Air Chemistry GISAT 112

Scientific and Technical Concepts Phases of airborne matter- gases, particles Inorganic and organic chemicals Balancing chemical equations, stoichiometry Combustion reactions Exposure, toxicity, and health risk Atmospheric and weather terms

Types of Matter Gases, liquids, solids Air pollutants –Gases –Particulate matter Can be either solid or liquid Often a combination of both

Chemistry in Context: Applying Chemistry to Society, 3e. A Project of the American Chemical Society. Copyright © 2000 by the American Chemical Society. All Rights Reserved. Table 1.5 Classification of Matter

The Parts of an Atom Atoms normally have –1 or more protons (+1 charge) –0 or more neutrons (no charge) –1 or more electrons (-1 charge) Chemistry depends on the number of electrons, which in turn depends on the number of protons Thus, the identity of an element is determined by the number of protons in its nucleus

Number of protons Rounded, approximates the “Mass Number” – # of protons and neutrons

Inorganic and Organic Compounds Organic compounds: carbon-based chemicals Some examples- –C 3 H 8 (propane; a hydrocarbon) –C 2 H 5 OH (ethanol; an alcohol) –CH 3 COOH (acetic acid; an organic acid) –C 6 H 4 Cl 2 (dichlorobenzene; a chlorinated hydrocarbon) Inorganic compounds: chemicals other than those based on carbon (although some contain carbon, e.g., as carbonates) Some examples- –NaCl (table salt) –H2O–H2O –SO 2 (sulfur dioxide) –CaCO 3 (limestone) –Fe 2 O 3 (rust)

Reactions and Equations Reactants  Products For example, consider combustion of carbon: C (s) + O 2 (g)  CO 2 (g) Note that matter and number of atoms are conserved!

Characteristics of Chemical Reactions Note: these aren’t necessarily true for nuclear reactions!

Balancing Chemical Equations Since the number of atoms of each element involved is conserved, the equations describing chemical reactions must be “balanced” E.g.: ___ CH 4 + ___ O 2 → ___ CO 2 + ___ H 2 O Hint: Start with the most restricted terms. 1212

Atmospheric and Weather Terms Troposphere: the part of the atmosphere from ground level to km height. Comprises about 80% of the mass of the atmosphere and nearly all of the water vapor, clouds, precipitation, and air pollutants. Stratosphere: the layer of the atmosphere from km above the surface of the Earth. Contains about 20% of the mass of the atmosphere. Few air pollutants get this far (but there are important exceptions). Here is where oxygen and ozone filter out most of the dangerous UV light from the sun.

Atmospheric and Weather Terms (cont.) Plume : the zone downwind of a pollutant source where the concentration of pollutants is higher than background. Atmospheric stability : the level of air velocities due to wind (mostly horizontal movement) and temperature differences (that can create vertical movement). Pollutants tend to stay concentrated in stable air and disperse rapidly in unstable air. Inversion : in the troposphere, the air temperature generally decreases with height above ground. Under inversion conditions, the temperature increases. Inversions are temporary, but they create stable air and cause pollution to become more concentrated while they last.

Supporting Concepts Units of concentration, units conversions Ideal gas law, molar volumes Significant figures

Units for NAAQS ppm, ppb for gases μg/m 3 for particles (and gases) (μg/m 3 = micrograms per cubic meter)

Parts Per Million (or Billion) For gases, the basis is VOLUME –i.e., volume of pollutant per volume of air For example, 1 ppm of a gaseous pollutant is 1 m 3 of pollutant per 1 million (10 6 ) m 3 of air Or, 1 liter of pollutant per million liters of air, etc. Similarly, ppb is per billion (10 9 ) volumes of air

Conversion: % to ppm Multiply both sides by 10,000:

Moles and Atomic Weight A mole of anything is Avogadro’s Number (6.022 x ) of those things. A mole of a chemical is therefore x molecules. –For reference: the universe is thought to have existed for about seconds! The atomic weight of an element is the weight of one mole of those atoms –We can find this on the periodic table –E.g. the atomic weight of carbon is about 12 g/mole

Molecular Weight The molecular weight (sometimes called “formula weight” ) of a chemical substance is the weight of one mole of its molecules in grams. –The molecular weight of water is 18 g/mole. Why?

Molecular Weight The molecular weight (sometimes called “formula weight” ) of a chemical substance is the weight of one mole of its molecules in grams. –The molecular weight of water is 18 g/mole. Why? –One mole of water (H 2 O) is therefore 18 grams of water. –The molecular weight of table salt (NaCl) is 58.5 g/mole. One mole of table salt weighs 58.5 grams.

Ideal Gas Law The Ideal Gas Law describes quite accurately how most gases, under typical environmental conditions, respond to changes in those conditions. PV = nRT P = total pressure (in atmospheres) V = volume (in liters) n = number of moles of the gas R = gas constant ( L·atm/mole·K) T = absolute temperature (in K, which is °C + 273)

Molar Volume The molar volume of an ideal gas at 1 atm and 0°C is 22.4 liters. For more typical environmental conditions of 20-25°C, the molar volume is approximately 24 L/mole.

Unit Conversions: ppm or ppb μg/m 3 For most calculations, we can assume the gaseous pollutant behaves as an ideal gas At typical ambient temperatures (20-25°C) and atmospheric pressure, one mole of an ideal gas occupies about 24 L (22.4 L at 0°C) [molar volume  24 L/mole] One mole of any chemical substance weighs its molecular weight in grams (e.g., MW of benzene = 78; 1 mole = 78g)

Unit Conversions: ppm or ppb μg/m 3 Example: convert 10 ppm of benzene  ? μg/m 3 –MW of benzene = 78 g/mole (or 78 x 10 6 μg/mole) –10 ppm = (10L benzene/10 6 L air) –Similarly, 10 ppb  32 μg/m 3 For more precise calculations and significantly different temperatures or pressures, use the Ideal Gas Law