Acid deposition lecture outline

• Gases (e.g., 0₃)
• Vapors ( e.g., HNO₃)
• Particulates/aerosl
  • Dry (e.g, dust)
  • Liquid (e.g., H₂SO₄)

Sources of atmospheric trace chemicals:

• Natural
  • Volcanoes
  • Sea spray
  • Plants
• Anthropogenic

Once in the atmosphere, the pollutant can be deposited out in either wet or dry form. Wet deposition can be either as rainout or washout

Atmospheric transport of pollutants can occur over short or long distances, depending on atmospheric structure and dynamics, and may depend on:

• Atmospheric boundary-layer stability
• Persistent temperature inverslons
• Geographic constraints
• Recirculations

For acidic deposition/precipitation we consider sulfur and nitrogen compounds

Sulfur compounds:

• Sulfur dioxide SO₂
• Hydrogen sulfide H₂S (converted to SO@ in days to weeks)
• Carbonyl sulfide COS or OCS
• Dimethyl sulfide CH₃SCH₃
• Methyl mercaptan CH₃SH
• Carbon disulfide CS₂
• Dimethyl disulfide CH₃SSCH₃

SO₂ oxidizes to S0₄-- at a rate of up to 4%/hr

Nitrogen compounds

• Nitric oxide NO
• Nitrogen dioxide NO₂
• Nitrous oxide N₂0
• Nitrogen trioxide NO₃
• Ammonia NH₃

Significant acids: H₂SO₄ and HNO₃

pH: measure of acidity

pH is the negative logarithm of the H+ ion concentration

• O<= pH <= 14
• pH = 7 is neutral acidity
• pH < 7 is acidic
• pH > 7 is basic

Natural precipitation is in equilibrium with dissolved CO₂ and has pH = 5.65. Acid precipitation has pH < 5.6

Entrained soil particles in western US tend to make natural pH higher (more basic) whereas entrained soil particles in the eastern US tend to make precipitation acidic. Effects on vegetation

Crops: No evidence of significant agricultural or horticultural damage due to acid precipitation on a large scale. Simulated acid precipitation levels must be high for observing damage.

Forests: Isolated cases of severe tree damage near known high volume sources of SO₂ point to the possibility of significant local damage. Large-scale damage is much more difficult to identify because of complex soil interactions. There is a possible benefit due to nutrient enhancement as well as detrimental effects.

Soils: Controversial. Agricultural soils generally are well buffered and not affected much by typical acidic precipitation levels, compared to acidifying effects of fertilizers and liming practices that reduce acidity. Furthermore, SO₂and N¡_x are plant nutrients and may be beneficial. Forest soils may be more affected.

Water:

• Input of acid substances
• directly from the atmosphere
• indirectly through runoff from watershed
• internal generation of H+ ions within the watershed

A "natural" lake has a natural buffering effect based on its alkalinity which is a measure of the concentration of bicarbonate (HCO^3-) ions. (Water is neutral at pH = 7, has greatest variety of aquatic species between 6 and 7). As acidity increases, alkalinity or buffering capacity decreases. At pH = 5.2, the natural buffering is exhausted.

H + + HCO^3- = H₂0 + CO₂

Lakes in areas of limestone availability are well buffered

Three phases of acidification of a lake

• 1) Alkalinity decreases with only small change in pH as acid rain and runoff enter
• 2) With bicarbonate ions depleted, pH drops; acidic surges occur in spring and fall, some during vulnerable spawning times
• 3) pH stabilizes around 4.5, humus and aluminum react to absorb free hydrogen ions. Aluminum ions increase dramatically, poisoning many organisms. Water becomes clear.

Case study of effects on forests: Schulze, E.-D., 1989: Air pollution and forest decline in a spruce forest. Science 244, 776-784.