited in t"e nasal area and in t"e upper parts o( t"e respiratory tract. Smaller particles (ollo2 t"e air(lo2 to t"e deeper parts and "ave a "ig" probability o( depositing by di((usion. Aiten particles, smaller t"an *.1 5m in diameter, represent by (ar t"e largest number o( particles and play a role in atmosp"eric processes, suc" as cloud (ormation, rainout and 2as"out. Aerosols are de(ined di((erently in various disciplines. 3"ey are relatively stable suspensions o( particles, and may also be termed mist, (og, (umes, and smoe. 3"e size o( particles determines dynamic properties, be"avior and (ate during transport. ;arge particles are mainly o( crystal origin, and (rom natural sources. 3"e "ig"est level o( concentration o( trace elements and to/ins (rom ant"ropogenic sources and radioactivity (rom natural sources is related to t"e very small particles. Particle p"ysical and c"emical properties bear a relation to t"e sources generating t"e particles. 3"is "elps to identi(y t"e parameters or t"eir ranges t"at s"ould be speci(ically targeted (or various types o( emission sources operating in t"e environment under investigation. Atmosp"eric aerosol concentrations range (rom: ▪
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5g m 9! in polar regions< 1* 5g m 9! as bacground value< !* 5g m 9! (or remote and rural locations< 1&* 5g m 9! (or polluted urban areas up to 1** *** 5g m 9! in sand storms=
$. Sour%es
Coarse particles are mainly (rom mec"anical processes, including grinding, breaing and 2ear o( material, transport, and dust re9suspension, and contain largely eart" crust compounds. 4ine particles are mainly (rom combustion, p"otoc"emical processes, and gas to particle conversion. 3ypically 3ypically t"ey contain soot, organic compounds, c ompounds, and acid condensates, i.e. sul(ates and nitrates, as 2ell as trace metals and ot"er to/ins.
Some processes generate particles 2it" broad size distributions, covering bot" (ine and coarse ranges, e.g. in (orest (ires t"ere are airborne combustion products, as 2ell as large diameter particles t"at are entrained into t"e smoe column as a result o( t"e turbulence and buoyancy generated by t"e (ire. In most cases, "o2ever, (ine and large particles result (rom totally di((erent generation processes. 3"us (ine and coarse airborne particles, or particle number and mass are not necessarily correlated. It is evident t"at (rom t"e measurement o( total particle mass only limited in(ormation can be obtained. Primary biological aerosol particles are discussed in Indoor in Indoor ir !ualit" !ualit" #onitoring and Control . ▪
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o o o o
Condensed 2ater aerosols comprise "aze, (og, and clouds. +aindrops, sno2 and "ail are very s"ort9lived. >ydrometeors are atmosp"eric particles 2it" 2ater as a dominant component. Clouds can be subdivided into: cumulus, cumulus, clouds 2it" considerable vertical development< stratus: stratus: layer clouds< nim$us: nim$us: rain clouds and cirrus: cirrus: (ibrous ice clouds.
3"ese are (urt"er combined to subgroups, e.g. strato%cumulus e.g. strato%cumulus.. In "aze "orizontal visibility is bet2een 1 and 1* m< in (og it descends belo2 1 m. Particle properties strongly depend on bot" particle (ormation and post (ormation processes. 4ine and coarse mode particles "ave di((erent c"emical composition and origins, are transported and removed by di((erent mec"anisms, and reuire di((erent detection tec"niues. 7any e((orts "ave been made to correlate emission sources and immission o( particulate, on a basis o( composition, elemental and isotopic ratio, and ot"er c"aracteristics, termed source marers. &. P'ys(%!) C'!r!%ter(st(%s C'!r!%ter(st(%s
Some important p"ysical properties o( particles 6individually, or as a group8 include: ▪
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umber and number size distribution< 7ass and mass size distribution< Speci(ic sur(ace area< S"ape< ?olatility< >ygroscopicity< Electrical c"arge.
3"ere are no instruments t"at can measure t"e entire particle size range, (rom nanometers to tens o( micrometers and usually t"ere is a size range selected (or investigations t"at depends on t"e ob0ectives o( t"e investigations. ?arious ?arious classi(ications and terminologies "ave been used to de(ine particle size ranges. &.". Nu*er !nd Nu*er D(str(*ut(on
;arge numbers 6up to millions per cubic meter8 are associated 2it" Aiten particles and Cloud Condensation uclei. A second mode 6i.e. a ma/imum in t"e distribution curves8 is generally
generated by (ine or coarser aerosol particles, caused by emissions and by atmosp"eric c"emical reactions o(ten involving ammonia and sul(uric acid. Coarse particles are generally blo2n up by t"e 2ind and are o( crystal origin. &.$. P!rt(%)e S(+es !nd S(+e D(str(*ut(on
Size is c"aracterized by particle diameter, or (or irregular s"apes 9 euivalent particle diameter, i.e. t"e diameter o( a sp"ere "aving t"e same value o( a p"ysical property as t"e particle being measured. Euivalent diameter relates to any property, e.g. inertia, electrical or magnetic mobility, lig"t scattering, radioactivity, -ro2nian motion, or to c"emical or elemental concentration, cross9sectional area, and volume to sur(ace ratio. Particles (rom most atmosp"eric sources "ave a lognormal size distribution: a particle concentration versus particle size curve is normal 6bell s"aped8, 2"en t"e particles are plotted on a logarit"mic scale. Particle size distributions o(ten contain several distinct peas, called modes. A 2ide9ranging aerosol collector 6@+AC8 provides an estimate o( t"e (ull coarse mode distribution. Inlet restrictions o( t"e "ig" volume sampler (or 3SP, t"e P7 1* sampler, and t"e P7.$ sampler reduce t"e total mass reac"ing t"e sampling (ilter. &.&. Dro,)et S(+e
Droplets are appro/imately sp"erical in s"ape, alt"oug" elongation occurs under industrial (lo2 conditions. Depending on t"e application at "and t"e representative droplet size is selected di((erently. Droplet size is (reuently denoted by a mean or median diameter. Eac" nomenclature (or describing droplet sizes leads to di((erent numerical values. 7ass ?olume 7edian Diameter is t"e largest o( all and number median diameter is t"e smallest. 3"e di((erences bet2een t"e various mean and median diameters provide a 2ay o( conveniently speci(ying t"e spread o( droplet sizes produced. I( all droplets "ad been uni(orm in size, t"e mean and median diameters 2ould "ave been identical 2it" t"e uni(orm size. Droplet size "as been measured 2it" a PDPA 6P"ase Doppler Particle Analyzer8 system. T!*)e "- Usu!) Met'ods o Des%r(*(n/ Dro,)et S(+e
Source: Dr. Alan +a2le, B-asic Principles o( Particle Size AnalysisB, 3ec"nical Paper, 7alvern Instruments ;td.
Me!n 0!)ue
De(n(t(on
Arit"metic
@eig"ted average o( t"e diameters o( all individual droplets in t"e spray sample
Sur(ace
Diameter o( a droplet 2"ose sur(ace area, i( multiplied by t"e total number o( droplets, 2ill eual t"e total sur(ace area o( all droplets in t"e spray sample
?olume Sur(ace
Diameter o( a droplet 2"ose ratio o( volume to sur(ace area is eual to t"at o(
or Sauter
t"e entire spray sample T!*)e ": sual 7et"ods o( Describing Droplet Size
7ass 6?olume8 7edian Diameter: t"e diameter, 2"ic" divides t"e mass 6volume8 o( spray into t2o eual "alves.
volume o( mi/ture
void (raction 2"ere & ' number o( particles V g volume o( gas p"ase V p volume o( particulate p"ase
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&.1. Nu*er Fre2uen%y D(str(*ut(on
4igure 1: umber 4reuency Distribution A+EA 4raction o( particles bet2een
and
6!8 umber mean 6#8 umber variance
6$8
&.3. M!ss Fre2uen%y D(str(*ut(on
4igure : 7ass 4reuency Distribution A+EA 6%8 4raction o( particulate mass associated 2it" particles bet2een
7ass mean 6&8 7ass variance
6'8 #. C"emical C"aracteristics Important c"emical properties are:
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Elemental composition, including mineral dust and 2ater. Secondary inorganic ions, mainly sul(ate, nitrate and ammonium. Carbonaceous compounds 6organic and elemental carbon8. Frganic composition.
1.". E)eent!) Co,os(t(on
Combustion results in emissions o( a cluster o( di((erent trace elements, present in t"e (uel or t"e lubricants used 6motor ve"icles8. Since most trace elements are associated 2it" ultra (ine particles and are less prone to c"emical trans(ormations, t"ey undergo long9range atmosp"eric transport. 7ec"anical processes, suc" as mining, mineral processing, uarrying, breaing, grinding, dust re9suspension, etc, generate particles predominantly containing crustal elements. 3able relates common outdoor particle sources to common suites o( elements. T!*)e $- E)eents e(tted ro #!r(ous ,!rt(%)e sour%es 4!%%ord(n/ to 5HO6. EC 7 E!rt' Crust
Source: Adapted (rom Buidelines (or concentration and e/posure9response measurement o( (ine and ultra (ine particulate matter (or use in epidemiological studiesB, Ed. D. Sc"2ela et al. @orld >ealt" Frganization, **.
E(ss(on Sour%e
C'!r!%ter(st(% E)eents E(tted
Ro!d tr!ns,ort
-r, Pb, -a, 7n, Cl, Gn, ?, i, Se, Sb, As
7otor ve"icle emissions
4e, Al
Engine 2ear
+are eart"s, Pt
Catalytic converters
GnF, carbon blac
3yre 2ear
EC, Al, Si, H, Ca, 3i, 4e, Gn
+oad side dusts Industr(!) !%()(t(es
?, i
Fil (ired po2er plants
Se, As, Cr, Co, Cu Al, S, P, a
Coal combustion
?
Fil +e(ineries
As, Sb, Cu, Gn, Pb, Cd, >g
on(errous metal smelters
Gn, Pb
Iron and steel mills
Cu, Gn
Copper re(inery
Reuse (n%(ner!t(on
Gn, Pb, Cu, Cd, >g, H
M(ner!) !nd !ter(!) ,ro%ess(n/
Si, Al, Ca, 7g, H, Sc, 4e, 7n.
Se! s,r!y
a, Cl, S, H
Resus,ended so()
Si, Al, Ca, 7g, 4e, 3i, Sr, 7n, Sc
T!*)e $: Elements emitted (rom various particle sources 6according to @>F8, EC Eart" Crust 1.$. S,e%(!t(on S,e%(!t(on is t"e identi(ication o( c"emical (orms o( an element. Di((erent c"emical (orms "ave di((erent p"ysical and c"emical properties and "ealt" e((ects. Processes suc" as environmental transport and bioavailability are species dependent.
Arsenic, beryllium, cadmium, c"romium, manganese, nicel, lead, tin, selenium, titanium, vanadium and zinc all are enric"ed at t"e sur(ace o( particles. Also, t"eir load in (ly as" increases in concentration 2it" decreasing particle size. >ence, conventional bul analyses o( particles provide but a poor measure o( actual concentrations o( many to/ic trace elements in t"e e/ternal environment o( t"e particle. 4or a particle o( an aerodynamic diameter o( one 5m, as muc" as '* o( t"e trace elemental mass is 2it"in t"e e/tractable sur(ace layer. 1.&. Se%ond!ry P!rt(%)es
3"e main atmosp"eric source o( secondary particles is t"e o/idation o( SF and F/. F/idation o( SF al2ays results in t"e (ormation o( aerosols, due to t"e lo2 vapor pressure o( >SF#. In contrast, >F! is distributed bet2een bot" t"e gas and aerosol p"ases. C"emical trans(ormation o( gases to particles depends on p"ysical (actors suc" as plume mi/ing and dispersion, reaction inetics, o/idant concentration, sunlig"t, catalytic aerosol sur(aces, etc. Conversion rates o( SF are generally around one 9 t2o percent per "our and about 1* times "ig"er (or nitrate. SF is emitted bot" by natural and ant"ropogenic sources, mainly (ossil (uel emissions o( SF converted to >SF#. Fver continents, > SF# (orms still acidic >#>SF# and neutral 6> #8SF# 2it" ammonia mainly (rom animal (arming and catalytic car mu((lers. Secondary pollutants, 2"et"er in t"e vapor p"ase or aerosols, are also (ormed (rom organic compounds, derived (rom engine e/"austs, (ugitive emissions and evaporation o( solvents, or natural in origin. Ft"er sources o( particulate organic carbon include pollen, lea( abrasion, suspension o( biological debris, as primary sources. Secondary particulate organic carbon results (rom gas9 particle conversion o( certain ?FC o/idation products.
1.1. Hu(d(ty
Atmosp"eric particles generally gro2 2it" relative "umidity 6"ygroscopicity8. @ater certainly contributes signi(icantly to t"e outdoor P7 mass concentration. Aerosol particles t"ere(ore contain a signi(icant amount o( 2ater. 4rom gro2t" (actor measurements, it can be calculated t"at a particle o( >#6SF#8 contains about !* o( 2ater in mass at $* relative "umidity. 1.3. C!r*on!%eous Aeroso)s
3"e carbonaceous (raction comprises carbonate, a 2ide range o( organic compounds and elemental carbon, 2"ic" is an inert tracer o( primary combustion. 3"is (raction can be de(ined (rom optical properties or (rom t"e analytical conditions in 2"ic" it evolves as CF . Combustion aerosols contain organics, more in particular products o( incomplete combustion, or PICs 6see (ollution Control through Efficient Com$ustion )echnolog"8. 1.8. P!rt(%)e Or/!n(% Co,os(t(on
Combustion sources generate large amounts o( volatile and semi9volatile organic compounds. Semi9volatile organic compounds in t"e air are eit"er in vapor or in particle (orm 6solid or liuid8. E/posure to organic compounds "as been associated 2it" various types o( "ealt" e((ects. Po)y!renes9 Po)y%y%)(% Aro!t(% Hydro%!r*ons 4PAH6 , some strongly carcinogenic, are a class o( compounds contained in t"e organic (raction o( (ine particulate matter. PA> compounds gro2 into large molecular structures in lo29o/ygen environments, suc" as t"e (uel9ric" region o( t"e (lame. I( e/iting incompletely combusted (rom t"e (lame zone, t"ey are released and condensed or are adsorbed onto t"e sur(ace o( t"e particles.
A compilation o( "ealt" e((ects o( selected PA> may be (ound in publications by @>F and t"e S EPA. Se(:#o)!t()e %o,ounds include alip"atic "ydrocarbons, (rom diesel and gasoline engines. uaiacol and derivatives result (rom t"e pyrolysis o( 2ood lignin and appear to be relatively stable in t"e atmosp"ere< t"ey serve as tracers o( 2ood smoe. A strongly carcinogenic compound present in diesel emissions is !9nitrobenzant"rone. Still, most o( t"e "uman burden derives (rom t"e (ood c"ain. Or/!n(% !%(ds, emitted (rom combustion o( (ossil (uels and biomass can contribute to source apportionment and "ave been lined to "ealt" e((ects.
4rom a "uman "ealt" vie2point it seems relevant in 2"at p"ysical (orm t"e semi9volatile compounds are in"ale: in vapor (orm, or associated 2it" particles o( speci(ic sizes. 3"ere is little in(ormation available, mainly due to t"e di((iculties in investigating organics in small amounts o( mass. Summary o( t"e most common volatile and semi9volatile organic compounds emitted (rom combustion sources may (ound in t"e specialized literature. 3. Sour%e A,,ort(onent
Source c"aracterization and uanti(ication o( emissions (rom individual pollution sources is necessary (or dressing inventories at local, regional and national levels and developing management and control strategies related to air uality and its impact on "ealt".
3"e principal primary sources o( emissions in t"e H are road transport, stationary processes 6coal burning8 and industrial processes, i.e. acid processes, cement production, petroleum re(ining and incineration. 3.". Atos,'er(% Aeroso)s
In atmosp"eric aerosols t"e dry (ine particulate is dominated by t"e (ollo2ing species: ▪
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sul(ates, mainly eit"er 2it" ammonia or acidic< nitrates, mainly 2it" ammonia< organics< carbon< soil dust.
nder ambient conditions 2ater is an important component o( aerosol particles. Depending on atmosp"eric "umidity aerosol particles eit"er gro2 or evaporate. Fne publication, (or instance, cites p"ase transition points (rom solid to liuid 6deliuescence8 (or a number o( salts, ranging (rom %1.' 6> F # !8 to '#. percent 6a SF#8. Fnly > #>SF# is e/tremely deliuescent, at !) percent. Salts may eit"er be non9volatile 6aCl8 or rat"er volatile 6>#Cl8. @et deposition is damaging to auatic and terrestrial ecosystems. 3.$. Sour%e M!rkers !nd Sour%e S(/n!tures
In order to control t"e sources it is necessary to "ave an understanding o( 2"ere particles are coming (rom, and "o2 muc" individual sources contribute at t"e receiving end. 7any sources emit a very large number o( compounds. 4or e/ample, over #*** di((erent compounds can be (ound in tobacco smoe. It is not only impossible, but also unnecessary to monitor all t"ese compounds. Instead some so called source signatures and source marers are used as (ingerprints, i.e. p"ysical or c"emical c"aracteristics speci(ic (or particular emissions, t"roug" 2"ic" t"eir source can be identi(ied. An ideal marer s"ould be easily, accurately, and cost9e((ectively measurable. Source marers are used (or uanti(ication o( e/posures to a particular type o( a source. A suitable marer (or uanti(ying concentrations o( emissions (rom a particular source s"ould be: niue or nearly uniue to t"e source, Similar in emission rates (or a variety o( (uels used, Easily detected at lo2 concentrations, Proportional to compounds t"at "ave e((ects on "uman "ealt", +elated to organic compounds, but not evaporating during transport (rom source to receptor ▪
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In practice, "o2ever, source identi(ication and apportionment is di((icult: ▪
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o o
Futdoor air contains a dynamic mi/ture o( pollutants. 3"e latter is emitted (rom numerous and various sources. 3"is mi/ture undergoes continuous c"ange in time as interactions tae place bet2een t"e pollutants, and as components are removed (rom t"e air to various sins.
Fnly rarely emission c"aracteristics are uniue to a particular source. 7ore o(ten emissions (rom ot"er sources display some o( t"ese c"aracteristics as 2ell. ▪
Source signatures include speci(ic suite o( elements or compounds and speci(ic ratios o( elements, isotopes, or compounds. 3.&. S,!t(!) !nd Te,or!) 0!r(!t(on
4ollo2ing (ormation and emission, pollutants undergo a range o( p"ysical9c"emical processes, 2"ic" c"ange c"emical composition, p"ysical c"aracteristics and concentration in t"e air. 3"e residence time o( emission products in t"e atmosp"ere varies (rom seconds or minutes to days or 2ees, sometimes even years. Some combustion related emission products are "ig"ly dynamic, 2"ile mec"anical dust is less so. Particles measured a2ay (rom t"e emission site, or particles generated indoors and measured some time a(ter emission, "ave di((erent c"aracteristics to t"ose measured immediately a(ter (ormation. ;arger particles are removed t"roug" gravitational settling, smaller particles by dry and especially 2et precipitation or di((usion deposition. Close to t"e emission point certain compounds could create signi(icant "ealt" e((ects. 3"ere is still little in(ormation on t"e (ate o( (ine and ultra (ine particles in t"e air. In an urban environment, motor ve"icle emissions usually constitute t"e most signi(icant source o( (ine and ultra (ine particles. Fne study e/amining P7 1*, P7.$, F, blac smoe, and benzene at a distance (rom a ma0or motor2ay concluded t"at F and blac smoe concentrations decrease 2it" increased distance (rom a road up to !** m, 2"ereas t"ere is no signi(icant decrease in concentrations o( P71*, P7.$, and benzene. Apart (rom ve"icle emissions, also re9suspended road dust could contribute signi(icantly to P71* or P7.$ and t"e degree o( t"is contribution is very site speci(ic. +e9suspension is signi(icant (or 2ind speeds rising above $.# m s 91. 3.1. Co*ust(on Aeroso)s
Combustion results in t"e emission o( non9reactive, reactive gases and aerosols. 3"e relative abundance o( t"ese t"ree groups o( compounds depends on t"e uality and c"aracteristics o( bot" combustion processes and (uels. Several distinct processes may e/plain aerosol (ormation: ▪
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Polymerization o( reactive (uel products< Dismutation o( carbon mono/ide, generating elemental carbon< Secondary reactions, including ageing.
4res" combustion9derived particles may be presented as a carbonaceous matri/ 2it" inclusions o( various trace elements and compounds. +elevant are: ▪
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t"e initial composition o( t"e (uel matri/, t"e distribution o( compounds bet2een volatiles and residue 6i.e. (i/ed carbon8, primary and secondary conversion reactions.
In t"e atmosp"ere aerosols interact 2it" ot"er species, undergoing coagulation and secondary reactions. Electrolytes 6sul(ates8 and semi9volatile compounds 6PA>s, dio/ins8 are collected onto t"e particles.
5.4.1. Markers 3"e carbon content may be de(ined in di((erent 2ays. 3"e 3otal Carbonaceous (raction o( aerosols is t"e residual carbon remaining a(ter acidic treatment (or removing carbonates. Frganic Carbon, FC, can be separated (rom -lac Carbon, -C, by a t"ermal treatment. 3"e material evolving at a temperature belo2 !$*J C corresponds to volatile, primary and secondary organics, t"e balance, at !$*J C, is o(ten considered to be -C. 3"ere is a continuous evolution in time and temperature o( suc" products< t"e residual blac carbon is not elementary, grap"itic carbon, since it "as its sur(ace coated 2it" organic groups. 3"e state o( t"ese compounds can be c"aracterized by t"eir: ▪
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>ydrogenKCarbon +atio< ?olatility< Analysis, a(ter e/traction< Aromaticity and color.
A number o( compounds "ave been proposed as marers, e.g.: potassium, long9c"ain alanes, n9alanoic acids or alanols, terpenic compounds, p"enols, arising in t"e t"ermal degradation o( lignin9based compounds in *(o!ss %o*ust(on < alanes, PA>9compounds, benzant"racene 6!utoot(#e8 and benzo(luorant"enes 6 d(ese)8. ▪
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PA>s "ave been proposed to apportion sources. 3"ere is, "o2ever, some ambiguity in t"e literature data, because FC, -C, soot, smoe, elemental carbon are used too loosely. Scanning electron microscopy can be used to study t"e morp"ology o( carbonaceous particles and 3ransmission electron microscopy 9 o( smaller particles. Electron energy loss spectroscopy can reveal t"e inner structure, and L9+ay analysis allo2s detection o( elemental marers 6potassium, vanadium, sul(ur8. Enric"ment in 1!C and 1#C "as been also used in discrimination. Ft"er salient properties are: ▪
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Particle sur(ace reactivity< Fptical properties< Sensitivity to "umidity< Atmosp"eric c"emical impact on particles.
5.4.2. Heavy Metals 7any trace metals are ubiuitous in (ossil (uels 6i, >g, ?8 and ores, as 2ell as in some industrial products 6>g in caustic soda8. ▪
Some elements are "ig"ly volatile, as t"e element proper, its o/ides, or "alogenides 6mainly c"loride and muc" less 9 bromide8. ▪
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Some elements 6Ag, As, Cd, Pb, Sb, Se, Gn8 are strongly enric"ed on (ine particulate. ;ong9range transport is possible.
3.3. Asso%(!t(on to Ot'er Po))ut!nts
In addition to particles, many emission sources, particularly combustion sources, generate gases and vapors: carbon mono/ide, carbon dio/ide, nitrogen o/ides, sul(ur dio/ide and volatile organic compounds. 7any o( t"ese pollutants "ave been independently, but also in combination 2it" particles, associated 2it" "ealt" impacts. 3.8. Re)e#!n%e or He!)t' Re)!ted Issues.
3able ! summarizes some c"aracteristics measured because o( potential lins bet2een e/posureKdose and "ealt" e((ects. T!*)e &- Potent(!) )(nks *etween e;,osure
Source: Adapted (rom Buidelines (or concentration and e/posure9response measurement o( (ine and ultra (ine particulate matter (or use in epidemiological studiesB, Ed. D. Sc"2ela et al.,@orld >ealt" Frganization, **.
C'!r!%ter(st(%s
>ust((%!t(on 4!*r(d/ed6
Particle mass
E((ect on "ealt", representative o( e/posure to coarse particles, only occasional correlation 2it" ot"er particle p"ysical c"aracteristics
P71* P7.$ P71
E((ect on "ealt", represents respirable (raction o( particles, in some environments, "o2ever, good correlation 2it" P7 1* E((ect on "ealt", representative o( e/posures to combustion particles, only occasional correlation 2it" ot"er p"ysical c"aracteristics
Particle number
E((ect on "ealt"< lac o( correlation 2it" ot"er p"ysical c"aracteristics suc" as mass.
Particle sur(ace area
Intermediate bet2een number and mass, available amount o( a to/ic species is related to t"e sur(ace area.
Particle number size distribution
Since most particle number is present in t"e ultra(ine range, (or many investigations particle size distribution does not need to be measured on a continuous basis, but on campaign basis to acuire general in(ormation (or t"e study area.
C"emical composition
+ecent evidence o( its e((ect on "ealt", only occasional correlation 2it" ot"er particle p"ysical c"aracteristics< t"ose elements and compounds t"at are associated 2it" local emission sources and t"eir relation to "ealt" e((ects s"ould be particularly targeted<
Detailed size (ractionated c"emical composition
Di((erent areas o( deposition in t"e lung result in di((erent bio9 availability o( to/ic and carcinogenic substances and t"us in di((erent "ealt" e((ects
T!*)e &: Potential lins bet2een e/posureKdose and "ealt" e((ects and 0usti(ication (or t"e recommendation o( (urt"er study.
3"e e((ects o( e/posure to (ine and coarse particles di((er, not only in t"eir ability to penetrate to t"e di((erent parts o( t"e respiratory tract, but also 9 in t"eir signi(icantly di((erent c"emical and to/icological composition. 4or t"is reason studies aimed at lining "ealt" e((ects to e/posure to particles s"ould include c"aracterization o( atmosp"eric (actors as many as practically possible, and relations"ips bet2een particles and ot"er pollutants in air. 8. Dust S!,)e An!)ys(s 8.". S(e#(n/
Analytical sieving is conceptually a simple met"od (or deducing t"e particle9size distribution o( a po2dered solid: one passes t"e sample t"roug" 2ire mes"es t"at "ave openings o( various sizes and t"en measures "o2 muc" o( t"e sample is retained on eac" sieve. Sieving analysis is di((icult to per(orm (or oily, co"esive po2ders, or irregular9s"aped particles, because t"ey tend to clog sieve openings. Sieving, dry or 2et, typically applies to particles resulting (rom mec"anical operations, suc" as breaing and milling. Sieving is t"e most 2idely used met"od (or analysis o( aggregate or particles larger t"at $* 5m. 3"e lo2er limit o( traditional sieve analysis is !& or &# 5m, 2"ic" is too coarse (or treating most (ilter dusts. +arely sieves are used (or (ine particle analysis. Sieves are made (rom 2ire mes" sealed into t"e base o( an open cylindrical container. 3"e ideal sieve openings are apertures t"at are nearly suare. Particles are commonly classi(ied by placing t"e sample in a nest o( sieves and agitating in a standard (as"ion (or a (i/ed period o( time. In t"e (irst stage o( sieving, particles considerably smaller t"an t"e screen opening pass readily. In a second stage, particles close to t"e size o( t"e opening pass slo2ly< t"e second part o( t"e sieving process is a((ected by particle s"ape, moisture content, met"od o( s"aing, and amount o( sample. 3ypically $* to 1$* grams are placed in t"e top sieve, and a(ter 1* or 1$ minutes o( s"aing t"e amount o( particles retained on eac" screen is 2eig"ed. 3"e 3yler Standard Scale is 2idely used (or sieve sizes. 3"e standard is based on a 2ire clot" 2it" ** openings per linear inc", 2"ic" is no2n as a **9mes" sieve. 3"e size o( t"e opening o( t"is screen is &# 5m. 3"e ratio bet2een successive sizes o( t"e screen scale is t"e suare root o( t2o, so t"at eac" sieve "as opening areas t2ice t"ose o( t"e ne/t9(iner sieve. 3"e S Sieve Series 2as de(ined by t"e ational -ureau o( Standards using t"e same ratio as t"e 3yler Standard Scale, but based on an opening o( one 5m.
8.$. Sed(ent!t(on
Particle sizes can also be analyzed by studying t"e terminal velocity o( particles in a sedimentation medium, i.e. a gas or a liuid. Stoes ;a2 t"en yields an euivalent particle diameter, d , (or a sp"ere (alling at t"e same terminal velocity, ut, 6cm s918:
6)8 2it" * p particle density, g cm9! *( particle density, g cm9! gravityMs acceleration, i.e. )'1 cm s9 + absolute viscosity, g cm91 s91 3"e diameter, d , is also called t"e Stoes or t"e sedimentation diameter. 3"e euivalent aero9 or "ydro9diameter, also called reduced Stoes or sedimentation diameter, is t"e diameter o( a sp"ere o( unit density "aving t"e same settling speed as t"e particle. Stoes ;a2 is strictly valid only (or: ▪
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Sp"erical particles, n"indered settling, i.e. 2it"out interaction 2it" ot"er settling particles, Streamline or viscous (lo2, i.e. (or +eynolds umbers, e N 1.
61*8 Particles o( diameters eual or less t"an t"e mean (ree pat" o( t"e molecules 6see, inetic t"eory o( gases in Basic Concepts of the -as (hase8 slip bet2een t"e molecules o( t"e (luid and t"e molecules (all (aster by a Cunning"am correction (actor, C o(:
6118 2"ere . mean (ree pat", cm a numerical (actor, ca. *.) D particle diameter, cm
In sedimentation analysis one must distinguis" bet2een t"e settling o( a "omogeneous suspension or particles introduced at t"e top o( a column. Sedimentation met"ods measure: t"e c"anges in concentration at a particular level in a settling suspension 6pipette, diver, "ydrometer, p"oto9e/tinction met"ods8, t"e cumulative c"anges in t"e total concentration o( a settling suspension 6sedimentation balance, manometric met"od8. ▪
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3"e (irst met"ods yield directly t"e 2eig"t (raction o( particles smaller t"an t"e size t"at 2ould (all (rom t"e sur(ace to t"at dept", during a time counted (rom t"e commencement o( settling. 3"e second met"ods yield as a result a 2eig"t deposited vs. time curve, i.e. an integrated result. 3"ese met"ods are relatively (ast but s"ould only be used in samples 2it" a no2n, "omogeneous density. Anot"er delicate (actor is obtaining adeuate dispersion, because o( electrostatic c"arging in gas 0ets and poor per(ormance o( mec"anical stirrers in liuid systems. 3"ese procedures also (ail to satis(y t"e reuirement o( no mutual interaction o( particles: since it is inconvenient to study single particles, a compromise is made bet2een number and mutual inter(erence. 3"e e/istence o( convective currents creates ot"er operating problems. 8.&. Hydroeter K(t9 St!nd!rd Set
In t"e "ydrometer met"od t"e sample is cleaned (rom organic matter, dried and 2eig"ed. e/t, it is suspended in 2ater and sieved. 3"e solution t"at passes t"roug" t"e sieve is trans(erred to a measuring cylinder 2it" 2ater. Sedimentation time and "ydrometer readings, taen a(ter regular intervals, are used to determine t"e grain sizes according to t"e StoesM ;a2. A commercial "ydrometer it contains: a number o( "ydrometers, sedimentation cylinders, a t"ermometer, a glass container, a "eating element 2it" t"ermostat and stirrer, a soil stirrer and various accessories. 8.1. E)utr(!t(on
Elutriation is t"e entrainment o( (ine particles in a rising current o( (luid. A particle elutriation separator may be composed o( a number o( vertical columns, 2it" gradually rising cross9 section. A (ilter closes t"e column on top. Initially t"e sample is introduced at t"e bottom o( t"e set9up. 3"e particles distribute in an up2ard air stream over t"e various columns, t"e coarse particles remaining suspended in t"e lo2er, narro2er, but "ig" velocity zones, t"e (iner particles (inding an euilibrium position at lo2er rising air velocity. 32o main operating problems arise: ▪
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Particles become electrostatically loaded, and start agglomerating to gro2ing balls, or t"ey ad"ere to t"e 2all.
8.3. Centr(u/!) te%'n(2ues
Centri(ugal tec"niues similarly allo2 classi(ication o( particles, but at a multiple o( gravity acceleration, so t"at t"ey are muc" (aster. 3"is tec"niue classi(ies particles by depositing t"em at speci(ic locations corresponding to t"eir settling velocity. Commercial analyzers based on t"e principle o( liuid p"ase p"oto9sedimentation 2it" ma/imum rotational speed o( 1*,*** rpm "ave an analytical capability (rom *.*1 to !**9micrometers. -ul samples can be analyzed using (or instance, a centri(ugal classi(ier, a combined centri(uge9elutriator consisting o( a rotor assembly driven by a totally enclosed electrical motor. It yields accurate and reliable results in a size range bet2een one and !* micrometers. 3"e instrument applies t"e principle o( terminal settling velocity to determine particle size distribution. 3"is met"od reuires a sample greater t"an 1* g (or analysis. 3"e motor creates precisely controlled air velocities 2it"in t"e air spiral and si(ting c"amber o( t"e centri(uge. 3"e sample is introduced into a spiral s"aped air current (lo2ing to2ard t"e center. It "as suitable tangential and radial velocities so t"at a certain part o( t"e sample is accelerated by t"e centri(ugal (orce to2ard t"e perip"ery o( t"e 2"irl, t"e ot"er part o( t"e sample being carried by t"e air current to2ard t"e center o( t"e 2"irl. Size, s"ape, and 2eig"t o( t"e particles determine 2"ic" direction t"ey 2ill tae in t"e air current. ?arying t"e air9 (lo2 allo2s to adapt t"e terminal velocity limit o( division and t"us t"e material can be divided into a number o( (ractions 2it" limited terminal velocity ranges. 3"e classi(ied sample can be recovered in 2"ole (rom t"e instrument (ollo2ing analysis. 8.8. F()tr!t(on
4ibrous (ilters are unsuitable (or particle counting or size determination, as particles penetrate too deeply into t"e (ibrous mass, eluding microscopic evaluation. 7embrane (ilters range in pore size (rom *.**$ to 1* 5m and can be used (or microscopic study o( (ine particulate. Smaller particles are retained only partly, but retention is virtually complete (or particles above t"is nominal size. 4ilter media must e((ectively retain particles at t"e lo2er side o( t"e size range o( interest, s"o2 uni(orm e((iciency and an acceptable resistance to air9(lo2. ;eas bet2een (ilter and "older must be avoided. 8.?. Inert(!) I,!%tors
Inertial impingers, e.g. t"e onimeter, t"e F2ens 0et dust counter, t"e reenburg9Smit" impinger, etc. "ave been in use (or many years. 3"e latter collects particles onto a plate immersed in a collecting liuid. >istorically, impingers "ave (irst been used in evaluating mineral dusts related to pneumoconiosis. Cascade impactors are based on a step2ise increase in classi(ier air velocity, by sending t"e same (lo2 t"roug" ori(ices step2ise reduced in size. Particles may be collected by 2a/9coated plates, and 2eig"ed be(ore and a(ter use. 3"e Anderson cascade impactor contains a series o( (our or more impinger stages o( decreasing 0et 2idt". As velocity increases t"roug" t"e decreasing 0et apertures t"e collected particles progressively are (iner.
3"e raseby9Andersen Cascade Impactor is used (or isoinetic sampling o( particulate, 2"ile using inertial separation to size segregate particulate samples (rom t"e gas stream. Eac" o( t"e successive separation stages gives a cut9point based on aerodynamic diameter o( t"e particles. 3o eep t"is cut9point constant, t"e impactor operates at a constant (lo2 rate. During sampling, t"e particles are 0et9driven to2ard a collecting sur(ace 2"ere t"ey may cling. Impaction occurs 2"en t"e particleOs inertia overcomes t"e aerodynamic drag. -y c"anging t"e velocity 6ori(ice size o( t"e 0et8, t"e size o( t"e particles collected is controlled. 3"e size o( t"e 0ets 2it"in eac" stage is constant, but (or eac" succeeding stage t"e 0ets get smaller. At eac" stage, t"e particle impacts on a desiccated, 2eig"ed @"atman glass (iber mat. 4or modeling purposes, it is convenient to e/press t"e be"avior o( an irregularly s"aped particle as i( it 2ere a sp"erical particle. 3ypically, t"e density o( a particulate sample is not no2n during (ield sampling. All calculations are t"ere(ore assuming unit particle density. 3est time (or particle size distribution is dependent on t"e actual grain loading o( t"e gas stream. 3"e 7arple 9 7iller Impactor is a cascade impactor used (or testing aerosol drug delivery devices 6metered dose in"aler and dry po2er in"aler8, and determines t"e aerosol size distribution. It is easy to operate and "as s"arp cut9point c"aracteristics and lo2 internal particle "oldup. 8.@. Pre%(,(t!t(on
Electrostatic precipitation and t"ermal precipitation are ot"er means o( producing dust samples. In t"e Casella t"ermal precipitator dust deposits in a rectangular pattern on eac" cover glass. A(ter e/amination (or uni(ormity o( deposition, t"e particulate is counted using a microscope. ?. P!rt(%)e E#!)u!t(on ?.". O,t(%!) M(%ros%o,(% E;!(n!t(on
Particles smaller t"an 1** 5m can advantageously be e/amined using an optical microscope. 3"e t"eoretical resolution in a lig"t microscope is ca. *.# 5m. Prior to t"is e/amination t"e particles can be dispersed dry on a slide, randomly, 2it"out bias or (ractionation. Alternatively, a drop o( 2ater or solvent can be placed on t"e slide, and dust added using t"e end o( a toot"pic. 3"e mi/ture is spread over t"e slide as a t"in (ilm. 3"e liuid evaporated dust dispersion resides on t"e slide. 3o (i/ t"e sample permanently Canada balsam dissolved in /ylene may be used. Quartz particles, "o2ever, "ave a re(raction inde/ very close to t"at o( balsam, and can no longer be seen. 3"ermal precipitation samples collected dry may be e/amined immediately. Samples collected on membrane (ilters are placed on slides, dust side do2n. Adding immersion oil 2ill mae t"e (ilter transparent. ?.$. O,t(%!) S(+(n/
7icroscopic images allo2 linear and area measurements. 3"e pro0ected diameter is t"e diameter o( a circle eual in area to t"e pro(ile o( t"e particle 2"en vie2ed normal to its
position o( greatest stability. Suc" areas are determined by comparison 2it" calibrated circles, using a micrometer, or 2it" a planimeter measuring pro0ected areas on a screen or a p"otograp". Particles can be evaluated using a 7artinMs or a 4eret diameter< t"e (irst is smaller, t"e second larger t"an t"e pro0ected diameter. 3"e ratio o( bot" diameters tends to be constant (or a given material. Statistical evaluation reuires e/amination o( at least ** particles. 3"e pro0ected diameter corresponds more closely to t"at derived (rom sedimentation tests.
4igure !: 7artinMs and 4eretMs diameter measuring
4igure #: Statistical distribution o( particles using 4eretMs diameter 3"e electron microscope allo2s direct observation do2n to around *.**1 5m. ?.&. Auto!t(% Cou)ter ,!rt(%)e %ounters
Coulter in 1)$% (irst developed a "ig"9speed met"od (or blood counts t"at 2as later applied to accurate particle counting 2it"in a given size range. Particles suspended in a slig"tly conducting liuid move (rom one c"amber into anot"er, t"roug" a calibrated ori(ice. Eac" time a particle passes t"roug" t"e ori(ice a c"ange in resistance is recorded, permitting to count t"e particles passing. ;arger particles produce a deeper dip in conductivity and tae longer to pass t"e ori(ice. Fbviously, t"e ori(ice s"ould be adapted to t"e particle range. 3"e instrument is calibrated using mono9disperse particles. ?.1. P'ot(% sens(n/
Ft"er use(ul met"ods to detect particles are p"otic sensing, involving t"e (or2ard and side2ays scattering o( lig"t by particles, or t"eir s"ado2ing. A lig"t source sends an intense beam o( lig"t in t"e sensing zone 2"ere aerosol particles are introduced. Instruments e/ist (or bot" gas and liuid p"ase measurement. 3"e lig"t scattered at an angle is piced up by a p"otomultiplier, resulting in pulses being a measure o( concentration. Size is related to t"e amplitude o( t"e pulses. 7inimum particle sizes (or detection by suc" met"ods are *. *.# 5m in t"e gas p"ase, a (e2 5m in liuids. 7a/imum values vary bet2een 1* and &* 5m in t"e gas p"ase.
Advantages o( suc" instruments are: particles are counted as t"ey occur in air and no sampling or sample preparation are reuired< c"anges are detected almost instantaneously, 2"ic" is important in process control< very lo2 concentrations can be detected and monitored. ▪
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A ma0or di((iculty is relating t"e signal to actual concentration numerical (igures, since t"e relations"ip bet2een signal and geometry, optical systems, and particle or aerosol c"aracteristics 6size, s"ape, re(ractive inde/ and absorption coe((icient8 are almost impossible to establis". sing uniue inco"erent lig"t scattering t"eory, a devise called RCrystalsizer is able to measure dry po2ders (rom *.& to $** microns. A ne2, measuring set9up in t"e RCamsizer 2it" t2o digital cameras as an adaptive measuring system, improves and re(ines t"e principle o( digital image processing. Its t"eoretical measurement limits are 1$ 5m and )* mm. ?.3. L!ser Aeroso) P!rt(%)e S(+e S,e%troeter
;aser di((raction at present is t"e most 2idely used tec"niue (or particle size analysis over a broad size range and in a variety o( dispersion media. A representative cloud o( particles passes t"roug" a broadened beam o( laser lig"t, 2"ic" scatters t"e incident lig"t onto a 4ourier lens. 3"is lens (ocuses t"e scattered lig"t onto a detector array and, using an inversion algorit"m, a particle size distribution is in(erred (rom t"e collected di((racted lig"t data. Sizing particles using t"is tec"niue depends upon accurate, reproducible, "ig"9resolution lig"t scatter measurements to ensure (ull c"aracterization o( t"e sample. 3"e laser beam is broadened by t"e use o( a spatial (ilter in order t"at it may interact 2it" a large number o( particles in t"e sample cell. It needs to be aligned relative to t"e detector array. 7ain areas o( concern are: Inter9instrument reproducibility, determined by "o2 accurately t"e laser can be aligned, 2"ereas measurement reproducibility on one instrument is primarily determined by "o2 consistently t"e alignment can be maintained. I( t"e laser is not accurately aligned, t"e lig"t scatter pattern 2ill be observed at t"e incorrect angle, and "ence systematic errors 2ill creep into every measurement. ▪
;imited +esolution 6missing s"oulders, tails and sub9populations in particle size distributions8.T ▪
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Submicrometer c"aracterization
7.5.1. Resolution 3"e resolution o( a laser di((raction particle size analyzer can be de(ined as t"e ability o( a system to di((erentiate bet2een similarly sized particles and t"ere(ore measure t"e correct s"ape and true 2idt" o( t"e particle size distribution. +esolution is primarily determined by "o2 2ell t"e angular scattering pattern is measured. Fptimal size distribution results are ac"ieved by covering a 2ide angular range 2it" a su((icient number o( detectors to ensure t"at important in(ormation (rom t"e scattered lig"t is not lost or overlooed.
7.5.2. Particle Size Distribution ;ig"t, scattered by particles, (orms a series o( concentric rings o( alternating ma/imum and minimum intensities o(ten called t"e Airy dis. 3"e (irst minimum 6closest to t"e center o( t"e Airy dis8 provides in(ormation to determine t"e mean size o( t"e distribution. Subseuent ma/ima and minima contain in(ormation on t"e s"ape and 2idt" o( t"e distribution, including any s"oulders and tails. It is t"is series o( ma/ima and minima t"at needs to be accurately measured in order to report t"e true s"ape o( t"e particle size distribution.
7.5.3. Caracterisin! Sub"icro"eter Particles C"aracterisation o( submicrometer particles poses additional problems. Particles scatter lig"t at an angle t"at is inversely proportional to t"eir size< t"ere(ore, as t"e size o( t"e particles decreases t"e scattering angle increases. 7ost laser di((raction instruments are only able to measure t"e angular intensity pattern up to a ma/imum angle o( about #$ degrees, limited by bot" t"e sample cell design and t"e 4ourier lens size and uality. n(ortunately, submicrometer particles scatter most intensely at "ig"er angles t"an t"is, so not all t"e scattered lig"t is collected. 7oreover, t"ere are very small di((erences in t"e laser di((raction scatter patterns (rom di((erently sized submicrometer particles. 3"ere(ore, it is almost impossible to c"aracterize submicrometer particles by laser di((raction alone.
7.5.4. #ackscatter $nalysis -acscatter analysis gives a little more in(ormation, but does not overcome t"e (undamental limitation t"at t"e intensity o( scattered lig"t is virtually constant in all directions (or submicrometer particles.
7.5.5. Polarization %ntensity Di&&erential Scatterin! Polarization Intensity Di((erential Scattering 6PIDS8 uses t"ree di((erent 2avelengt"s o( lig"t 6#$*, %** and )** nm8 in t2o planes o( polarization 6vertical and "orizontal8 to irradiate t"e sample and give "ig" submicrometer resolution. 3"e PIDS detectors are placed at angles o( up to 1$* degrees to collect t"e "ig" angle scatter data. 7odem laser di((raction instruments use 7ie 3"eory as t"e basis o( t"eir size calculations. As 7ie 3"eory covers all lig"t scatter (rom sp"erical particles bot" PIDS data and laser di((raction data can be processed into a particle size distribution using one continuous algorit"m.
7.5.'. %S( 1332)*1 +aser Di&&raction Meto,s 3"e ISF Standard 1!!*91 6Particle Size Analysis ;aser Di((raction 7et"ods Part I: eneral Principles8 establis"ed ne2 guidelines (or t"e understanding and interpretation o( laser di((raction analysis. 3"e (ollo2ing (actors are important to resolution and sensitivity: ▪
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umber, position, and geometry o( t"e detector elements< Signal9to9noise ratio< 4ine structure in t"e measured scattering pattern< Di((erence in scattering pattern bet2een size classes< Actual size range o( t"e particulate material< Adeuacy o( t"e optical model< Smoot"ing applied in t"e deconvolution procedure.
3"e particles o( interest are irradiated by electromagnetic radiation o( a speci(ic 2avelengt", eac" particle acting as a scattering center. 3"e resulting (lu/ pattern, i.e. lig"t intensity as a (unction o( angle, is t"en analyzed mat"ematically to determine t"e size distribution. 7easuring lig"t intensity as a (unction o( angle is crucial to t"e success(ul interpretation o( t"e particle size distribution. early all laser di((raction instruments use silicon9based p"otodiodes in t"eir detector arrays. 3"e number, position, and spacing o( t"ese elements are crucial in c"aracterizing t"e scattering pattern. 3"e more detectors t"e array "as, t"e greater t"e resolution o( t"e scattering pattern. Indeed, t"e (irst9dra(t copy o( t"e ISF standard stated t"is clearly. 3"e position and geometry o( t"e detector elements is also o( vital importance to ensure adeuate c"aracterization o( t"e (lu/ pattern. Commercial models cover an ultra92ide measurement range, (rom *.* to *** microns (or a variety o( laboratory use, suc" as p"armaceuticals, ceramics, batteries, abrasives, emulsions, resins etc. 3"e main applications are in: ▪
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7easuring particle size distributions 6spray, dust, po2der8< 4ilter testing and c"aracterization< Estimation o( (ractional e((iciency< C"aracterization o( p"armaceutical aerosols< Aerosol researc".
?.8. Dro,)et An!)ys(s
Uears ago, t"e accepted tec"niue 2as to collect droplets on glass slides, but t"e process 2as slo2 and droplet splatter raised doubts about accuracy. 3oday, advanced tec"niues are based on laser lig"t signals. Suc" instruments use a signal9processing algorit"m supposing t"at t"e signal comes (rom a sp"erical particle. Any signal, 2"ic" deviates (rom t"is (orm, is re0ected. 3"is "as signi(icant limitations since it does not cope 2ell 2it" large or non9sp"erical droplets (ound in industrial environments. A direct video imaging system pioneered by -ete o((ers many signi(icant advantages, since imaging systems also report in(ormation about t"e droplet morp"ology. ?.?. TEOM
3"e 3EF7 6tapered element oscillating microbalance8 measures t"e ambient particulate mass concentration o( P71*, P7.$, P71 and 3SP 6total suspended particulate8 collected on a (ilter in real time. 7ass concentration data are reported in micrograms per cubic meter at standard averaging times. E/posed collection (ilters can be analyzed (or "eavy metals using standard laboratory tec"niues, suc" as AA and ICP. 3"is patented monitor is used in ambient air monitoring net2ors, indoor air uality assessment, and e/posure studies. 3"e instrument "as t"e S EPA euivalency designation (or P7 1*. 3"e erman EPA also approves t"e monitor as an euivalent continuous monitoring tec"niue (or 3SP 6total suspended particulate matter8 and P71*. 3"e instrument is suitable (or a range o( ot"er applications reuiring time9resolved, mass measurement o( t"e suspended particulate matter concentration.
@. E)uent Dust An!)y+ers @.". S%o,e
Dust is an important parameter in numerous industrial emissions, e.g. pulverized coal (iring, cliner production in cement ilns, metallurgical processes, or in p"ysical operations, e.g. drying, pneumatic conveying, pulverizing. +a2 o((9gas typically contains one to 1** g m 9! o( dust and clean gas bet2een one to 1** mg m 9!. Immission values, depending on natural sources and ant"ropogenic pollution, are e/pressed in 5g m 9!. Some industrial spaces must be (ree o( dust, e.g. clean rooms in electronics or p"armaceutical industry. Dust in air or gases is monitored using: ▪
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Dust 4ilters, gravimetrically or by V9radiation attenuation, as opacity, 2it" laser beam, transmitted9 or scattered9lig"t, 3ribo9electrical sensors.
Dust particles may be collected (rom an air or gas stream upon, e.g. silica or cellulose (ilters and a(ter "aving treated a certain volume o( gas, t"e (ilters are eit"er dried and 2eig"ed 6gravimetric met"od8, or e/amined under an optical or scanning electron microscope. 3"e latter not only yields a picture o( t"e dust, but possibly also 9 its elementary c"emical composition 6e/cept (or t"e lig"ter elements8 using appropriate tec"niues 6e.g. L+4 or E+D8. 3"e latter may be produced (or a certain area o( t"e sample, or a speci(ic (eature o( its sur(ace< moreover t"e particle interior can also be scanned, a(ter sputtering a2ay successive sur(ace layers. 3"is allo2s establis"ing a composition gradient, starting (rom t"e sur(ace and entering deeper and deeper into t"e sample. In environmental samples t"e sur(aces is o(ten enric"ed in pollutants, and it is important to no2 2"et"er an analytical met"od addresses eit"er t"e sur(ace or t"e bul o( t"e particle. Anot"er (reuently occurring problem is evaluating soot (ormation in combustors. 3"e Wtrue9 spotM smoe tester 2as developed (or t"at purpose. 3"e operating principle is simple: using a "and pump a given amount o( gas is aspired t"roug" a (ilter, 2"ic" in t"e process turns to various grades o( gray or blac. 3"e latter are visually compared to a scale, e.g. t"at o( +ingelmann, or o( -acc"arac", but t"e result is not al2ays accurate. @.$. Isok(net(% s!,)(n/
@"en collecting dust particles (rom a gas stream upon a (ilter only a small stream o( gas is selected (or establis"ing t"e concentration o( dust. >o2ever, t"e latter may be aspired so strongly t"at t"e gas stream to be analyzed becomes enric"ed in t"e particles, compared to t"e bul o( t"e gas stream. Conversely, a "ig" resistance (ilter 2it" 2ea aspiration o( t"e gas may recover (e2er particles t"an normally available. In bot" cases, t"e sample gas lacs representativeness. reat care s"ould be taen t"at t"e particles are aspired at t"eir WnormalM speed, so t"at t"ey are neit"er enric"ed nor reduced in number. 3"is is t"e principle o( (sok(net(% s!,)(n/. Isoinetic sampling is only possible in a suitably long, straig"t duct 6e.g. a stac8, since duct curvature as 2ell as local turbulence bot" lead to uneven and unpredictable particle distributions. 7oreover, dust sampling occurs according to a predetermined grid o( sampling locations t"at is representative (or t"e entire cross9section o( t"e duct.
3"is can be per(ormed according to several international standards, e.g. ISF, ?DI, EPA, AS37.
4igure $: A dust sampling system F((9stac analyzers only measure t"e dust load at a speci(ic sampling point, needing an isoinetic and labor9intensive 2ay o( dust sampling, resulting in e/tra di((iculties (or industrial plant "aving unstable dust emissions. 7etallurgical plant, (or e/ample, o(ten operates batc"2ise and 2it" "uge di((erences in dust load, depending on t"e part o( t"e process cycle 6loading, "eating, melting, re(ining, disc"arging8 considered. @.&. :R!d(!t(on Attenu!t(on Sensors
3"e ra2 gas is taen t"roug" a (ilter, e.g. during ten minutes. 3o prevent condensation, it is sometimes diluted 2it" puri(ied air be(ore entering t"e (ilter. 3"e amount o( collected dust is determined by evaluating t"e attenuation o( V9radiation: t"e loaded (ilter is measured against a blan (ilter. 3"e captured radiation intensity is measured using a eiger97Xller counter.
3"e t"icness and composition o( t"e collected dust layer and t"e used (ilter type all in(luence upon t"e absorption o( radiation. In a given plant, t"e absorbing capacity o( t"e collected dust does not vary very muc"< "ence, t"e resulting reading is a direct (unction o( t"e t"icness o( t"e collected dust layer. A(ter t"e measurement t"e (ilter9tape runs idle (or a 2"ile until a ne2 measuring cycle starts, (iltering t"e gases t"roug" anot"er part o( t"e (ilter9tape. @eig"ing t"e (ilter, a(ter cutting out its dust9loaded part, allo2s calibration. -y 2eig"ing blan (ilter pieces, "aving t"e same sur(ace, t"e real dust load can t"en be calculated. @.1. F)ue /!s o,!%(ty on(tors
4lue gas opacity is commonly monitored in9stac, as a measure o( dust concentration in o((9 gas (lo2s. 3"e system uses eit"er transmitted or scattered lig"t. Fpacity measurements in commercial analyzers are conveniently e/pressed WeuivalentM to a certain dust concentration, in mg m9!. An obvious advantage o( t"ese monitors t"e use o( a representative, (ull stac traverse as a measuring zone. 3"ere is no direct contact bet2een t"e measuring device and t"e (lue gas and "ence less need (or cleaning. >o2ever, a WstandardM dust uality must be assumed (or calibration, since t"e reduction in lig"t transmission or its scattering also depends on t"e nature o( t"e particles 6size, brig"t or dar color, absorptive and re(lective properties8 in t"e gas. 3"us t"e response o( 2"ite as" or dar soot particles di((ers maredly. A disadvantage o( opacimeters is "ence t"eir need (or e/ternal calibration using comparative measurements. Eac" c"ange in dust c"aracteristics reuires a ne2 calibration o( t"e instrument, eac" calibration needing at least ten comparative measurements, pre(erably at maredly di((erent levels o( dust concentration.
-.4.1. rans"issio"etry @"en a beam o( lig"t passes t"roug" a gas its intensity is 2eaened by t"e particles, as a result o( bot" attenuation and scattering. 3"e more particles present and t"e longer t"eir traveling distance in t"e lig"t beam, t"e greater 2ill be t"e attenuation. 3"e particle content can be calculated by comparing t"e intensity o( t"e initial and t"e emerging lig"t beams. 3"is tec"niue is suitable (or medium and "ig" dust concentrations, and can even be used in ducts 2it" very large diameters.
4igure %: 7easurement o( ;ig"t 3ransmission 3"e transmittance ) , t"e ratio I K I * o( t"e transmitted intensity I to t"e original intensity I * o( t"e lig"t source, depend on nature and size o( t"e particles, t"e dust concentration and t"e instrumental pat" lengt". 3"e relation can be e/pressed by t"e -eer9 ;ambert euation: Transmittance
) e
618
a/c/l
@"ere a size and nature o( particles c dust concentration l pat" lengt" Opacity 1
)
Care(ul calibration, using comparative gravimetric measurements, is necessary (or uanti(ication o( t"e dust concentration or emission rate.
-.4.2. Scattere,*li!t (/aci"etry A lig"t transmitter emits a beam o( visible or in(rared lig"t, scattered by t"e particles in t"e gas (lo2. 3"e scattered lig"t is detected and uanti(ied by a "ig"ly sensitive optical detector. 3"e strengt" o( t"e emitted beam and t"e angle o( t"e reception (ield de(ine t"e volume reuired in t"e gas duct (or ac"ieving an accurate measurement.
Due to its "ig" sensitivity, t"is measurement principle is suitable (or lo2 concentrations, even (or large duct diameters.
4igure &: 7easurement o( Scattered ;ig"t
-.4.3. +aser Dust Monitors 3"e a(orementioned tec"niue can also be applied using laser beams. ;aser bac9scatter systems are used in cross9stac continuous measurements, even (or stacs up to * m diameter.
-.4.4. riboelectric sensors A rat"er ne2 tec"niue (or indicating particle concentrations in a gas (lo2 is based upon triboelectric e((ects. 3"ese occur 2"en dust particles in t"e gas stream impact upon t"e sensor, causing t"e trans(er o( a small electrical c"arge (rom t"e particles to t"e conducting sensor sur(ace. 3"e c"arge is measured a(ter suitable multiplication. 3"e signal depends on t"e number o( particles "itting t"e sensor and also on t"e properties and composition o( t"e dust. Important advantages o( t"e system are simplicity, compactness and lo2 cost, including installation e/penditure and maintenance reuirements. 3"is tec"niue is most commonly used (or controlling t"e e((ectiveness o( (ilter devices, e.g. (or veri(ying t"at bag "ouse sleeves are (ree (rom pin"oles. 3"ey are less suitable (or measuring dust emissions, because too many di((erent (actors a((ect t"e results. o prior calibration is reuired (or controlling pu((s o( dust, escaping (rom a (ilter. Damaged (ilter elements can easily be detected, merely by relating t"e occurrence o( dust signals 2it"
t"e cleaning cycle o( speci(ic ro2s o( sleeves. 3"is simpli(ies maintenance, allo2s avoiding product losses and preventing a visible e/"aust gas trail (rom t"e stac. . Dust S!,)(n/ Pro%edure .". Met'od
A representative sampling o( particles under real industrial conditions is very di((icult because o( erratic variations in dust load and composition, settling and inertial separation o( solid particles, gyrating (lo2, (ollo2ing bends in industrial ducts, and a "eterogeneous distribution o( dust in t"e cross9section at sampling level. 3o minimize t"ese problems, t"e sampling procedure "as to satis(y t"ree basic rules: ▪
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3"e c"oice o( an appropriate measuring location, 3"e structuring o( t"e section, Isoinetic sampling.
?ertical (lo2 is pre(erred over "orizontal, avoiding problems o( dust sedimentation. 3"e measuring section must be (ree o( s2irling (lo2 and local turbulence. 3"e measuring section must be located at t2o t"ird o( t"e total lengt" o( a straig"t duct, counted (rom upstream. 3"e duct s"ape and cross9section must be constant and its lengt" at least si/ times t"e "ydraulic diameter D", de(ined as
61#8 3"e measurement section is subdivided in segments 2it" identical sur(ace. 3"e center o( gravity o( eac" o( t"ese partial sur(aces constituting t"e cross9section is sampled (or at least ten minutes, t"e same time in eac" measurement point. 3"e reuired number o( measurement areas varies 2it" t"e diameter o( t"e duct. 4or a circular duct 2it" a diameter o( !.$* m, t"is amounts to $ points organized according to t"ree a/es. -ecause o( t"e limited lengt" o( most sampling probes it is impossible to cover t"e 2"ole diameter in once. 7easuring t"e speed o( t"e gas is important in per(orming isoinetic sampling, be(ore starting t"e actual sampling. 3"e local gas velocity varies 2it" temperature 6indicated by a t"ermocouple8, t"e di((erential pressure in t"e duct 6measured by a Pitot9tube at t"e level o( t"e measurement section8 and t"e volumetric mass o( t"e gas, varies 2it" t"e possibly unno2n composition o( t"e gas. .$. D!t! !%2u(s(t(on !nd redu%t(on
A series o( p"ysical or c"emical parameters must be measured to determine t"e pollutantsM concentration in t"e (lue gas. Some data is directly available by on9line analyzers 6e.g. concentrations8 and ot"ers must be computed 6e.g. (lo2 rates8. A mobile laboratory is euipped 2it" a data acuisition system recording all necessary parameters. 3"e (ollo2ing values are available: ▪
Average over one minute,
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7inimum value, 7a/imum value.
Astronomical numbers o( numeric data are generated during a measurement campaign. Care s"ould be taen t"at instruments are regularly calibrated and c"eced. A study o( time trends or statistical correlations bet2een parameters learns a lot on a particular plant operation. .&. Isok(net(% S!,)(n/ Tr!(n
3"e isoinetic sampling train is composed (rom: A "eated sampling probe, adapted to t"e stac dimensions, 2it" an aperture, adapted to t"e gas speed in t"e duct and t"ere(ore to t"e c"aracteristics o( t"e sampling pump. ▪
A (ilter, "eated to above 1**J C, to avoid condensation o( 2ater in t"e euipment and retaining a representative sample o( t"e dust. 3"e (ilter is 2eig"ed and t"e additional dust 2eig"t is reported to t"e gas volume aspired by t"e sampling pump. A(ter 2eig"ing t"e dust sample is available (or (urt"er analysis, e.g. by Scanning Electron 7icroscopeKElectron +ay Di((raction. ▪
a condenser system 2it" a circulating re(rigerant (ollo2s t"e dust (ilter. 3emperature and pressure are measured to correct (or residual 2ater vapors in t"e e/"aust gas. ▪
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A membrane pump.
An ori(ice dry gas (lo2 meter. 3emperature and pressure are measured to correct actual readings to standard conditions. ▪
4igure ': Isoinetic sampling o( particles ". Con%)us(ons
A survey is given o( met"ods to de(ine and c"aracterize particle dimension and properties, o( source apportionment, dust sampling and analysis. Re)!ted C'!,ters
Clic >ere 3o ?ie2 3"e +elated C"apters )oss!ry Aeroso) -a (airly stable dispersion o( tiny solid or liuid particles in a gas. A(tken -atmosp"eric particles smaller t"an *.1 5m in diameter. ,!rt(%)es Ant'ro,o/en(% - particles originating (rom "uman activities. ,!rt(%)es B!%%'!r!%' -c(. +ingelmann scale.
s%!)e C!s%!de - particulate classi(ication device, based on a step2ise increase in classi( ier air (,!%tors velocity, by sending t"e same (lo2 t"roug" ori(ices step2ise reduced in size. E)utr(!t(on -entrainment o( (ine particles in a rising current o( (luid. In'!)!*)e - particles t"at penetrate into t"e respiratory system, o(ten e/pressed as P71* ,!rt(%)es Isok(net(% -representative sampling o( particulate by aspiring a (lo2 at an undisturbed s!,)(n/ local gas (lo2 rate. PM"9 PM$.39 - particulate smaller in size t"an 1*, or .$ 5m. Pr(!ry -natural or ant"ropogenic particles introduced into t"e air. ,!rt(%)es P)ur(od!) -6e.g. particle size8 distribution 2it" several ma/imum values. d(str(*ut(on R(n/e)!nn -various grades o( gray or blac o( a (ilter, e/posed to soot and as" (rom (lue gases. s%!)e Se%ond!ry - particles (ormed in t"e air by gas9to9particle conversion o( precursors. ,!rt(%)es Sour%e -identi(ication o( t"e potential origins o( atmosp"eric particulate. !,,ort(onent Stokes or - particle diameter (or a sp"ere 2it" t"e same terminal (alling velocity as a sed(ent!t(on particle in a sedimentation medium, i.e. a gas or a liuid. d(!eter Tot!) -tiny airborne particles or aerosols 2it" diameters less t"an 1** microns. Sus,ended P!rt(%)es 4TSP6 Tot!) -mass o( all particles con(ounded per unit o( gas volume. sus,ended !tter 4TSP6 Tr(*oe)e%tr(% -sensor sensitized by t"e impact o( particles. sensors B(*)(o/r!,'y
1 -aron, P.A. and @illee, H. 61))8 Aerosol 7easurement: Principles, 3ec"niues and Applications, e2 Uor, Yo"n @iley Z Sons, IS-: *#&1'#*%' 3reatise on various aspects o( aerosols, (ocusing on t"eir c"aracterization. Cac"ier >. 61))'8, RCarbonaceous Combustion Aerosols, tmospheric (articles, Ed. +.7. >arrison and +. ?an rieen, C"apter ), pp. )$9!#', Yo"n @iley Z Sons ;td., IS- * #&1 )$)!$ ) -oo, 2it" c"apter on t"e title topic,and (iguring in an e/cellent compilation o( te/ts on Atmosp"eric Particles, as supplied by a variety o( aut"ors. ! Cooper Y. 61))'8. Particle Size Analysis 3"e ;aser Di((raction 3ec"niue. #aterials 0orld , 8 618, $9& Paper on a ma0or met"od o( Particle Size Analysis. # "ttp:KK222.cleanair.comKServicesKAnalyticalKServ[PricesKba"co[size."tml Particle sizing 2it" a -a"co Centri(ugal Classi(ier, including a determination o( resistivity and particle size analysis. $ "ttp:KK222.2"o.intKpe"K uidelines (or concentration and e/posure9response measurement o( (ine and ultra (ine particulate matter (or use in epidemiological studies. Ed. Sc"2ela, D. et al. Publis"ed in ** by 3"e @orld >ealt" Frganization 6@>F8 on be"al( o( 3"e European Commission Interesting treatise on various aspects o( aerosols, including t"eir "ealt" e((ects.
