Workshop on Atmospheric Deposition: Processesand Environmental Impacts May 2012

Transcription

1 Workshop on Atmospheric Deposition: Processesand Environmental Impacts May 2012 Processes in wet deposition: In-cloud and below cloud scavenging Karine Sellegri Laboratoire de Meteorologie Physique LaMP CNRS Clermont Ferrand France

2 Processes in wet deposition: In cloud and below cloud scavenging Karine Sellegri Laboratoire de Météorologie Physique LaMP/CNRS Clermont Ferrand, France

3 Relative contribution of in cloud and below cloud scavenging Aerosol mass transfer using Mesoscale cloud model with detailed microphysics DESCAM 3D: balance after one hour Adapted from Flossmann (1998), in Flossmann and Wobrock, Atm. Res. 2010

4 General processes involved in clouds H 2 O vapour Soluble gases Condensation 100%RH Accretion µm µm µm Dry particle Hydrated particle Cloud droplet Rain drop

5 In cloud scavenging: Cloud droplet formation Köhler theory: Equilibrium between aqueous solution and humid air Surface tension Soluble fraction Particle radius S P P sat Saturation vapour pressure/flat surface Curvature (Kelvin) Effect: the saturation vapour pressure increases with increasing curvature exp 2M e 3 s M e r p RTr g e M s e (r 3 g r 3 p ) Droplet radius Solute (Raoult) Effect: the presence of solutes in the drop decreases the saturation vapour pressure

6 Curvature (Kelvin) effect The smaller the droplet, the greater the supersaturation (with respect to a flat surface) is needed to keep the droplet from evaporating

7 Raoult effect The vapor pressure for a solution drop is less than that for a plane of pure water The vapor pressure required to maintain equilibrium decreases as the drop radius decreases. This is opposite of the effect for curvature.

8 Köhler curve Initially the solution effect dominates, but as the drop gets bigger, the curvature effect takes over. When the drop is very large, neither effect dominates and the surface of the drop, to the water molecules, appears as a flat surface. Is this equilibrium theory observed in reality?

9 Processes in the subsaturation regime H 2 O vapour Soluble gases Condensation 100%RH Accretion µm µm µm Dry particle Hydrated particle Cloud droplet Rain drop

10 Aerosol watervapourinteractions: controlled humidification systems HTDMA: Humiditidy Tandem Differencial Mobility Analyser Aerosol Inlet CPC Drier DMA1(Dry) Humidifier DMA2(Wet)

11 Hygroscopic growth for a known inorganic aerosol Theory works well for simple inorganic aerosols, even for residence time of a few seconds

12 Water vapour uptake of a mixture of different aerosol types at a given size Concentration HUMIDIFICATION AT 85% RH Taille (en nm)

13 Example of results for atmospheric aerosols: time series at a background station (puy de Dôme)

14 Can hygroscopic growth be accurately predicted from chemistry? ZSR Theory (Zdanovskii- Stokes-Robinson): approximates the GF of a chemical mixture with the hypothesis that no interaction exist between chemicals, no surface and kinetic effects GF global ( agfa bgfb )

15 Can hygroscopic growth be accurately predicted from chemistry? Theory ZSR (Zdanovskii- Stokes-Robinson): approximates the GF of a chemical mixture with the hypothesis that no interaction exist between chemicals, no surface and kinetic effects The model predicts that the aerosol population is uptaking water as if only the more hygroscopic mode existed GF global ( agfa bgfb )

16 Can hygroscopic growth be accurately predicted from chemistry? BC hydrophobic (GF 90% =1.00) SO 4 +NH 4 +NO 3 (GF 90% =1.7) Organics : variable GF= Mode hydrophobic (BC + org) : combustion aerosol Mode hygroscopic (NH 4 +SO 3 +NO 3 +org): aged aerosol

17 Can hygroscopic growth be accurately predicted from chemistry? Freney et al. in prep Are there any surface or kinetic effects?

18 How does aerosol surface impact water uptake? Simulation of the ageing of sea salt with an hydrophobic substance (organic) NaCl HGF 90% =2.35 Paraffine HGF 90% =1.00 Comptage relatif (CPC2/CPC1) HGF= Diamètre (nm) Shit test NaCl (100nm) recouvert de paraffine (30nm) à RH=90% Coating of 30nm over a 100 nm particle 100nm 15nm HGF HGF paraffine non homogeneous coating GFα =2.35 et α=0.45 GFβ =1.00 et β=0.55 HGF ZSR : 1.86 Surface propertie does not seem to impact significantly water uptake on an aerosol particle at subsaturation. Does this apply at sursaturation?

19 Processes in the sursaturation regime H 2 O vapour Soluble gases Condensation 100%RH Accretion µm µm µm Dry particle Hydrated particle Cloud droplet Rain drop

20 Aerosol water vapour interactions using controlled humidification systems Aerosol DMA Size resolved CCN properties Puy de Dôme, France CCNc (with 2 supersaturati ons) CPC Activated fraction Diameter [nm]

21 Activation parameters Puy de Dôme E Activated fraction Dp50 Fitting: Diameter [nm]

22 Time series of aerosol activated fraction Fitted CCNc activation at SS=0.25%, puy de Dôme, France nm

23 Ability to predict CCN number under controlled conditions Puy de Dôme measurements Asmi et al. in prep

24 Studies in real clouds

25 Inlets: Separating activated aerosol from non activated aerosols Dp > 30 µm Heating Whole Air Inlet (WAI) CVI Dp > 5 µm Heating Cloud droplets (Cloud Droplet Impactor CVI) Residual particles and gazes RJI Dp < 5 µm Interstital inlet (Round Jet Impactor RJI) Interstitial particles and gazes

26 Activated size distribution Activated fraction at Puy de Dôme 85 nm Interstitial Cloud residual Mertes et al. (2005). Atmos. Env. 39, Aerosol particle larger than 100 nm form cloud droplets The are found are residual particles from evaporated cloud droplets The ratio activated/total gives the activation diameter

27 Droplet number prediction Asmi et al. in prep.

28 Activated fraction at Puy de Dôme Some large aerosols not activated CCN Chamber Real cloud Activated fraction Diameter [nm] Some mall aerosols activated

29 Chemistry of activated aerosols Activated aerosols Interstitial aerosols OC 17% OA 1% autres ions 7% OA 1% autres ions 2% NO3 5% SO4 4% NH4 3% CVI BC 1% NH4 12% NO3 37% BC 2% RJI SO4 25% OC 83%

30 Chemistry of activated aerosols 0.03µ<Dp<0.1µm OC 31% Residual Résiduel (activated) NO3-31% Interstitial Interstitiel (non activated) NO3-5% SO4 6% NH4 4% other ions 3% BC 2% BC 2% other ions 4% OC 28% NH4 16% SO4 16% NO3-30% OC 80% NO3-2% other ions SO4NH4 0% 3% 2% BC 2% 0.1µ<Dp<0.33µm 0.33µ<Dp<1µm BC 2% other ions NH4 3% 12% OC 22% NO3-8% SO4 25% SO4 15% OC 91% NO3-2% SO4 6% NH4 8% other ions 8% BC 19% other ions 18% NH4 18% OC 68% BC 8%

31 Evidence of complex mixture/role of chemistry Activated aerosol fraction Sellegri et al. JGR, 2003

32 Aerosol activation modeling Activated aerosol fraction modelled using EXMIX/measured Model internal mixing Model external Model external+condensation coeff Total measured Carbonaceous measured Taking into account chemical associations (external mixing) and kinetics of water vapour diffusion is necessary to model the real cloud mass transfer

33 Uptake of other gaz phase pecies H 2 O vapour Soluble gases Condensation 100%RH Accretion µm µm µm Dry particle Hydrated particle Cloud droplet Rain drop

34 Instrumental strategy CDI CVI RJI WAI Bulk water sampling Aerosol particle sampling Gaz phase sampling

35 Gazeous contribution to droplet chemical composition CH 3 COOH/CH 3 COO - Acetic acid HCOOH/HCOO - Formic acid 99% 97% Gaz phase 1% 3% Aerosol phase CH 3 COOH/CH 3 COO - HCOOH/HCOO - 88% 12% 12% 82% 17% 16% 0,1% 0,2% 1% 1% Sub sat Sub sat

36 Gazeous contribution to droplet chemical composition HCl/Cl - 90% 62% 56% - HNO 3 /NO % SO 4 Phase gaz Phase particulaire HCl/Cl- 38% 44% SO % 37% 11% 2% Equilibrium 87% 74% 13% 3% 7% HNO 3 /NO 3 - Equilibrium 91% 8% 45% 45% Sur sat 82% Sellegri et al. ACP, %

37 Below cloud impaction scavenging v t 2 pdp gcc( dp) ; 18 Cc 1 2 [ e dp 1.1dp ] Droplet d a, particle d b Sedimentation velocities v a, v b Num Conc. N a, N b Volume before colision: d d 2 v v t a 2 b 2 a b Number of particles impacted d d 2 va vb t Na a 2 b 2

38 Below cloud scavenging experiments

39 Sampling Methodology 12 Km Cloud sampling 800 m Puy de Dôme (1465m) Rain sampling Cézeaux (394m) Opme (660m)

40 Washout ratio W r C rain C aerosol With ρ,air volumic mass (1.2 kg/m 3) X X C rain (X), concentration in rain (µg/kg) (McNeary and Baskaran, 2003 Jaffrezo and Colin, 1988) C aerosol (X), concentration in aerosol, (µg/m 3) sampling the rain water/aerosol: can not separatebelow cloud scavenging from in cloud scavenging (activation) Which aerosol?

41 Rain and aerosol concentrations seasonal variation Rain : max summer pluie pdd Cx min winter Aerosol: Same at pdd Opposite at surface site (Cx) aérosol (µg/m 3 ) pluie (mg/l) Using aerosol concentration at cloud level more adapted? But using the Puy de Dôme aerosols, the ratio summer/winter higher in the rain (11.6) than in aerosol (1.4) 1 0 janvier février mars avril mai juin juillet aout septembre octobre novembre décembre Other factors influence rain composition

42 Effect of Boundary layer dynamics Puy de Dôme Cézeaux 0 janvier février mars avril mai juin juillet août septembre octobre novembre décembre h couche limite (m a.s.l.)

43 Effect of Boundary layer dynamics janvier février mars avril mai juin juillet août septembre octobre novembre décembre h couche limite (m a.s.l.)

44 Taking the Boundary layer height in the washout ratio calculation Wr V rain C X rain h BL X C aerosol NO 3 - SO 4 2- C 2 O 4 2- NH 4 + K + 7 Be 210 Pb 137 Cs Average variability 137% 82% 75% 222% 87% 212% 100% 179% 79% 116% 164% 87% 460% 122% 460% 109% Taking into account the BLH increases reliability of Wr Bourcier et al. 2012

45 Integrating all processes using modelling tools DESCAM 3D: a 3D cloud model with detailed microphysics Example: a cumulonimbus cloud Dynamics: NCAR Clark-Hall cloud scale model (Clark and Hall, 1991) Microphysics: DEtailed SCAvenging Model (Flossmann and Wobrock, 2010) We follow the spectra of : wet aerosol particles, drops, ice crystals aerosol mass in drops aerosol mass in ice crystals 5 x 39 size bins = 195 variables (Leroy et al. 2009, Atm. Res.)

46 Influence of the impaction scavenging DESCAM 1.5 D model Difference with/without impaction scavenging: Factor 2 in the aerosol mass deposited

47 Summary A large percentage of aerosol mass deposited by rain originates from aerosol incloud activation One can accurately predict the hygroscopic growth (RH<100%) of aerosol at a given RH from its size and chemical composition Surface effects does not influence the hygroscopic growth of aerosols at RH<100 At RH>100% aerosols are activated at sizes ranging from 50 to 150 nm for internally mixed pollutations N100nm is a good proxy of cloud droplet numbers for clouds studied at puy de Dôme However, chemistry play a major role when aerosol is externally mixed and some species will preferentially be incorporated in cloud droplets Gases also significantly contribute to the cloud droplet concentrations Below cloud scavenging contribution is difficult to separate from in cloud activation and scavenging when studying rain water composition experimentally but modelling tools indicate a factor 2 in the aerosol mass deposited The boundary layer mixing height should be taked into account when calculating wash out ratios but simplifactions/parametrizations are difficult and 3D modelling tools necessary