1、AT620 Review for Midterm #1,Part 2: Chapters 5-7 Brenda Dolan October 19, 2005,Chapter 5: Atmospheric Aerosols,Atmospheric Aerosols,Aerosol: Small liquid or solid particles suspended in a medium (the atmosphere). They are very small particles that do not have appreciable fall speeds.,Cloud Condensat
2、ion Nuclei (CCN): Aerosols that are activated to serve as cloud nuclei at realistic (low) supersaturations (S that would be found in atmosphere,Condensation Nuclei (CN): All aerosols in atmosphere, including those that are activated at high supersaturations an may not serve as cloud nuclei under nor
3、mal atmospheric conditions,Atmospheric Aerosols,Aerosol Production Processes:,1) Gas-to-particle conversion (mostly Aitken production)-vapors from plant exhaltations and combustion products -chemical reactions catalyzed by UV radiation-chemical reactions in small water droplets (clouds process air,
4、and thus there can be more concentrations of particles that have been processed by a cloud),Aerosol Production Processes:,2) Mechanical disintegration of the solid and liquid earth surface (mostly large and giant production) Solid earth-organic particulates by plants (pollen, seeds, waxes, spores)-m
5、echanical and chemical disintegration of vegetation free rocks and soils-volcanic emissions-particles injected into the atmosphere by industrial processes(paper mills, steel mills),Aerosol Production Processes:,2) Mechanical disintegration of the solid and liquid earth surface (mostly large and gian
6、t production)Ocean-production of spray droplets at the crest of breaking waves (minor)-bursting air bubbles that are present at oceans surface (few in number but fairly large in size)-Primarily water-soluble sulfates 3) Extraterrestrial sources (minor source of giant and large),Aerosol Production Pr
7、ocesses:,In terms of mass weighting, natural particles are the greatest sources of aerosols, while anthropogenic particles are minor sources. In terms of numbers, anthropogenic can be quite extensive.,Aerosol concentrations,Aerosol Distributions:,In general, aerosol concentrations drop off with heig
8、ht,Junge layer-Abrupt increase in aerosol concentrations in the lower stratosphere-Changes in time and season, but is observed world-wide-Possibly a result of volcanic eruptions,In the ocean, aerosol particles are not dominated by sea-salt particles, but rather oxidation of DMS Aerosol concentration
9、s can be variable over the oceans, but are significantly less than continental concentrations,Measuring aerosols:,1) Electrical aerosol analyzer (EAA)measure mass and size of aerosols based on their measured mobility in applied electric field,2) Optical counters and nephelometersconcentration and si
10、ze distribution of aerosols is determined by the amount (intensity) of scattered light.,3) Direct impaction instrumentscoated slides are swept through volumes of aerosols. Used for large particles (0.1 m),4) X-ray techniquesevaluate composition of aerosols depending upon the radiation given off,5) A
11、itken nucleus counter expansion chamber used to create high supersaturations, then they are counted optically.,Aerosol removal processes:,Aitken particles Coagulation: brownian motion causes particles to collide and self-collect 2) Capture by cloud droplets: either by condensation of vapor on surfac
12、e, or direct impact on aerosol by a cloud droplet,Giant particles 1) Sedimentation: Dry deposition due to relatively large fall velocities 2) Precipitation scavenging: collection efficiencies are large,Aerosol removal processes:,Large particles: The Greenfeld gap Large aerosols have the longest life
13、 because there is no efficient sink for them. Their fall velocities are not large enough in most cases for dry deposition, and they are in the size range where coaguation is not efficient. 1) Some dry deposition 2) Some precipitation scavenging,Aerosol removal processes:,Coagulation Brownian motion:
14、 irregular movement of aerosol particles due to thermal bombardment by air molecules Smoluchowskis equation for Coagulation,gain,loss,Collection kernel,Smoluchowskis equation for Coagulation Describes the change in size spectrum of aerosols since particles that are moving irregularly have a finite p
15、robability of colliding and coagulating with one another and particles with relatively large mobilities collide and coagulate more readily, we need to define some efficiency that is related to mass. This is the diffusivity, D. Diffusivity is inversely proportional to r and proportional to T,Aerosol
16、removal processes:,Wet Removal Mechanisms Phoretic effects Condensation and evaporation of vapor molecules can effect the collection of aerosols, because aerosol particles being bombarded by vapor molecules experience a force directed toward the droplet surface, which enhances the coagulation betwee
17、n cloud droplets and aerosol particles. Diffusiophoresis: Enhanced diffusion of aerosols to drop, enhancing the collection kernel Thermophoresis: Diffusion of heat away from growing droplet, which inhibits collection of aerosols,Aerosol removal processes:,Wet Removal Mechanisms Phoretic effects Phor
18、etic effects are most important for aerosol particles between 0.1 m diffusiophoresisThis results in a reduced rate of aerosol particle scavenging by a cloud droplet growing by vapor depositionThis results in an enhanced rate of aerosol particle scavenging by a cloud droplet that is evaporating,Aeros
19、ol removal processes:,Aerosol removal processes:,DRAW,Aerosol removal processes:,Hydrodynamic capture A large drop settling through smaller drops will sweep out a volume and collect aerosols with some efficiency, E,Depends on size of drops and size of aerosols Most efficient for large and giant aero
20、sols duet to large Vt and cross-sectional area,Cloud Condensation Nuclei:,1% of aerosol mass serve as CCN in continental air, while 10-20% serve as CCN in maritime Chemical composition determines the best CCN hygroscopic wettable solubility,Cloud Condensation Nuclei:,CCN measurement techniques Therm
21、ogradient diffusion chamber: Two wetted plates are held at different temperatures, molecular diffusion not convection leads to: linear variation of T between plates linear variation of vapor pressure (e) between plates Saturation vapor pressure varies exponentially with T Saturation can be changed b
22、y changing the temperature of the two plates,Cloud Condensation Nuclei:,DRAW,Cloud Condensation Nuclei:,World-wide measurements of CCNcontinental air masses are richer in CCN than maritime air massesConcentrations of CCN increase with supersaturation as expectedRemote ocean air contains the fewest C
23、CN Typically NCCN100 cm-3 at 1% supersaturation for maritime airmasses The relationship between CCN and supersaturation is exponential:,Cloud Condensation Nuclei:,Spatial and temporal variation of NCCN CCN can vary over several orders of magnitude over short periods of time proximity to CCN sources
24、wind direction and wind speed (air mass could switch to maritime or continental) precipitation (cloud formation depletes CCN, precipitation scavenges CCN) CCN concentrations diminish with height away from ground; but inversions could trap CCN,Cloud Condensation Nuclei:,Properties of CCN Theory: NaCl
25、 and large particles serve as CCN Observations: not NaCl, but sulfides and sulfur compounds; even particles down to 0.02 m can serve as CCN Type of could system can influence type of activated CCN (low S, weak verticall motion, etc.) In reality, atmospheric nuclei are composed of a mixture of partic
26、les Number of CN and CCN are not well correlated,Chapter 6: Observed Microstructure of Warm Clouds,Cloud droplet distributions,CSk combined with radiative cooling through ascent Distribution is no just due to aerosols type or air mass, but also depends on velocity and liquid water content when drop
27、concentrations are smaller, drops can grow larger when there are lots of CCN, they grow smaller (competing for the water) higher vertical velocities lead to higher concentrations Activated spectra bimodal distribution non-activated spectra mono-modal distribution Continental: mean=11.2 m, mode=12 m,
28、 more narrow droplet size-spectra Maritime: larger mean and modal diameters, broader droplet-size spectra, but lower concentrations,Raindrop size spectra,Marshall-Palmer distributionSlope depends on rainfall rate Assume that N0 can be specified Generalized gamma distributionThe concentration of rain
29、drops is much smaller than the concentration of cloud droplets Rain drops are obviously much larger than cloud droplets This implies that only a few cloud droplets make it into raindrops,LABEL,Fog size distributions,DRAW,Chapter 7: Theory of Cloud Droplet Growth,Growth by vapor deposition,Growth by
30、vapor deposition (Diffusional growth) Assumptions steady stateno accumulation of vapor densitysurface of drop is exactly saturated,Growth by vapor deposition,Heat budget for a drop growing by vapor deposition: Internal energy:,No heat storage (du=0):,Three mechanisms for heating 1. Condensation2. Mo
31、lecular diffusion3. Radiative heating (cooling),Growth by vapor deposition,Total heat budget (thermodynamic equation):,Combined equation for growth by vapor deposition:,Growth by vapor deposition,Combined equation for growth by vapor deposition: Rate of change of radius decreases as drop gets bigger
32、 (doesnt favor growth of large droplets) Growth rate increases if saturation ratio increases Growth rate increases over a solution Growth rate decreases due to curvature Radiation can either increase or decrease the growth rate net effect of this is that drops can cool enough at the top of the could
33、 to grow by vapor deposition. Bigger droplets cool more by OLR,Growth by vapor deposition,Combined equation for growth by vapor deposition: Assumptions made in deriving this equation: Transfer of heat and moisture are by steady-state diffusionThe vapor density at the droplet surface is that under wh
34、ich the droplet persists in equilibriumThere is no disturbance of vapor field by neighboring dropletsThere is no disturbance of vapor field by motion of the droplet There is no additional source of heat to or from the droplet other than radiationHeat storage on the droplet is negligible,Growth by va
35、por deposition,Combined equation for growth by vapor deposition: Also, this is an assumption that this continuous diffusion rather than discrete. Thus we can modify the diffusion coefficient with a condensation (accommodation) coefficient, and similarly, the thermal diffusion coefficient. Large drop
36、s also ventilate, which can enhance evaporation and condensation Large drops can also evaporate,Growth by vapor deposition,Growth example:,DRAW,Narrows the droplet spectrum in time shows 1/a dependence solution effects enhance growth,Growth by vapor deposition,Growth of a population by condensation:
37、,As drops grow, they remove S, but as air rises, S increases,Growth by vapor deposition,Growth of a population by condensation:,In fog, S is lower and only the most chemically active and huge aerosols are activated Small drops get “starved” of H2O, never reach S large enough to grow If updraft incre
38、ases, peak saturation would also increase (cool air faster, takes drops long time to use up H2O) thus smaller drops also activate In general, it takes days to grow drops to precipitation sizes by vapor deposition alone! TOO SLOW!,Collision-Coalescence Growth,E1: Coalescence efficiency E2: Collision
39、efficiency,Collection kernel: (units m3/s),Collision-Coalescence Growth,E is very small (especially for small drops) at the beginning because small drops sweep around droplet (following streamlines) E1 for a broad range of a1/a2 ratios Can have efficiencies greater than 1 E drops off as a1/a2 approa
40、ches 1 E spikes as a1/a2 is very nearly 1. This is due to wake capture wake capture: as drops are close to same size, hydrodynamic flow fields interfere and drops slip around each other. But this really doesnt matter because when a1 and a2 about equal, the difference in their terminal fall speeds is
41、 so small it decreases the collection kernel.,DRAW,Collision-Coalescence Growth,Continuous Growth Model or Accretion Model,if we also assume that a1ai and v1vi,Assume coalescence efficiency of unity Continuous accretion model is applicable when collector droplet is much larger than collected droplet
42、s Fails because it requires an initial broadening of droplet spectra to get drops large enough to be efficient collectors A given droplet will always grow to the same size when falling through the same droplet population,Collision-Coalescence Growth,Quasi-stochastic model,Uses Smolokoskies equation
43、to predict a unique spectrum after some time dt Use a Monte Carlo distribution a type of “bin” model that predicts the time rate of change of mass or volume (not radius),gain,loss,Collision-Coalescence Growth,Quasi-stochastic model,Similar to aerosols, but K increases as you get to larger droplets (
44、K decreases for smaller aerosols) Integration limit on Gain term accounts for combinations (dont want to double count) Implies that higher droplet concentrations = higher rate of collection but for a given LWC higher concentrations lead to smaller dropletsie: for same LWC, a cloud with less concentr
45、ation will grow larger drops than one with more concentration aerosol # can really affect the cloud/precipitation processes polluted clouds = much smaller drops Increasing the LWC can greatly accelerate the collection process,Collision-Coalescence Growth,Problem of Initial Broadening,Problem: How to
46、 get droplets to a size where collision-coalescence can kick in Initial droplet spectra is 4m to 12m, so how do we get to sizes 25-30m to make collision-coalescence productive? Growth by vapor deposition tends to narrow the droplet spectrum Need broad spectrum of sizes or else the velocities and siz
47、es will be too similar for Collision and Coalescence collection kernel. Thus problem of initial broadening is not just creating drops large enough for Collision-coalescence to begin,Collision-Coalescence Growth,Problem of Initial Broadening,1) Turbulence influences on condensation growth via fluctua
48、tion supersaturations mixing process is inhomogeneous (get pockets of clear airspaghetti strings) parts of cloud may be rising and others falling on a small scale leading to evaporation of some drops, leading to relatively larger drops in some areas Fine scale eddies can centerfuge particles out of
49、regions, increasing the S leading to faster growth of particles that remain 2) Role of GCCN Can act as “Coalescence Embryos” if soluble, wettable, and large very small concentrations (similar to raindrop concentrations) depends on CCN concentration if GCCN is important, because lower CCN clouds driz
50、zle actively without the presence of these GCCNmaritime clouds are prolific collision and coalescence machines and GCCN presence doesnt matter,Collision-Coalescence Growth,Problem of Initial Broadening,3) Turbulence influences on droplet collision and coalescence Enhance collision efficiencies (smal
51、l drops can cross streamlines) Enhance collection kernels (accelerated by air movements) Producing inhomogeneities in droplet concentrations 4) Radiative broadening assumes droplet stays around top of cloud for a long enough time (strat/fog) cooling decreases satruation vapor pressure at the surface
52、 of the drop, leading to faster growth than drops in the middle of the couldcan offset the 1/a dependence since larger drops radiate more limited to certain classes of clouds, but it is most easy to quantify Broadening mechanisms are difficult to quantify because of difficulty of studying turbulence.,
