Chapter Overview:
composition of air
structure of the atmosphere, atmospheric pressure
elements, compounds, mixtures
chemical formulas
atoms, molecules
chemical reactions, equations
combustion, balancing equations
air pollutants
risk assessment
Composition of Air
dry air: 78% N2, 21% O2, 1% Ar, small amounts of other gases
humid air: up to 6% water vapor
carbon dioxide (CO2) 350 ppm ( = 0.035%)
about 4% CO2 and 4% H2O in exhaled air (N2 75%, O2 16%)
relative concentrations pretty constant with altitude but total pressure drops
see Fig. 1.2 for pressure/altitude correlation
rough rule is atmospheric pressure drops by half every 5 km (~ 3 miles)
Minor components of air -- air pollutants
CO, O3, SO2, SO3, NO, NO2 (sometimes called SOx and NOx )
see Table 1.2 for typical concentrations in ppm (sometimes ppb for O3 and NOx )
Classifications of Matter
mixture - a physical combination of two or more substances
the mixture may vary in composition (like air - dry/humid, inhaled/exhaled)
a mixture may be separated physically (but it may not be easy - air is separated by cooling until different components freeze or liquefy)
element - a substance that cannot be broken down into simpler matter chemically
there are 110 elements currently known (about 90 are naturally occurring)
elements are arranged in the periodic table according to nuclear composition
there are standard one - or two-letter symbols for all the elements
you will need to know the most common elemental symbols:
H, C, N, O, F, Na, Si, P, S, Cl, Ar, K, Ca, Fe, Ni, Cu, Zn, Br, Ag, I, Au
compound - a pure substance made up of two or more elements in a fixed composition
a compound can be chemically (but not physically) converted to elements or
other compounds
pure substance: different samples of a compound always behave identically
fixed composition: CO2 means one carbon and two oxygens (carbon dioxide)
CO means one carbon and one oxygen (carbon monoxide)
different fixed compositions means different compounds
there are over ten million different known compounds, and theoretically no limit
why make new compounds? new materials, new drugs
Atoms and Molecules
classifications of matter describe what can be manipulated (the macroscopic level)
the microscopic view (actually submicroscopic) visualizes what the individual
particles are like and why they are different and behave as they do
see Table 1.4, which distinguishes observables and theory
atoms - the smallest individual unit of an element
atoms consist of smaller (subatomic) components - protons, electrons, neutrons
atoms can only be changed by nuclear (not chemical) reactions
atoms are exceedingly small (discuss Take A Breath )
molecules - a combination of a fixed number of atoms held together by chemical
bonds in a well-defined geometry
compounds are made up of molecules
some elements exist as molecules, e.g., N2, O2
nomenclature - compounds and molecules go by the same name, usually derived
from the atomic constituents
know the prefixes in Table 1.5
Chemical Reactions and Equations
chemical changes convert one kind of molecule into another kind
example: combustion of carbon (charcoal)
C + O2 ----> CO2 (a balanced chemical equation)
C + O2 ----> CO (an unbalanced chemical equation)
2 C + O2 ----> 2 CO (balanced)
actual combustion (of any carbon-containing compound) gives both CO and CO2
CO2 is predominant in the presence of excess O2 (most common case)
ratio of CO/CO2 indicates combustion efficiency
combustion of other materials:
S, CH4, C3H8, C8H18, etc.
Air Pollution
comparison data (1975 & 1991), same data are shown in Table 1.7 and Figure 1.6
London smog
caused mainly by coal burning, worsened by a thermal inversion
SO2 - from small amount of sulfur in most coal
particulates - from small amounts of minerals in coal (ash)
effects of catalysts - speed up a given reaction
e.g., oxidation of SO2 to SO3 on ash particles
SO3 is a special problem because it reacts with water to form sulfuric acid
remediation: particulates can be mainly removed by filtration on smokestacks
sulfur oxides remain a problem (Acid Rain, Chapter 6, returns to this issue)
LA smog
caused mainly by combustion in automobiles, worsened by a thermal inversion
lead - from leaded gasoline (helps smooth combustion)
CO - from incomplete combustion
partially oxidized hydrocarbons - from incomplete combustion
NOx - from partial oxidation of nitrogen gas during combustion
ozone - from photochemical reactions involving NOx and O2
catalytic converters - metal catalysts (expensive) convert CO to CO2
remediation: CO decreased strongly due to catalytic converters
lead decreased strongly since all new cars require unleaded fuel
NOx and ozone - still a serious problem (Ozone, Chapter 2)
Air Quality - Risk / Benefit Analysis
risk assessment - statistical evaluation of likelihood of an effect
exposure - concentration in air x amount of air inhaled (Consider This 1.1)
toxicity - statistical effects of a substance on a standard population
studies may be done on humans, animals, or bacteria
(risk of the studies decreases but the relevance also decreases)
Scientific Notation - The Size and Abundance of Molecules
one breath ~ one liter ~ 2 x 1022 molecules
NOx permissible limits 53 ppb (from Table 1.2)
2 x 1022 molecules per breath x 53 molecules NOx per 109 molecules
= 106 x 1013 molecules NOx per breath
= 1.06 x 1015 molecules NOx per breath
~ 1 x 1015 molecules NOx per breath
other examples to illustrate that molecules are very tiny yet very numerous
estimate about 1 million grains of sand in 100 mL ( ~1/2 cup)
then 1 ppm = 1 grain of sand in 100 mL
ozone air standard = 0.12 ppm = 1 molecule in 8 million
one grain of sand in 800 mL (~ 1 quart)
one inch in 125 miles
one breath ~ 1022 molecules
~ 1022 grains of sand in the whole Oregon beachfront
(1 km wide x 1 km deep x 1000 km long)