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Atmospheric chemistry

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  1. Atmospheric chemistry Lecture 1: Introduction & Overview Structure of the atmosphere Atmospheric Transport Dr. David Glowacki University of Bristol,UK

  2. Our goals in these lectures… • Atmospheric Chemistry is fascinating because it is spans a range of fascinating subjects • In one week, I hope to: • give you an overview of atmospheric chemistry • teach you some of the key principles • provide you sufficient background to understand the details of two key arctic atmospheric phenomena: • arctic haze • Polar ozone holes

  3. Useful Reading Materials… • Daniel Jacob, Introduction to Atmospheric Chemistry, 1999, available on the web at • Seinfeld & Pandis, Atmospheric Chemistry And Physics: From Air Pollution to Climate Change • Progress and Problems in Atmospheric Chemistry, edited by John R. Barker • G Marston, “Atmospheric Chemistry”, Annu. Rep. Prog. Chem., Sect. C, 1999, 95, p 235-276 • G.E. Shaw, “The Arctic Haze Phenomenon”, Bull. Am. Met. Soc., 1995, 76(12), p 2403-2413 • P.S. Monks, “Gas phase radical chemistry in the troposphere”, Chem Soc. Reviews, 2005, p 2–21

  4. Where I live…

  5. What I do… • I work on the frontier where chemistry meets theoretical physics • I use the mathematical tools of quantum & classical mechanics to understand what molecules do • Most of my research involves massively parallel computers • A lot of what I do concerns how to make more accurate approximations to solving the full quantum mechanical equations • Lots of applications: • Atmospheric chemistry • Combustion • Materials Science • Biochemistry

  6. My background… • During my PhD, I did atmospheric chemistry experiments: • Instrument design • Laser spectroscopy • Optics • Chemical kinetics

  7. Before my PhD… • MA in religion and Cultural Theory at the University of Manchester: • Undergraduate degree at the University of Pennsylvania in Philadelphia: • Major in Chemistry with lots of work in math, physics, and Humanities subjects • That’s where I met Mark Hermanson • We worked together to teach an environmental chemistry class • Originally from Milwaukee, WI

  8. Our Plan for Today’s Lecture • The general structure of the atmosphere Vertical Mixing in the Atmosphere • Variation of Pressure with altitude • Variation of Temperature with altitude Horizontal Mixing in the Atmosphere • Coriolis Forces • Hadley Circulation

  9. Atmospheric Temperature and pressure variations z Heating by exothermic photochemical reactions Convective heating from surface. Absorption of IR (and some VIS-UV) radiation from the sun

  10. Vertical Mixing Processes

  11. Variation of pressure with Altitude: The hydrostatic equation [P(z)-P(z+dz)]A P(z+dz) • Consider a column of air at altitude z • A cross section of the air has width dz, • It has two opposing forces: • Upward direction: [P(z)-P(z+dz)]A • Downward direction: -ρgAdz • If the air parcel is in equilibrium, then: • [P(z)-P(z+dz)]A = -ρgAdz • [P(z)-P(z+dz)] = -ρgdz dz z P(z) -ρgAdz Rewriting gives the hydrostatic equation:

  12. Combining the hydrostatic equation with the Ideal gas Law: the Barometric equation [P(z)-P(z+dz)]A • Ideal gas law tells us that • PV=nRT • The Density of a gas, ρ, may thus be written as: • r = m(n/V) = m(P/RT) • where m is the molecular weight of the gas P(z+dz) dz z P(z) -ρgAdz • Plugging this expression for density into the hydrostatic formula gives: • Rearranging and integrating we obtain the Barometric equation:

  13. Properties of the barometric equation • Hs is termed the scale height • It is the altitude over which the pressure falls by a factor of 1/e (0.37) {hint: you can see this by setting z = Hs} • The Barometric equation written above: • - Assumes T is constant (remember that T actually depends on z!) • - May be compared with a Boltzmann distribution • - Has an average Mair = 28.8 g mol-1 • - Hs = 6 km for T = 210 K; and ~8,5 km for T = 290 K. • - Species with a smaller molecular mass would have a larger scale height; however, because of turbulent mixing, this separation is not important in the troposphere and stratosphere

  14. A simple application of the barometric equation: sea breeze • Fluids flow from regions of high density (pressure) to low density (pressure) Mass conservation

  15. Variation of Temperature with altitude: the dry adabiatic lapse rate Δ work (pressure – volume) Δ Heat Δ System Energy 1st law of Thermodyamics (Conservation of Energy) Δ enthalpy The air parcel doesn’t exchange heat with the surroundings (adiabatic process) Tells how much energy we have to put into the system to change its temperature From the hydrostatic equation (dp/dz=-ρg) Dry Adiabatic Lapse Rate

  16. The adiabatic lapse rate • As air parcels rise, they expand and cool • On earth, g and Cp combine to give • Gd ~ 9.8 K km-1 • The actual atmospheric temperature gradient, Λ, is defined as: • Λ = -(dT/dz)atm • The adiabatic lapse rate may be less than or equal to Λ • This affects vertical mixing, giving rise to either stable or unstable conditions

  17. Adiabatic lapseStable AtmosphereConvective Atmosphere • If Gd > Λ, the atmosphere is stable & little mixing occurs • As a rising air packet A expands, it cools faster than the surroundings • At the same pressure, TA(z+dz) < TATM(z+dz), making A cooler and denser (r α P/T) than its surroundings, slowing its rise • If Gd < Λ the atmosphere is unstable & convection occurs. • As a rising air packet A expands, it cools slower than the surroundings • TA(z+dz) > TATM(z+dz), making A warmer & less dense than its surroundings, accelerating its rise Γd is constant (slope = Γd) (slope = Γd) (slope Λ) (slope Λ) Λ changes depending on conditions Fast vertical mixing - convective Little vertical mixing

  18. The Planetary Boundary Layer • The subsidence inversion creates stability & inhibits mixing, often leading to bad pollution build-up in large cities • Planetary Boundary Height = 500 – 3000 m. • Mixing near the surface is always fast because of turbulence

  19. The Planetary Boundary Layer: diurnal variations • During the day, the earth heats the surface layer by conduction and then convection mixes the region above in the convective mixed layer. There is usually a small T inversion (dT/dz >0) above this which marks the top of the BL. This slows transfer from the BL to free troposphere (FT). Traps pollutants. • Night – surface cools, dT/dz > 0 in surface layer – surface inversion. Confines pollutants to surface layer. • Can get extreme inversions in the surface layer in winter that can lead to severe pollution episodes. High build up of pollutants.

  20. Vertical Mixing • Average atmospheric lapse rate is 6.5 K km-1, giving moderately stable conditions • Turbulence, most important near the surface, increases mixing • Solar heating also makes the atmospheric unstable & increases mixing (accounts for different mixing between night and day) • Water vapor and clouds complicate all these things • The stratospheric Temperature inversion significantly limits vertical mixing between the troposphere & Stratosphere, limiting transport of many ground level VOCs to the stratosphere (The polar regions are special though!) • Tropospheric/stratospheric mixing times are on the order of years! • The Temperature profile of the stratosphere means it is much more stable than the troposphere

  21. Atmospheric Transport • Random motion – mixing • Molecular diffusion • Molecular diffusion is slow, diffusion coefficient D ~ 2x10-5 m2 s-1 • Average distance travelled in one dimension in time t is ~(2Dt) • Molecular diffusion more important at very high altitudes & low pressures • Air Parcel diffusion • In the troposphere, eddy diffusion of air parcels is more important with a diffusion coefficient Kz ~ 20 m2 s-1 • Takes ~ month for vertical mixing (~10 km). This has implications for short and long-lived species. • Directed motion • Advection – winds & geostrophic flow • Occurs on a number of different scales • Local (e.g. offshore winds & sea breeze – see earlier) • Regional (weather events) • Global (Hadley circulation)

  22. Horizontal Mixing Processes

  23. Global circulation – Hadley Cells • To a first approximation, horizontal mixing within the atmosphere is well described as sea breeze circulation driven by the T difference between the hot equator and cold polar regions • Hadley circulation model developed in the 18th Century Intertropical conversion zone (ITCZ) – rapid vertical transport near the equator.

  24. Coriolis Forces • Longitudinal winds are well described by a coupling of Hadley type circulation to Coriolis forces • What is a Coriolis force? 3d example 2d example

  25. Coriolis Forces & Hadley Cells

  26. Geostrophic Flow: A balance of Coriolis Forces & Pressure Gradients The theoretical flow that would result if the system was no more complicated than Coriolis forces and parallel isobars High pressure to the right of the flow Low pressure to the left of the flow

  27. The general circulation: Hadley Cells coupled to Coriolis Forces Polar high pressure region Westerlies High pressure latitudes (location of major deserts) Ferrel cell Trade Winds (easterlies) ITCZ Trade Winds (easterlies) High pressure latitudes (dry areas) Stronger westerlies Roaring 40s & Screaming 50s (less friction) Polar high pressure region

  28. Horizontal transport timescales

  29. Summary • Atmospheric chemistry depends on atmospheric structure & transport dynamics • Some simple physics gives us basic insight into some of the principles that determinate atmospheric structure and transport dynamics • The barometric equation describes the relationship between pressure & altitude • The adiabatic lapse rate helps us understand the atmospheric vertical T dependence, and vertical transport • To a first approximation, global circulation may be thought of as sets of sea breeze cells coupled to Coriolis forces • Mixing processes are coupled to chemical change, which we will learn about tomorrow

  30. Some Questions to Consider • Using your knowledge of (1) the adiabatic lapse rate and (2) the temperature profiles of the troposphere and stratosphere, explain why vertical mixing in the stratosphere is much slower than in the troposphere • Using (1) a diagram involving adiabatic lapse temperature profiles and (2) your knowledge of stability/instability, explain the origins of the planetary boundary layer • Should footballers worry about Coriolis forces when they are taking a north to south 20m free kick that travels 50 km/hr? (hint: see Jacob equation 4-3) • Based on the model of general circulation: • Why are the timescales for east west transport shorter than those for north south transport? • Using your knowledge of Hadley transport, the Sea Breeze model, and Coriolis forces: • Explain why the Arctic is so windy • What direction would you expect the wind to blow on the ground in Longyearbyen?