The AtmosphereAtmospheric structureTropopause altitudePlanetary boundary layer (PBL)Atmospheric pressureThe Standard AtmosphereAtmospheric compositionTrace constituentsCO2 concentrationsThe Ozone LayerSlide 11Slide 12Atmospheric circulationOzone holeOzone is not just in the stratosphere..Atmospheric stabilityAdiabatic coolingSlide 18Slide 19Slide 20Slide 21Water in the atmosphereTemperature inversionsPollution trappingRadiosondesRadiosonde soundingsStüve diagramsSkew T-log p--ExampleWind barbsSlide 31Radiosonde sounding – Green BaySlide 33Ideal Gas LawIdeal gasesQuantification of gas abundancesSlide 37Vertical profile of ozoneConversion of abundance unitsAbundance units for trace gasesUnit conversion exampleColumn densityDobson UnitsSlide 44Slide 45Lifetimes of trace gasesThe AtmosphereAtmospheric structure• Atmospheric layers defined by changes in temperature• Troposphere – contains 75% of atmospheric gases; temperature decreases with height• Tropopause – boundary between troposphere and stratosphere; location of the jet stream• Tropopause altitude varies from ~8 km (Poles) to ~17 km (Tropics)• Stratosphere – contains the ozone layer, which causes the temperature to increase• Thermosphere: highly energetic solar radiation (UV, X-rays) absorbed by residual atmospheric gasesTropopause altitude• Tropopause altitude is dependent on latitude – it is highest in the tropics where convection is strong• The tropopause is not a ‘hard’ boundary – it can be defined thermally, dynamically or chemicallyCumulonimbus cloud over Africa(photo from International Space Station)Planetary boundary layer (PBL)• The PBL is the lowest part of the atmosphere – directly influenced by contact with the planetary surface• Responds to changes in surface forcing rapidly (hours)• Quantities such as flow velocity, temperature, moisture show rapid variations (turbulence) and vertical mixing is strong• PBL winds are affected by surface drag, as opposed to winds in the ‘free troposphere’ above which are determined by pressure gradients PBL height300 m – 3 km• Influenced by convection• Varies diurnallyAtmospheric pressure• Atmospheric pressure is the weight of the gases surrounding the earth. It is a function of height, density and gravity. • Energy (motion) at the molecular level creates atmospheric pressure and prevents the atmosphere from collapsing on itself• At ground level it is recorded as 101.32 kilopascals (kPa) ; equal to 14.7 lbs. per sq. inch or 760 mm Hg (also 1 atmosphere, 1 bar, 1000 millibars etc.)• Atmospheric pressure decreases exponentially with altitude: at 18,000 ft. (~6 km) it is halved and at 33,000 ft., (~11 km) quartered • Note that in water atmospheric pressure doubles at at a depth of 33 ft€ h =RTglnP1P2 ⎡ ⎣ ⎢ ⎤ ⎦ ⎥Hypsometric equationh = layer thickness (m)R = ideal gas constant (8.314 J K-1 mol-1)T = temperature (K)g = gravitational acceleration (9.81 m s-2)P = pressure (Pa)The Standard Atmosphere• Standard (or model) atmospheres facilitate comparison of radiative transfer models• They represent ‘typical’ atmospheric conditions for a particular region/season• Used whenever an actual sounding (measurement of the atmospheric state) is not available• At least 7 standard model atmospheres are in common use: tropical (warm, humid, high tropopause), midlatitude summer, midlatitude winter, subarctic summer, subarctic winter, arctic summer and arctic winter (cold, dry, low tropopause)Atmospheric compositionNitrogen (N2) 78% (780,840 ppmv)Oxygen (O2) 21% (209,460 ppmv)Argon (Ar) 0.93% (9340 ppmv)Carbon dioxide (CO2) 0.04% (383 ppmv)Neon (Ne) 0.002%Helium (He) 0.0005%Methane (CH4) 0.0001%Krypton (Kr)Hydrogen (H2)Nitrous oxide (N2O)Ozone (O3) 0-0.07 ppmvWater vapor (H2O) 1-4% at surfaceppmv = parts per million by volume = volume mixing ratioComposition of dry atmosphere, by volumeTrace constituentsSome atmospheric trace gases of environmental significanceCO2 concentrationsMeasurements of atmospheric carbon dioxide at Mauna Loa Observatory, Hawaii (Keeling curve)The Ozone Layer• The stratospheric ozone layer is a consequence of molecular photodissociation• UV-C radiation dissociates molecular oxygen:O2 + hv (λ < 0.2423 µm)  O + O• The large amount of oxygen in the atmospheric column absorbs most solar radiation at λ < 0.24 µm by this mechanism• The free oxygen atoms from the above reaction then combine with other O2 molecules to produce ozone:O + O2  O3• Ozone is then dissociated by UV radiation:O3 + hv (λ < 0.32 µm)  O + O2• Ozone is also destroyed by this reaction:O3 + O  O2 + O2The Chapman ReactionsThe Ozone Layer• Fortunately for life on Earth, ozone absorbs strongly between 0.2 and 0.31 µm via electronic transitions – removing most UV-B and UV-C not absorbed by O2• UV-A radiation (λ > 0.32 µm) is transmitted to the lower atmosphere• Plus a small fraction of UV-B (0.31-0.32 µm) – responsible for sunburn • Widening of this UV-B window (due to ozone depletion) would have serious impacts on life• Absorption of solar radiation by ozone also locally warms the atmosphere to a much higher temperature than would be possible if ozone was absent – hence the increase in T in the stratosphere• Hence in an atmosphere without free oxygen, and hence without ozone, the temperature would decrease with height until the thermosphere. There would be no stratosphere, and weather would be vastly different...The Ozone Layer• Most of the ozone production occurs in the tropical upper stratosphere and mesosphere, but the ozone maximum occurs at mid-latitudesAtmospheric circulationOzone holeAntarctic ozone hole on Sept 11, 2005Observed by Ozone Monitoring Instrument (OMI)• Ozone destruction peaks in the Spring, as UV radiation returns to the polar regions• Catalyzed by the presence of CFC compounds (which supply chlorine), and by polar stratospheric clouds (PSCs) at very cold temperaturesOzone is not just in the stratosphere..• The UV-A radiation that reaches the troposphere is a key player in tropospheric chemistry• Photochemical reactions involving unburned fuel vapors (organic molecules) and nitrogen oxides (produced at high temperatures in car engines) produce ozone in surface air (tropospheric ozone)• Ozone is good in the stratosphere, but a hazard in the troposphere (it is a strong oxidant that attacks organic substances, such as our lungs)• Ozone is a major