METO 637 Lesson 13 Air Pollution Air Pollution

METO 637 Lesson 13

Air Pollution

Air Pollution

Air Pollution

The Troposphere • In the Stratosphere we had high energy photons so that oxygen atoms and ozone dominated the chemistry. • In the troposphere we have lower energy photons, and the chemistry is dominated by the OH and NO 3 radicals. • OH is generated photochemically (i. e. only during the day), NO 3 is rapidly photolyzed during the day, so it can only survive at night. O(1 D) + H 2 O → OH + OH : NO 2 + O 3 → NO 3 + O 2 • NO 3 is generally less reactive then OH, its peak concentration is higher. • OH provides an efficient scavenging mechanism for both natural and anthropogenic trace constituents

Dry and Wet Deposition • Dry deposition – removal of gases and particles by a direct transfer from the atmosphere to the surface. • Wet deposition – removal of gases and particles carried to the surface in water – rain, snow, fog etc. • Dry deposition is known for SO 2, O 3, CO 2, and SO 3. • Wet deposition of gaseous species requires that they be water soluble. Terms used are rainout, or washout. • Acid rain is an example of the rainout of sulfurous and nitric acids, produced in polluted atmospheres.

Dry Deposition • Three separate steps (1) Species must be transported close to the surface (2) Species must cross to the surface (3) Species must be taken up on the surface. • Dampness of the surface can be critical for some species, e. g. SO 2, O 3, HNO 3

Wet deposition • Wet deposition – incorporation into falling precipitation (washout), or into cloud droplets (rainout). • Chemical conversions can occur within the droplets.

Dry and Wet Deposition

Boundary Layer • Lower limit of the troposphere is seen as a layer and not the surface – known as the boundary layer • As the winds flow across the surface they undergo friction – which generates turbulent eddies in the atmosphere. • This region of high turbulent mixing is generally confined to altitudes up to 0. 5 – 2 km in the day, and lower at night • The region above the boundary layer is known as the free troposphere • In the free troposphere the kinetic energy is associated with large scale disturbances – periods of a day or more. • In the boundary layer the periods are of the order of minutes

Hadley Cell

GLOBAL CIRCULATION • George Hadley first suggested in 1735 the general concept of atmospheric circulation. • He used a single cell, to explain the existence of the easterly winds at the surface • Cold air at the pole - high pressure at the surface. Warm air at the equator - low pressure at the surface. Pressure gradient force at surface will move air from pole to equator at surface. Return path at high altitudes. • The action of the Coriolis force on this southerly flow at the surface will produce easterly winds. • But we find westerly winds at mid-latitudes – why?

GLOBAL CIRCULATION • In reality the atmosphere cannot maintain a single cell, instead there are three cells, with boundaries at about 30 and 60 degrees latitude. • This results in sinking air at 30 N and 30 S. But sinking air suppresses cloud development and precipitation. Hence most of the worlds deserts occur along these latitudes. • At mid-latitudes the prevailing wind in the free troposphere is westerly (mid-latitude westerlies) • Between the equator and 30 degrees we get easterlies – trade winds. • The free troposphere also has lower temperatures than at the surface – reaction rates are slower, and pollutants can travel large distances before they are removed chemically.

Fig. 7. 6

Column tropospheric ozone