Partitioning Processes in the Atmosphere. Terry F. Bidleman

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1 Partitioning Processes in the Atmosphere Terry F. Bidleman Centre for Atmospheric Research Experiments Science & Technology Branch, Environment Canada

2 Roadmap Particles in the atmosphere Semivolatile organic compounds (SVOCs) Models of particle/gas distribution Sampling methods Comparison of measurements and models

3 Air-Surface Exchange Processes

4 Particle-Gas Interactions free vapour vapour-surface adsorption absorption liquid-surface adsorption non- exchangeable material liquid-like like layer aerosol core

5 Particles in the Atmosphere Whitby, K., Sverdrup, G., Adv. Environ. Sci. Technol. 10, 477 (1980)

6 Distributions of Aerosol Properties number of particles surface area volume or mass Whitby, K., Sverdrup, G., Adv. Environ. Sci. Technol. 10, 477 (1980)

7 Semivolatile Organic Compounds (SVOCs) Vapour pressures roughly between Pa Many classes of compounds Key physicochemical properties Liquid-phase vapour pressure, p o L, Pa Liquid-phase water solubility, S o L, mol/m3 K AW = C air /C water K OW = C octanol /C water K OA = C octanol /C air

8 Semivolatile Organic Compounds (SVOCs) gas phase K AW K OA aqueous dissolved phase K OW organic dissolved phase Key relationships: Henry s law constant (H, Pa m 3 /mol) = p o L /So L K AW = C air /C water = H/RT, where R = 8.31 Pa m 3 /mol K K OA = K OW /K AW

9 Some Semivolatile Organic Compounds (SVOCs) polycyclic aromatic hydrocarbons (PAHs) polychlorinated biphenyls (PCBs) Cl Cl Cl Cl Cl pyrene polychlorinated naphthalenes (PCNs) Cl Cl Cl Cl 2,2,3,3,4,5 -hexachlorobiphenyl organochlorine pesticides Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl 1,3,4,7,8-pentachloronaphthalene trans-chlordane

10 More Semivolatile Organic Compounds (SVOCs) polychlorinated dibenzo-p-dioxins (PCDDs) polychlorinated dibenzofurans (PCDFs) Cl O Cl Cl O Cl Cl O Cl Cl Cl 2,3,7,8-TCDD 2,3,7,8-TCDF polybrominated diphenyl ethers (PBDEs) modern pesticides Br O Br Br Br 2,2,4,4 -tetrabromodiphenyl ether chlorpyrifos

11 Models of Particle-Gas Interactions Adsorption: Junge Pankow ( ) Absorption: Pankow (1994) Absorption: K OA ( ) Combined: Dachs Eisenreich (2000) PPLFERs (polyparameter linear free energy relationships) (~2001)

12 Junge_Pankow: Langmuir Adsorption Fixed number of sites on the adsorbent that can hold a molecule. Low partial pressures: number of occupied sites proportional to partial pressure, expressed as a fraction of saturation pressure, p/p o ). High partial pressures: all sites covered, maximum adsorption. C ads adsorption of POPs p/p o

13 Junge-Pankow Adsorption Model φ = cθ p o L + cθ Assumes linear Langmuir adsorption φ = fraction of total air concentration on particles Pankow, J.F. Atmos. Environ. 21, (1987)

14 Junge-Pankow Adsorption Model φ = cθ p o L + cθ c is related to the energetics of adsorption

15 Junge-Pankow Adsorption Model c = Pa cm = 10 6 N S RTexp[(Q d -Q v )/RT] N S = moles of adsorption sites/cm 2 aerosol Q d = heat of desorption of the chemical from the aerosol, J/mol. Q v = heat of vapourisation of the chemical, J/mol R = 8.31 Pa m 3 /mol K = J/mol K 10 6 = cm 3 /m 3 c is usually assumed to be a constant = 17.2 Pa cm, but it is really not constant! It should vary with the chemical (through Q d and Q v ), but there are few measurements of Q d for verification of this.

16 Junge-Pankow Adsorption Model φ = cθ p o L + cθ θ is related to surface area of the aerosol

17 Junge-Pankow Adsorption Model θ = cm 2 aerosol/cm 3 air Air type θ TSP, μg/m 3 clean continental background Average background background + local sources 0.42 x x x urban 11 x Whitby, K., Atmos. Environ. 12, (1978); Bidleman, T.F., Environ. Sci. Technol. 22, (1988).

18 Junge-Pankow Adsorption Model φ = cθ p o L + cθ p o L is the saturation liquid-phase vapour pressure of the chemical

19 Solid- and liquid-phase vapour pressures Ln p o L = ΔH vap /RT + b Ln p o L p o L melting pt. hypothetical extension below melting pt. p o S Ln p o S = ΔH sub /RT + b lower vapour pressure due to crystal lattice energy 1/T(K)

20 Solid- and liquid-phase vapour pressures When sorbed to aerosols, soils, etc., molecules are isolated from each other. No crystal lattice energy present. p o L is a better descriptor than po S.

21 Estimating liquid-phase vapour pressures below the melting point p o L for nonpolar chemicals can be measured by gas chromatography. p o L for any chemical can be calculated from: Ln p o L /po S = (ΔS fus /R)(T m T)/T Δs fus is the entropy of fusion, calculated from ΔH fus /T m Measured for some chemicals. For others, can be estimated from: ΔS fus = 56.5 J/mol K

22 Estimating liquid-phase vapour pressures below the melting point Ln p o L /po S = (ΔS fus /R)(T m T)/T What is p o L at 293 K for pyrene, for which p o S = 2.9 x 10-4 Pa and T m = 156 o C (429 K)? ΔS fus is known for pyrene: 54.8 J/mol K. Ln p o L /po S = (54.8/8.31)( )/293 = 3.15 p o L /po S = 21.3 p o L = 21.3(2.9 x 10-4 ) = 6.1 x 10-3 Pa pyrene

23 Finally, we are ready for a J-P calculation! What is φ for pyrene in urban air at 20 o C? p o L = 6.1 x 10-3 Pa (from previous figure) θ = 1.1 x 10-5 c = 17.2 Pa cm φ = cθ/(p L + cθ) = 17.2(1.1 x 10-5 )/[6.1 x (1.1 x 10-5 )] = % on particles, 97% in vapour phase

24 How does this prediction change with temperature? What is φ for pyrene in urban air at -10 o C (263 K)? Ln p o L = ΔH vap /RT + b For pyrene, ΔH vap = J/mol and b = p o L = 2.1 x 10-4 Pa (from above) θ = 1.1 x 10-5 c = 17.2 Pa cm φ = cθ/(p L + cθ) = 17.2(1.1 x 10-5 )/[2.1 x (1.1 x 10-5 )] = % on particles, 53% in vapour phase

25 Influence of volatility and aerosol surface area on φ 1.0 Fraction on particles (φ) average background air urban air Ln p o L (Pa)

26 Particle-gas partition coefficient, K p K p = C p /C g = m 3 /μg C p = concentration on particles, mass/μg C g = concentration in gas phase, mass/m 3 air In some literature, C p = particulate concentration, mass/m 3 air C g = concentration in gas phase, mass/m 3 air In the latter case, K p = (C p /TSP)/C g where TSP = μg/m 3

27 Relationship between φ and K p φ = K p (TSP) K p (TSP) + 1 φ = cθ p o L + cθ Log K p = log cθ/tsp log p o L

28 Particulate Carbonaceous Material 1. Primary organic aerosol (POA): e.g., plant waxes, soil organic matter, combustion products 2. Secondary organic aerosol (SOA): Particulate organic matter produced by oxidation and subsequent coagulation of vapour-phase compounds 3. Elemental carbon ( black carbon), e.g., from soot

29 Absorption Models SVOCs absorb into organic film on particles Raoult s Law: p L,i = p o L,i X i γ ι X i = mole fraction in solution, γ i = activity coefficient γ i 1 as X i 1

30 Pankow Absorption Model 1 K p = m 3 /μg = 10-6 RTf om /M om γ om p o L f om = fraction of organic matter in the aerosol (typically ). M om = molecular weight of the organic matter (g/mol) = g/μg Getting γ om is a problem, though some estimates have been made for natural om 2 and octanol Pankow, J.F. Atmos. Environ. 21, (1994); 2. Jang, M. et al., Environ. Sci. Technol. 31, (1997) 3. Xiao, H., Wania, F., Atmos. Environ. 37, (2003)

31 Octanol-Air Partition Coefficient Model 1-3 K p = m 3 /μg = (10-9 /ρ oct )[K OA f om (M oct /M om )(γ oct /γ om )] M oct /M om : ratio of molecular weights, octanol/organic matter. γ oct /γ om : ratio of activity coefficients in octanol/organic matter. ρ oct = 820 kg/m = kg/μg 1. Finizio, A. et al., Atmos. Environ. 31, (1997) 2. Harner, T., Bidleman, T.F., Environ. Sci. Technol. 32, (1998) 3. Pankow, J.F., Atmos. Environ (1998)

32 Octanol-Air Partition Coefficient Model 1-3 Assume: M oct /M om and γ oct /γ om are each = 1 K p = (10-9 /ρ oct )K OA f om ρ oct = 820 kg/m 3 Log K p = log K OA + log f om Finizio, A. et al., Atmos. Environ. 31, (1997) 2. Harner, T., Bidleman, T.F., Environ. Sci. Technol. 32, (1998) 3. Pankow, J.F., Atmos. Environ (1998)

33 Octanol-Air Partition Coefficient Model 1-3 Assume: M oct /M om and γ oct /γ om are each = 1 How good are these simplifying assumptions? Modelling 1 suggest that γ oct /γ om is ~2. Since solubility is 1/γ, this implies greater affinity for om than octanol. Average M om likely greater than M oct (130 g/mol) 2 1. Chandramouli, B. et al., Atmos. Environ. 37, (2003). 2. Lohmann, R., Lammel, G., Environ. Sci. Technol. 38, (2004).

34 Dachs-Eisenreich Sorption Model 1 Considers partitioning to two aerosol components: absorption into organic matter (om) adsorption to aerosol elemental carbon (EC) K p = m 3 /μg = [(f om /ρ oct )K OA + (f EC K SA )A EC /A SC ] f om and f EC are fractions of organic matter and elemental carbon. ρ oct = density of octanol, 0.82 kg/l comes from: 10-9 kg/μg x 10-3 m 3 /L 1. Dachs, J., Eisenreich, S.J., Environ. Sci. Technol. 34, (2000)

35 Dachs-Eisenreich Sorption Model 1 K p = m 3 /μg = [(f om /ρ oct )K OA + (f EC K SA )A EC /A SC ] A EC and A SC are specific surface areas (m 2 /g) of the aerosol elemental carbon and the soot carbon used to measure K SA. The D-E model assumes this ratio = 1. The model further assumes unity for ratios of molecular weights and activity coefficients (as in the K OA model). 1. Dachs, J., Eisenreich, S.J., Environ. Sci. Technol. 34, (2000)

36 Scanning electron microscopy of soot particles: Do you think specific surface areas are the same? Jonker, M., Koelmans, A., Environ. Sci. Technol. 36, (2002)

37 Where to get K SA? Presently, no measured values! Estimated from measured soot-water constants Log K SW for pyrene, L/g Soot type W-L J-K act. carbon 7.7 traffic 6.8 oil 6.0 wood 6.1 coal 5.7 diesel 6.4 ~10-fold variability among soots! K SA = K SW /K AW where K AW = H/RT Walters, R., Luthy, R., Environ. Sci. Technol. 18, (1984); Jonkers, M., Koelmans, K., Environ. Sci. Technol. 36, (2002

38 Predictions from the D-E Model K p = m 3 /μg = [(f om /ρ oct )K OA + (f EC K SA )A EC /A SC ] A EC /A SC = 1 f om = 0.2, f EC = 0.05 ρ oct = 0.82 kg/l Log K OA Log* K SW Log K AW Log K SA Log K p fluoranthene PCB K p,fla = [(0.2*1.3x10 9 /0.82) + (0.05*4.3x10 9 )] K p,fla = (3.2x x10 8 ) = 5.4x10-4 K p,pcb = (5.0x x10 7 ) = 5.1x10-4 (~40% to soot) (~2% to soot) *Jonkers-Koelmans value

39 Measurements Filter Adsorbent ( Hi-Vol ) Denuder

40 High Volume ( Hi-Vol ) Air Sampler Glass- or quartz-fiber filter, collects 99.9% of particles >0.3 μm. Solid adsorbent cartridge to trap vapours, polyurethane foam or XAD resins. Hi-Vols typically sample at ~ m 3 /min. Separate analysis of the filter and cartridge yields an operational particle/gas distribution.

44 Measured Log K p vs. Physicochemical Property Plots Log K p = m Log p o L + b Log K p = m Log K OA + b Log K p Log K p Log K OA Theoretical m = 1 Log p o L Theoretical m = 1

45 Particle-Gas Distributions in Chicago (Cotham, W., Bidleman, T.F., Environ. Sci. Technol. 29, (1995) PAHs single event PCBs single event Log K p PAHs all events PCBs all events Log p o L /Pa

46 The Reality Slopes of log K p vs. log p o L or log K OA often different from 1 or 1. Why? plots are Particles and vapours are not at equilibrium Air concentrations, temperature change over the sampling period. For adsorption: excess heat of desorption (Q d -Q v ) is not constant within a compound class. For absorption: γ om varies within a compound class, γ om differs from γ oct. Sampling artifacts (e.g., blow-off losses from particles). Pankow, J., Bidleman, T.F., Atmos. Environ. 25A, (1992)

47 Comparison of Hi-Vol and J-P Modelled Phase Distributions % Particulate (100 x φ) Log p o L/Pa Log p o L/Pa Bidleman, T.F. and T. Harner (2000). Sorption of persistent organic pollutants to aerosols. In: D. Mackay and R.S. Boethling (eds.) Estimating Chemical Properties for the Environmental and Health Sciences, A Handbook of Methods. Lewis Publishers, Chelsea, Michigan,

48 Comparison of Hi-Vol and J-P Modelled Phase Distributions % Particulate (100 x φ) Log p o L /Pa

49 Field measurements and K OA model predicted K p for PAHs Lohmann, R., Lammel, G., Environ. Sci. Technol. 38, (2004) Log K p, measured FLE PHEN FLA,PY CHRY B[a]P Log K p, predicted

50 Field measurements and D-E model predicted K p for PAHs Lohmann, R., Lammel, G., Environ. Sci. Technol. 38, (2004) Log K p, measured FLE PHEN FLA,PY CHRY B[a]P Log K p, predicted

51 Absorption into environmental tobacco smoke Log K p, m 3 /μg Log p o L /Torr Liang, C., Pankow, J.F., Environ. Sci. Technol. 30, (1996)

52 Where do we stand with models and hi-vol measurements? J-P model represents measured PAH phase distributions well in urban air, less so in rural air. Greater discrepancy for organochlorines,with J-P giving higher results for both urban and rural air. K OA model works fairly well for organochlorines, but tends to underestimate PAH distributions. Improvements for PAHs can be made by a dual model, with K OA accounting for absorption and a second term which describes adsorption to soot carbon (D-E). Both models have serious errors when applied to polar compounds.

53 Is there a better approach coming down the pipe? You bet! They are called: (Purple) Polyparameter Linear Free Energy Relationships (PPLFERs) Why polyparameter? Single parameter relationships (such as to p o L and K OA ) Are OK for nonpolar chemicals. They cannot account for The multiple and varied interactions of polar compounds.

Schwarzenbach 2,3 1 Harvard School of Public Health 2 Swiss Federal Institute of Aquatic

54 Beyond K ow and Vapor Pressure Christine M. Roth 1,2,3 Kai-Uwe Goss 2,3, Christian Niederer 2,3, Hans Peter Arp 2,3, René P. Schwarzenbach 2,3 1 Harvard School of Public Health 2 Swiss Federal Institute of Aquatic Science and Technology (EAWAG) 3 Swiss Federal Institute of Technology ETH Zurich SETAC Baltimore, Nov. 15 th 2005 HARVARD SCHOOL OF PUBLIC HEALTH

55 Intermolecular Interactions van-der-waals interaction ed/ea interaction (hydrogen bond) - electron-acceptor - electron-donor surface: mineral bulk: organic particle

56 poly-parameter LFERs Adsorption: van-der Waals property surf van-der Waals property i e-donor property surf e-acceptor property i e-acceptor property surf e-donor property i logk isurf/air = vdw surf vdw i + ED surf ea i + EA surf ed i + const van-der-waals interaction electron-donor/ acceptor interactions

57 poly-parameter LFERs Adsorption from air: logk isurf/air = vdw surf vdw i + ED surf ea i + EA surf ed i + const Absorption from air: logk i bulk/air =vdw bulk vdw i +ED bulk ea i +EA bulk ed i +Coh bulk Vol i +s π+c Cavity formation Dipolarity

58 Compounds covered

59 Compounds covered vdw i vdw i + ed i vdw i + ed i + ea i

60 Adsorption on Diesel Soot -2-2 log Kdiesel soot/air (m) experimental R 2 = 0.87 nonpolar polar log Kdiesel soot/air (m) experimental n = 74, R 2 = 0.94 nonpolar polar 50% RH, 15 C log p i * L (Pa) log K diesel soot/air (m) calculated Roth et al., 2005

61 Absorption into Humic Acid 10 8 unpolar nonpolar polar Linear (unpolar) (nonpolar) 10 8 unpolar nonpolar polar n=177 R 2 =0.79 rmse= x=y n=158 R 2 =0.943 rmse= logk octanol,air [m 3 /m 3 ] K oa -model (red line: equation by Karickhoff) logk HA,air [L/kg] pp-lfer (line: 1:1-line) Niederer et al., unpublished data 2005

62 PPLFER Model of Sorption to Aerosols OCs: most are associated with organic matter. Götz et al., ES&T 2007

63 PPLFER Model of Sorption to Aerosols Polar pesticides: most are associated with mineral matter. Götz et al., ES&T 2007

64 For more about PPLFERs, see: Goss, K., Schwarzenbach, R. Environ. Sci. Technol. 35, 1-9 (2001). Nguyen, T. et al., Environ. Sci. Technol. 39, (2005). Roth, C. et al., Environ. Sci. Technol. 39, (2005). Roth, C. et al., Environ. Sci. Technol. 39, (2005). Götz, C. et al., Environ. Sci. Technol. 41, (2007).

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