Why are we interested in aerosols?

Transcription

1 Chemistry an Aerosols Bernhar Vogel Chemistry an Aerosol

2 Why are we intereste in aerosols? Aerosols have an imact on human health, BUT they have also an imact on climate an weather. by: moifying the atmosheric raiation irect effect, moifying clou formation inirect effect, an mixtures of both. AN: They are changing the chemical comosition of the atmoshere.

3 Why are we intereste in aerosols? The role of natural an anthroogenic aerosol articles for the atmoshere is not well unerstoo. This is ocumente in the ICC 007 reort for the global scale. On the regional scale the situation is even worse as shown by recently ublishe an contraictive finings on the effects of aerosol articles on raiation an clou formation e.g. weekly cycles of meteorological variables, Bäumer an Vogel, 007a,b.

4 efinitions: efinitions Aerosol: Susension of small articles liqui, soli, mixtures in gases. rimary Aerosol: articles emitte irectly into the atmoshere Seconary Aerosol: Forme by conversion from the gas hase articles iffer by: Size Surface Volume or Mass Chemical Comosition

5 Sources, Sinks an rocesses

6 Interactions SO HNO 3 VOC HONO N O 5 NH 3

7 Size Range

8 ictures Quelle: M. Koyro, AIA team

9 article Formation Mechanisms: rimary article formation results from mechanical isrution of the earth s surface. irect generation of san/ust aerosols Sea-sray aerosols resulting from bursting bubbles of entraine air. Combustion article formation Results from the incomlete combustion of fuel soot carbon etc Seconary article formation from gas-to-article conversion Homogeneous nucleation Aqueous hase chemical reaction oxiation of SO to aerosol SO 4

10 article Comosition: rimary aerosol: soil ust: iron, calcium, silica sea-sray: soium, chlorie, calcium, sulhate, otassium, etc Seconary aerosol: sulhates, nitrates, ammonium, organics, halogens, Combustion aerosol: sulhates, nitrates, elemental carbon, organic carbon, soot Sulhates are erive from oxiation of SO while nitrates are erive from oxiation of NO x Organic acis are erive from the oxiation of VOC emitte from lants an by fossil fuel burning

11 Global Emissions rimary

12 Sources of Seconary Aerosols

13 Number-, Area- an Volume- ensity istributions Nucleation N/Log r cm -3 Aitken Akkumulation Coarse Giant 0 5 Number Surface Area Col vs Col 4 Volume Raius nm in µm

14 Size an Number Number Anzahl ,0 0, 0,4 0,6 0,8,0 in µm

15 escrition of Size istributions The size istribution function n N is efine as: the number of articles er cm -3 of air having iameters in the range to therefore the total number of articles, N, is N in cm -3 N 0 n if Nn N is the number of articles between to then n N N/ in µm - cm -3 In terms of aerosol surface area: π in µm - cm -3 an Volume π 3 in µm -3 cm -3 S V n n N N Similar arguments follow for exressing the size istribution in ln or Log : N e nn ln ln n e N ln ln number of articles in the size range ln to ln ln. where the units are now cm -3.

16 roerties of Size istributions: The mean article iameter of the istribution is given by N 0 n N an the variance of the istribution is given by σ N 0 n N The Normal istribution is: where 68% of the area below the curve is in the range: The Log Normal istribution is therefore: σ g ower Law: geometric stanar eviation N u u n u ex / π σ σ u u ± σ u N ln ln nln ex / π lnσ ln σ g C n o N log u α g g where C an α are constants. α -3 corresons to a Junge sloing istribution. In the volume istribution, this gives a constant volume er unit size interval. Moifie Gamma n o N c A b ex B where A, b, B an c are all ositive arameters.

17 Size istribution fitte by three log-normal istributions moal aroach in nm Versick, 006

18 Measure size istribution fitte by iscrete sections sectional aroach in nm Versick, 006

19 Measure size istribution fitte by iscrete sections sectional aroach in nm Versick, 006

20 Single article ynamics The average istance travele by a article between a collision is calle the mean free ath. A imensionless number escribing the relative length scales is the Knusen number, Kn, Knλ BB / The mean free ath of a ure gas is: λ BB µ B 8M / πrt B / Continuum regime Kn 0 Free molecule kinetic regime Kn Transition regime Kn.

21 rag on Single articles: As a article is moving in a flui, the flui exerts a rag on the article. To calculate the rag on a article, the equations of flui motion must be solve to etermine the velocity an ressure fiels aroun the article. The normal force is obtaine by integrating the erenicular ressure comonent to the surface: F rag 3πµ u were u is the ustream velocity of the flui an µ is the flui viscosity.

22 Stokes Law an Non-Continuum Effects: A sli correction factor has to be introuce: F rag 3πµ u /C c C c λ ex λ C c is known as the Cunningham Sli correction factor iameter µm C C

23 Gravitational Settling: The equation of motion of a article of mass m is governe by balance forces on the articles: m v t i F i where v is the velocity of the article an F I is the i-th force on the article. For a falling article, in a flui, there are two forces: the gravitational force m g an the rag force F rag.. Therefore, for Re<0., the equation of motion becomes: m v t m 3πµ g C u v where the secon term is the correct Stokes Law for a article moving with velocity v in a flui having velocity u. v The equation can also be written as τ τg u v t mcc τ where 3πµ is calle the relaxation time. For a 0. µm article, τ 9 x 0-8 secon an for a 50 µm article, τ 7.7 x 0-3 secons. For a nonmoving flui u0, an the velocity v is in the irection of z, where ownwars is ositive, the solution the the equation is V z tτg[-ex-t/τ]. mcc g For t>>τ the article attains its terminal velocity: v t τg or vt since m π/6 3 ρ. 3πµ C

24 article Settling Velocity

25 article iffusion article iffusion Coefficient: ktcc 3πµ Comarison of iffusion an Gravitational motion article size iffusion istance Gravitational settling 0.µm 0µm 4µm µm 4µm 00µm The major imact of iffusion is coagulation.

26 The eosition Velocity

27 n t n λ 0,, E K ε λ 0, 4 n E U π λ ε Collision factor Temoral change of number ensity: Collection rate 3 * * Re 4 Re 0,6 0,4 Re Re, S St S St Sc Sc Sc E W w a ρ ρ µ µ Collection Efficiency calculate following Slinn 983 Brownian iffusion Intercetion Imaction a a U µ ρ Re iff a a Sc ρ µ U St τ Re ln Re ln, * S a C b iff TC k 3πµ 0,435 ex 0,84,493 λ λ λ C C article size ro size Seinfel, anis 998 Treatment of Washout

28 n t n λ 0,, E K ε λ 0, 4 n E U π λ ε Collision factor Temoral change of number ensity: Collection rate 3 * * Re 4 Re 0,6 0,4 Re Re, S St S St Sc Sc Sc E W w a ρ ρ µ µ Collection Efficiency calculate following Slinn 983 Brownian iffusion Intercetion Imaction a a U µ ρ Re iff a a Sc ρ µ U St τ Re ln Re ln, * S a C b iff TC k 3πµ 0,435 ex 0,84,493 λ λ λ C C article size ro size Seinfel, anis 998 Treatment of Washout

29 Washout

30 Coagulation Coagulation occurs when two articles collie an stick together, forming a thir, larger article. Coagulation conserves the total mass but reuces the total number. By this rocess the iameter of the articles changes. Coagulation between a small an a large article is referre to a scavenging or collection, leaing to a comlete loss of the small articles whereas the volume of the large article oes not significantly change.

31 Coagulation of articles that iffer in size iffusion coagulation occurs when one moe of articles coagulate with another moe of articles. iffusive coagulation is escribe by: N K N N t

32 Brownian Coagulation

33 Conensation Growth The growth rate of articles is irectly relate to the concentration of conensable vaour an it s rate of conensation. This inter-relation can be exresse by r t M X rρ where r is article raius, M X is molecular mass of conensable vaour X, β m is transitional correction factor for mass flux, is iffusion coefficient, an ρ is article ensity, an C is the vaour concentration. β m C The transitional correction factor for mass flux, following Fuchs an Sutugin 97, is given by β m 0.377Kn Kn 4 α Kn 3 4 α 3 Kn

34 Nucleation Nucleation summarizes the rocesses by which material is transforme from one hase to another. Nucleation lays a funamental role whenever conensation, crystallization, sublimation boiling or freezing occurs. A first ste in the formation of a new hase is a suer saturation of the mother hase, usually create by external forcing cooling. The suer saturation can also be cause by a rouction of conensable material within a volume. The secon ste is formation of clusters in the mother hase by thermal fluctuations. The thir ste involves the growth of the clusters to critical sizes. Homogeneous nucleation occurs in the absence of foreign material. Heterogeneous nucleation occurs on foreign substances or surfaces.

35 Imact of Aerosol articles on Warm Clous Aerosol articles act as clou conensation nuclei CCN an therefore they etermine the size istribution of clou rolets. Imact on: clou otical roerties coagulation of rolets reciitation 35

36 Some Basic hotochemistry hotostationary State: NO h ν NO O O O M O 3 NO O 3 NO O 36

37 Some Basic hotochemistry rouction of OH: Oxiation of CO: O 3 h ν O 3 O O 3 h ν O O O M O 3 M O H O OH CO OH CO HO HO NO NO OH NO h ν NO O 3 O 3 O M O 3 M CO O h ν CO O 3 37

38 38 En of art

39 Self Coagulation Two articles susene in a flui often come into contact ue to Brownian iffusion. When articles collie, they coagulate into one article. As a result of each coagulation interaction, the number ensity is reuce by a. The equation escribing self coagulation within a moe may be written in terms of the moe s number concentration, N, an the self coagulation coefficient, K : N KN - t K is a function of the aerosol iffusion coefficient an the article cross-sectional area. Solving the above equation to give the number concentration of articles within a moe at some time, t secons, after the start of the rocess yiels: N 0 N KtN 0