Environmental Chemistry

Tropospheric Chemistry What are the primary air pollutants in the troposphere and their effects? What are the sources of these species? What are considered safe levels of each pollutant, and how do measured concentrations compare? Where are exceedances of the regulations common? What are the trends in these concentrations? Are they increasing or decreasing and why? What is the difference between ‘classical smog’ and ‘photochemical smog’? What is the chemistry that leads to the production of ‘photochemical smog’? What atmospheric conditions promote formation of smog? Based on the chemistry, what are some effective methods of controlling photochemical smog? What are some other environmental effects of atmospheric emissions?

Deposition processes Not all emitted material stays in the atmosphere Dry deposition – removal of reactive or soluble species directly from the gas phase by contact with surfaces at ground level Wet deposition – removal of soluble species from the atmosphere by loss of fog, cloud or rain droplets to the surface

SO2 and NO2 revisited US Standard (Atmospheric Concentration) SO2 0.14 ppm (24-hr) NO2 0.053 ppm (24-hr) Deposition limits (suggested) SO2 20-40 mmol/m2 yr (wet and dry deposition) NO2 40-80 mmol/m2 yr (wet and dry deposition)

SO2 emissions trend SO2 emissions are declining in the US and Europe But increased emissions in Asia have led to a renewed global increase after a decade of decline https://www.pnnl.gov/science/highlights/highlight.asp?id=907

SO2 emissions https://www.eia.gov/todayinenergy/detail.php?id=4410

What happens to SO2? SO2 - rapid dry deposition (less water soluble) SO42-, H2SO4 - almost entirely as aerosols (very soluble) Gas phase oxidation: Aqueous phase (cloud drops) oxidation:

Simple Acid Rain Model O3, OH, H2O2 H2O Dissolution Oxidation SO2 H2SO4 2H+ + SO42- SO2 Dry Deposition Wet Deposition after Parks (1985)

What happens to NOx? NOx can be dry deposited to the earth or can be oxidized in the atmosphere Oxidation product HNO3 is a highly soluble acid  wet and dry deposition Gas phase reactions (hours): Aqueous phase reactions (cloud/rain drops):

Simple Chemical Model O3, OH, H2O2 H2O Dissolution Oxidation NO2 HNO3 Dry Deposition Wet Deposition after Parks (1985)

Acid rain https://www.ec.gc.ca/eblt-tlws/default.asp?lang=En&n=64935A88-1

Sulfate and nitrate deposition

Rainwater pH trend pH of pure water? Natural rainwater pH ~5.6 Why?

Where do emissions go?

Acid Rain: The Problem 1950-1970’s: pH: dropped in natural waters Fish populations: showed signs of stress and dropped Ecosystems altered: e.g. distribution of producers and consumers Leaching of Ca2+ and Mg2+ (base cations) Mobilization of Al3+: from soils at lower pH Figure 7.1. Lake trout taken from Lake 223 in 1979 when the pH was 5.6 (A), and one taken in 1982 when the pH was 5.1 (B). Based on work described by Schindler et al. (1985). Photographs supplied by K. Mills and D.W. Schindler. Gradual changes occurring over 100’s of years had been missed

Acid Rain Effects Water Land Vegetation Others Of all acidic lakes (larger than 10 acres), 75% are acidic because of acid rain. This is true for 50% of acidic streams as well. Most effected lake areas are in mountainous regions. Effected streams are those which run over thin soil. Land Areas of thin soil (Northeast United States) display more effects from acid rain as there is little or no buffering of the acid. Nutrients are destroyed and toxic metals are released in acidic soil. Vegetation The leaves of trees and plants are stripped of nutrients by acid rain. Most effected are trees at high elevation which are constantly surrounded by acidic clouds. Others Metals, paints and stone exposed to acid rain are corroded. $61 million is spent each year on coating new cars and trucks sold in the US to prevent possible damage by acid rain. Sulfate particles account for 50 to 70 percent of the visibility reduction in the eastern part of the United States www.epa.gov

pH of lake water as a function of time

Aquatic organisms The diversity of an acidified body of freshwater drops significantly. Soft bodied animals like leeches, snails, and crayfish die with a very little change in acidity, which is often an indicator of acidification. A decrease in pH is often paired with an increase in toxic metals like aluminum and mercury. A decrease in pH and elevated aluminum concentration will increase fish mortality, decrease fish growth, decrease egg production and embryo survival, and result in physiological impairment of adult fish. Aluminum in the water can precipitate onto fish gills, which would inhibit diffusion and result in respiratory stress. Acid rain is extremely detrimental to amphibian populations. Most amphibians lay their eggs in small, shallow ponds which receive most of their water from rainfall. A very small amount of acidic rainfall would kill any embryos in these small ponds.

Lake effects

Soil effects

Soil effects Soils lose metal cations in sequence as acid is added The ability of soils to neutralize some or all of the acidity of acid rainwater is called “buffering capacity.” Total buffering capacity depends on the soil composition

Al Speciation

Al Concentration as a function of pH Al in fish gills inhibits Na and Ca transport - effects gas transport Al also precipitates in the gills as Al(OH)3 (s)

Role of soil composition High buffering capacities lead to soils that do not become acidified. Midwestern states like Nebraska and Indiana have thick, alkaline soils that are well buffered. Alkaline Soils neutralize the acid directly and have the highest buffering capacity. Granite soils and volcanic rocks are not good at neutralizing acid. Places like New York‘s Adirondack and Catskill Mountains, have thin soils with low buffering capacity.

Damage To Trees Acidic water dissolves the nutrients and minerals in the soil and washes them away before trees and plants can absorb them out of the ground for use. Acid rain releases toxic substances such as aluminum into the soil which in very small amounts are very harmful to trees. Trees high up in the mountains are more at risk to receive acid, from acidic clouds and fog. Acid eats away at the waxy protective coating on the leaves. After this occurs, the leaves cannot perform photosynthesis and the trees are left unhealthy, weak, and usually die from disease or from insect attacks

Sulfate and nitrate deposition

Rainwater pH trend