The Atmosphere All of it. In one hour. Mikael Witte 10/27/2010
Outline Structure Dynamics - heat transport Composition Trace constituent compounds Some Atmospheric Processes Ozone destruction in stratosphere Aerosols and clouds Precip. scavenging & rainout
0.0001 0.001 Mesosphere 0.01 0.1 pressure (mb) 1 10 Ozone Layer 100 1000
Vertical Thermal Structure Without reradiation, the whole atmosphere would be like the stratosphere/thermosphere With it, the troposphere is warmest at the surface and cools to the tropopause Dry adiabatic lapse rate Γ D =9.8 K km -1 Lapse rate changes as water vapor added
Greenhouse Effect Incoming energy from the sun is shortwave radiation with a peak in visible range Some energy is reradiated from earth s surface in the infrared band Of that energy, some molecules absorb and reradiate the IR energy, heating up the atmosphere
Greenhouse gases absorb and emit IR radiation Greenhouse Gases CH 4 and N 2 O
Greenhouse Gases Water is most important - warmer climate, more water vapor CO 2 not as powerful as others, but so much of it. Methane oxidizes to CO 2 - double whammy. Others: N 2 O, tropospheric ozone, CFCs
Heat Transfer Two main modes in atmosphere: Convection: an object transfers energy with its environment by fluid motion Conduction/diffusion: happens between objects in physical contact Convection way faster than conduction but requires a thermal gradient, so only in the troposphere!
Direct heat transport ITCZ Indirect heat transport In the midlatitudes, heat is moved poleward by turbulent eddies You can see them in the real world!
Water ranges from ppm to 5% volume
Residence Time and Variability CO 2 τ~5 yrs Note departure of He from line noble gas! Inter-hemispheric mixing time: 1-2 yrs
CO 2 Variability
Sulfur Mostly in the form of SO 2 or H 2 SO 4, some OCS Very important in cloud formation (above along with dimethylsulfide)
Nitrogen Need to convert N 2 to something more useful Lightning (small source) N-fixation by bacteria, e.g. N 2 to NH 3
Carbon Found in a nearly unlimited number of compounds - unlike N and S Soot (or black/refractory/elemental C) inert so transported long distances, adsorbs lots of stuff Whole lot of organics
Carbon, Cont. Several hundred organic compounds Many anthropogenic compounds with no natural source, e.g. CFCs
Oxygen Two forms just as important as O 2, both formed by photolysis in the stratosphere Ozone, O 3 : protects surface from UV Hydroxyl radical, OH Responsible for (basically) all oxidation!
Ozone Requires high energy photons λ<242 nm O 2 + hν 2O O + O 2 + M O 3 + M O 3 + hν O + O 2 Reactions with NO x, HO x and ClO x also create/destroy ozone
Ozone Hole CFCs made by humans have no sink in troposphere since there s no biology to break them up, plus H 2 O insoluble Only sink: stratospheric photolysis Long residence time (40-150 yrs) But how does this affect ozone?
Ozone Hole Key is heterogeneous Cl reactions in polar stratospheric clouds: ice ClONO 2 + HCl Cl 2 + HNO 3 (ice) Each CFC leads to 2-3 Cl radicals Ozone hole forms during Antarctic spring - no sunlight to replenish Cl radicals kill O 3 like it ain t no thang
Aerosols Particulate matter - soil particles, sea salt, industrial crap, organics Held aloft in suspension by turbulent motion, transported long distances Important for both cloud formation (CCN) and radiative balance (albedo)
Cloud Formation When air rises, it cools, can hold less water. Air becomes oversaturated WRT water. Water condenses, releasing more heat and forming a cloud but it s really hard to condense water out of thin air, so particles known as cloud condensation nuclei (CCN) act as a substrate for water to condense onto
Deposition Or, How to Sink the Aerosol Recall clouds are not pure water. Two ways to deposit with clouds: 1. Rainout: particles used as CCN are removed when drops precipitate 2. Washout: particles are scavenged below the cloud by falling drops, gases dissolve within drops Basically serves to clean the air
Deposition Without rain, particles deposit by gravitational sedimentation (r > ~1µm) Many nutrients (Ca, Na, K, Mg, and especially P) deposited by dryfall over land. Desert dust major nutrient source for oceans (Sahara, Chinese duststorms) Vegetation absorbs some N- and S- containing gases directly
References Schlesinger, Chap. 3 (online) Charlson et al., Earth System Science, Chap. 7 (maybe online soon?) Wallace and Hobbs, Atmospheric Science: An Introductory Survey Take EART121: The Atmosphere!!!