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Meteorology 437 Atmospheric Physics II Cloud Microphysics and Chemistry

Meteorology 437 Atmospheric Physics II Cloud Microphysics and Chemistry. Email sheet, syllabus, survey distribution Prof’s introduction Students’ introduction My assumptions: You know each other. You all have access to the web. You receive and check email from your PSU account.

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Meteorology 437 Atmospheric Physics II Cloud Microphysics and Chemistry

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  1. Meteorology 437Atmospheric Physics IICloud Microphysics and Chemistry • Email sheet, syllabus, survey distribution • Prof’s introduction • Students’ introduction My assumptions: You know each other. You all have access to the web. You receive and check email from your PSU account. Each of you is: • a junior or senior Meteorology major • taking this course to satisfy a departmental requirement • taking (or have already taken) an atmospheric physics lab course. • taking (or have already taken) atmospheric thermodynamics. Meteorology 437

  2. Introductions • Students’ introductions, continued What are your names? Do you have anything in particular you would like to take away from this class? • Course web page • Survey Please complete one after class. • Syllabus Read it. We will review it now. Meteorology 437

  3. Motivation - why AtmPhy II? • Atmospheric chemistry describes stratospheric ozone depletion. • Cloud chemistry explains the creation of acid rain. • The presence of chemicals,aerosols and clouds has strong effects on radiation - hence climate (e.g. CO2, DMS, SO2,cirrus) and visibility (e.g. National Parks). • Aerosols and some chemicals are harmful pollutants (e.g ozone, nitrogen oxides, sulfate aerosols). • Cloud particle growth leads to precipitation! • Collisions of cloud particles lead to the electrification of clouds (lightning). • Radar and lidar propagation through the atmosphere depends on aerosol and cloud particle size and number. Meteorology 437

  4. Spatial Scales • Scales relevant to clouds which we will not study • Scales relevant to clouds and chemistry which we will study Meteorology 437

  5. Overview: What is the overall structure of the course? There are two major components of the class, and one important review section. These topics will be covered in the following order: • Atmospheric composition and chemistry • Review of thermodynamics • Cloud physics The chemistry and cloud physics are linked. Trace chemicals and aerosols are critical to cloud microphysics. The presence of clouds alters atmospheric chemistry. We will try to make these links apparent. Meteorology 437

  6. Vertical structure of the atmosphere • What are the names of the vertical layers of the atmosphere? • What defines the vertical layers in the atmosphere? • What do you know about the dynamics and stability of each layer? How do you know this? • Where are clouds located? • Why are there clouds in these locations? • Where is the water vaporin the atmosphere? • How do you identify the presence of clouds on a skew-T diagram? • What is the approximate % by mass of each layer? • See the figure. Also C2, Jacob; C1 W&H. Meteorology 437

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  8. Composition of the atmosphere (also see Jacob, Table 1-1) *ppmv = parts per million by volume Meteorology 437

  9. Composition, continued • Is the composition of the atmosphere uniform? Don’t heavier molecules settle closer to the earth? • Recall the derivation of the pressure structure of the atmosphere based on the hydrostatic equation and the ideal gas law (Wallace and Hobbs pg 52-54, or Jacob, chapter 2). • Scale height, H, varies inversely with molecular weight, therefore heavier molecules are more concentrated at the earth’s surface. Correct? Meteorology 437

  10. Composition, continued • Turbulent mixing competes with gravitational settling. • Close to the earth a molecule’s mean free path is short, and it takes a very long time to settle via molecular diffusion. Atmospheric turbulence keeps the composition very well mixed. Homosphere. Up to about 100km above ground. Here we use the “apparent molecular weight of dry air.” • Higher in the atmosphere the air density is lower, molecular collisions are more rare, and gases can settle according to their molecular weights. Heterosphere. Meteorology 437

  11. Chemistry and the temperature structure of the atmosphere • High in the atmosphere, high-energy radiation breaks down molecules into component parts, e.g. O2 -> O. This is typical of the ionosphere and heats the thermosphere. • In the stratosphere, O2 + O -> O3 creates the ozone layer. O3 absorbs UV radiation and heats the stratosphere. • Why is there a temperature maximum at the earth’s surface? Meteorology 437

  12. Units! • Always specify units. Use SI (system internationale) units - MKS - whenever possible. • Length: m, cm (10-2) , mm (10-3) , mm (10 –6), nm (10-9) • Mass: kg (103), g, mg • Time: s, min, hr Meteorology 437

  13. Units: Concentrations/Densities Concentrations express stuff per unit volume. • Mass concentration (kg m-3): Also called density. • Number concentration (molecules m-3, moles m-3, moles L-1) • What’s a mole? A mole is Avogadro’s number of molecules. One mole weighs one molecular weight. Beware gram vs. kg molecular weight! • 1 Liter (L) = (10 cm)3, 1 mL = 1 cm3 Meteorology 437

  14. Units: Mixing Ratio Sometimes mixing ratio means H2O mixing ratio by default in meteorology – not here. Mixing ratios express quantity i per unit quantity of the medium. • Mass mixing ratio: kg kg-1, or g kg-1. E.g. water vapor, is often given in g H2O / kg “dry air” (mixing ratio - w), or g H2O / kg “moist air” (specific humidity – q). • Molar mixing ratio (Ci): mol mol-1. This is the same as the ratio of number densities. Prove this. Therefore, this is the same as mixing ratios per unit volume, e.g. ppbv, e.g. [O3] = 40 ppbv = 40 O3 / 109 air molecules. • Volume mixing ratio: Same as molar for ideal gases. Meteorology 437

  15. Units: Partial pressures Chemical composition can also be expressed in terms of partial pressure. All gases (in the earth’s atmosphere) obey the ideal gas law to a very high level of precision. The ideal gas law is used to translate between concentrations/densities and partial pressure Forms of the ideal gas law include (R+Y, pg 2; Jacob, pg 6): • pV = NRT N=moles, R=8.31 J/mol/K • p = RrT/M r=kg/m3 • p = nkT n = mol/m3, k=R/NA Meteorology 437

  16. Units conversions: concentrations, densities, mixing ratios and pressures mi/V = ri m ~ kg, V ~ m3 ni/NA = Ni/V n ~ mol/m3, N ~ moles Ni*Mi/V = ri M ~ g/mole or kg/kmole ri/rair = wi w ~ kg/kg ni/nair = ri r ~ mole/mole wi*Mair/Mi = ri ri*106,9,12 = ri (ppm, ppb, ppt) pi*Mi/RT = ri p ~ kg m / s / m2, T ~ K pi*NA/RT = ni R ~ J / mole / K Meteorology 437

  17. Other handy things to remember when dealing with ideal gas mixtures • Pressures sum: ptotal = Spi • Densities sum: rtotal = Sri • Every individual gas (i) in the mixture obeys the ideal gas law according to its own temperature and pressure • For our applications, all gases in the earth’s atmosphere will share the same temperature, T. When could we have different temperatures, Ti? Meteorology 437

  18. Exercise • What is the volume of this room? • What is the mass of air within this room? • How many moles of air are in this room? • How many kg of H2O are in this room? • How many moles of H2O are in this room? • If T dropped to 0oC, how much water would change from gas to liquid phase? Meteorology 437

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