Composition and Structure The Atmosphere Composition and Structure

Outline Intro to air pollution Atmospheric Composition Measures of concentration Concentration of gases Concentration and composition of PM Structure of the Atmosphere Thermal stratification Characteristic vertical and horizontal mixing times Spatial variability of atmospheric composition Light Nature of light Interaction of light and matter Sunlight and its propagation through the atmosphere

Problems due to Air Pollution Question What are the major problems due to chemicals discharged into the atmosphere? List them. Stratospheric ozone depletion (due to CFCs, HCFCs, etc) Global climate change (due to GHGs, etc) Acid deposition (SO2, NOx) Smog (VOCs, NOx) Particulates (PM, especially “fine PM”) Other toxic air pollutants (eg, CO, Pb, Hg, PAHs and other toxic organics, etc)

Lecture Question What are the criteria pollutants? Carbon monoxide, CO Nitrogen dioxide, NO2 Ozone, O3 Lead, Pb Particulates, PM10 and PM2.5 Sulfur dioxide, SO2 What are the four most abundant components of dry air at sea level? Nitrogen (N2), oxygen (O2), argon (Ar), carbon dioxide (CO2)

Atmospheric Composition Chemical Species Concentration Source N2 78.08% volcanic, biogenic O2 20.95% biogenic H2O (gaseous) up to 4% (avg ~2.5%) volcanic, evaporation Ar 0.93% radiogenic CO2 0.037% (370 ppmv) volcanic, biogenic, anthropogenic Ne 18 ppmv volcanic (possibly) He 5.2 ppmv Kr 1 ppmv CO 50 – 200 ppmv biogenic, anthropogenic, photochemical CH4 1.7 ppmv biogenic, anthropogenic NMHC 5 – 20 ppbv CH2O 0.1 ppbv photochemical N2O 310 ppbv NH3 0 – 0.5 ppbv NOx biogenic, anthropogenic, lightning OCS 0.5 ppbv H2S SO2 0.01 – 1 ppbv volcanic, anthropogenic, photochemical DMS 0.01 – 0.1 ppbv all concentrations are in dry air (except, obviously, for H2O)

Measures of Concentration Relative concentration: volume mixing ratio (VMR) What is it? The mole (or volume) fraction. Units: %, ppmv, ppbv, etc Easy to understand Constant with altitude for inert gases like N2

Measures of Concentration Lecture Question List the current atmospheric concentration of CO2 in air in units of (a) % and (b) ppm. Answer in book: 375 ppm = 0.0375% by volume. According to NOAA (Jan 2007): Current average CO2 concentration (at Mauna Loa) is 383 ppm 383 ppm = 0.0383% by volume.

Measures of Concentration Absolute concentration Typical units Mass/volume (eg mg/L) Number density (cm-3), particularly for low concs Pressure units (torr, atm, bar, etc) Note: diameter of the Earth is 6400 km.

Particular Matter Concentration Background PM: 300/cm3

PM Mass Concentration Background PM: 1 mg/m3 Typical rural conc: 5 ug/m3 Typical urban conc: 30 ug/m3 Polluted urban conc: 100 ug/m3

Thermal Stratification of the Atmosphere Lecture Question List the major regions (layers) of the atmosphere, along with the typical altitudes for each region. Troposphere (0 – 15km) Upper limit (the tropopause) varies between 9 – 16 km depending on lattitude and season Stratosphere (15 – 50 km) Contains the stratospheric ozone layer, which (mostly) shields us from harmful uv light Mesosphere (50 – 100 km) Thermosphere (above 100 km) Above 60 km is the ionosphere, where there is a significant concentration of ions and electrons

Thermal Stratification of the Atmosphere Troposphere is heated by the ground Stratosphere and mesosphere are directly heated by ozone chemistry (solar energy) Tropopause varies in height (9–16 km) depending on latitude and season Vertical mixing in troposphere is rapid, but stratosphere is fairly stagnant Lower 1 – 3 km called the planetary boundary layer (PBL), which is rapidly mixed and often topped by a local inversion the tropopause is defined with the atmospheric lapse rate falls below 2 K/km. the thermosphere is heated mostly by absorption of O2. Temps are high because few atoms/molecules to spread the thermal energy around. Not in thermodynamic equilibrium. the homosphere includes the troposphere, stratosphere & mesosphere; it is the region where bulk mixing is complete, where molecular diffusion is not significant. Above the homosphere is the heterosphere, where there is some separation due to mass (ie heavier molecules settle more than ligher ones). The region above 500 km is the exosphere, where escape into space is significant for the lighter atmospheric consituents (eg hydrogen and helium. Above 60 km is the ionosphere, the region of the atmosphere above which there is some significant concentration of ions and electrons. It is further subdivided into D, E and F layers. It enables bouncing radio waves around the Earth’s curved surface.

Time Scales of Vertical Mixing Vertical mixing is due mostly to adiabatic heating/cooling of air parcels, as well as turbulent mixing from horizontal winds.

Time Scales of Horizontal Mixing Horizontal mixing is driven by pressure differences due to uneven heating by the Sun. MIxing along latitutdes (zonal mixing) is very fast; mixing along longitudes within a hemisphere (meridional mixing) is also pretty fast. Interhemispheric exchange is much slower.

Mixing vs Removal Two competing processes Effect on atmospheric composition Depends on relative rates of mixing and removal Species with higher rates of removal are more concentrated near their sources Removal mechanisms Water scavenging Dissolution into suspended water droplet (or water bodies on ground) Rate depends on solubility Reaction/decomposition Rxn with a reactive species (often OH) Photodissociation: absorption of light, followed by breaking of bond Adsorption to solid surface Either PM or surface on the ground Mixing mechanisms Buoyant mixing (vertical) Atmospheric circulation – ie, wind (horizontal)

Light: Electromagnetic Radiation Plays a critical role The vast majority of our energy arrives as sunlight Drives global circulation of atmosphere and water Many air pollution problems directly involve light Ozone depletion Global climate change Photochemical smog Drives atmospheric chemistry Through photodissociation Through generation of reactive species (OH, Cl, NO3, O3)

Light: Electromagnetic Radiation What is light? Light acts like a wave With oscillating electric and magnetic fields Can propagate through a medium or through vacuum animation here Light also acts like a particle Localized energy packet: a photon Massless...but has momentum Lecture Question What is the relationship between wavelength, frequency, and the speed of propagation?

Light: Electromagnetic Radiation Lecture Question What is the relationship between photon energy, wavelength, and frequency? Photon energy Is the minimum energy available in its interaction with matter

The Electromagnetic Spectrum increasing energy decreasing energy

The Electromagnetic Spectrum Can further subdivide the UV region UV-C is 200-280 nm (most energetic, completely blocked by ozone layer) UV-B is 280-320 nm (ground-level UV-B intensity most affected by ozone depletion) UV-A is 320-280 nm (least energetic, almost all gets through the ozone layer)

Incident Sunlight Top & Bottom of Atmosphere Lecture Question What fraction of incident sunlight is in the uv, visible and ir regions? 8% ultraviolet 39% visible 53% infrared