ATM 507 – Lecture 2
Fall 2012
• Reading assignment – Seinfeld and
Pandis, Chapter 2
• Other references
• Problem Set #1 – due Sept. 11
• Topics – Various concepts and definitions
are introduced for later use and reference
Outline
• In this lecture we will jump around a lot –
introducing a number of concepts useful to
the study of atmospheric chemistry. They
may not seem to “fit together” today, but
the ideas will return as the semester
progresses. It is essentially a set of
definitions to use as the course unfolds.
You may want to refer back to these
charts and tables later in the semester.
Ideal Gas Law
• Chemists write it as pV = nRT
– p = pressure in atmosphere
– V = volume in liters
– n = number of moles
– R = Gas Constant
– T = temperature in degrees Kelvin
• R has the value 0.08206 L•atm•mole-1•K-1
• Concentration has units moles/L or M
Ideal Gas Law (cont.)
• For a meteorologist – P = RT
– P = pressure in Pascal
– = concentration in moles/L
– R = Gas Constant = 8.31 J•mole-1•K-1
• For a physicist – p = nkT
– p = pressure in dyne•cm-2 (1 dyne•cm-2 = 0.001
millibar)
– n = concentration in molecules*cm-3
– k = Boltzmann constant = 1.38x10-16
erg•K-1 •molecule-1
Pressure Conversions
1 atm =
1013 mbar
101,300 Pascal (Newton m-2)
1,013,000 dyne cm-2
760 torr (mm Hg)
14.69 psi (pounds per sq. inch)
29.92 inches Hg
Concentration Units
• Absolute units:
– Moles per unit volume (i.e. moles L-1)
– Number per unit volume (i.e. molecules cm-3)
• Relative units (fraction of total):
– %, ppm, ppb, ppt (i.e., ppmv, ppbv, pptv)
– Mole fraction
– Mass Fraction, esp. ppmm
• Mass units:
– g L-1, µg m-3
Why so many units?
• Historical reasons
• Absolute units are preferred for
calculations, but watch the T and p
dependence (next slide).
• Relative units are more convenient when
looking at concentration profiles. They
scale with pressure (& T) and give a truer
sense of the change in concentration as a
function of height.
Conversion of Concentration Units
• Conversion from absolute to relative units (or vice versa)
requires knowledge of the temperature and pressure.
This can be confusing and requires attention to detail!
• 1 mole = 6.02x1023 molecules (Avogadro’s number)
• 1 m3 = 1000 L = 106 cm3
• Use the Ideal Gas Law (p/RT = n/V = C) to find
concentration if pressure and temperature are known.
• At 1 atm, 273 K, CM = 4.46x10-2 mole/L. (If this is
inverted, the STP molar volume is 22.42 L/mole.)
• To obtain number concentration from molar
concentration, multiply by Avogadro’s number and divide
by 1000:
• Cn = 2.69x1019 molecules cm-3 (Loschmidt’s number at
STP)
Example
• How does a number concentration at one
pressure and temperature convert to
another temperature and pressure?
• pV/(nRT) = p/(CRT) = constant
• p1/(C1RT1) = p2/(C2RT2), rearrange
• C2 = C1(T1/T2)(p2/p1)
Conversion of Concentrations at 1
atm and 298 K
1 atm, 298 K
1 ppb
1 ppt
1 torr
4.09x10-2 mole L-1
2.46x1019 molecule cm-3
106 ppm, 109 ppb, 1012 ppt
4.09x10-11 mole L-1
2.46x1010 molecule cm-3
4.09x10-14 mole L-1
2.46x107 molecule cm-3
133.3 Pa or 1.333 mbar
3.24x1016 molecules cm-3
1.32x106 ppb
Chemical Families
•
•
•
•
•
•
Oxygen
Hydrogen
Nitrogen
Sulfur
Halogens (Chlorine, Bromine, etc.)
Carbon and Hydrocarbons
“Simplest” Chemical Families
• Oxygen
– O2(almost all), O3, O atom, O2 dimer (O2*O2 or
O4)
– Text and other sources don’t even list the oxygen
“family”
• Hydrogen
– H2 and H2O (stable molecules), and OH (HO) and
HO2 (free radicals), H2O2
– Hydrogen bonds readily with many other species
– most notably carbon, to form, in this case,
hydrocarbons
Nitrogen Family (Major Species)
N2
Nitrogen
NO3
N2O
Nitrous oxide
N2O5
Nitrogen trioxide or
Nitrate radical
Dinitrogen
pentoxide
NO
Nitric oxide
HONO
Nitrous acid
HNO3
Nitric acid
NO2 Nitrogen dioxide
NOX
= NO + NO2
HO2NO2
Pernitric acid
NOY
= NOX + all
oxidation prod.
CH3C(O)
O2NO2
Peroxyacetyl
nitrate or PAN
NOz
= NOY - NOX
NH3
Ammonia
Nitrogen Family (Major Species)
N2
Nitrogen
NO3
N2O
Nitrous oxide
N2O5
Nitrogen trioxide or
Nitrate radical
Dinitrogen
pentoxide
NO
Nitric oxide
HONO
Nitrous acid
HNO3
Nitric acid
NO2 Nitrogen dioxide
NOX
= NO + NO2
HO2NO2
Pernitric acid
NOY
= NOX + all
oxidation prod.
CH3C(O)
O2NO2
Peroxyacetyl
nitrate or PAN
NOz
= NOY - NOX
NH3
Ammonia
Nitrogen Family (Major Species)
N2
Nitrogen
NO3
N2O
Nitrous oxide
N2O5
Nitrogen trioxide or
Nitrate radical
Dinitrogen
pentoxide
NO
Nitric oxide
HONO
Nitrous acid
HNO3
Nitric acid
NO2 Nitrogen dioxide
NOX
= NO + NO2
HO2NO2
Pernitric acid
NOY
= NOX + all
oxidation prod.
CH3C(O)
O2NO2
Peroxyacetyl
nitrate or PAN
NOz
= NOY - NOX
NH3
Ammonia
Sulfur Family
Reduced Sulfur compounds
Oxidized Sulfur compounds
Halogens (Esp. Chlorine & Bromine)
• “Non-reactive” forms: fully halogenated
hydrocarbons like Freons (CFCl3 or
CFC-11; CF2Cl2 or CFC-12), or CCl4; SF6
• Slow reacting and natural forms: CH3Cl,
CH3Br, CH3CCl3, CHCl3
• More reactive forms: Cl2, HOCl, HOBr,
BrCl, ClONO2, BrONO2, HCl
• Free radicals (very reactive forms): Cl,
ClO, Br, BrO
Carbon Compounds
• Here’s where things can get really out of hand!
• Reduced forms of carbon (next slides) include
fossil fuels and natural and man-made
emissions
• NMHC = Non-methane hydrocarbons
• These reduced forms are quickly oxidized in
combustion and slowly oxidized in the
atmosphere
• Partially oxidized forms include carbon
monoxide (CO), formaldehyde (HCHO) and
other oxygenated hydrocarbons
• Fully oxidized carbon is carbon dioxide (CO2)
Hydrocarbons
Hydrocarbons (cont.)
Oxygenated Hydrocarbons
Carbonyls (-C=O), Alcohols (-C-OH),
and Organic Acids (-C=O-OH)
Particulate Matter
• Complex mixture of phases, sizes and
composition
• Table 2.18 lists terminology used for
atmospheric particles
• Often measured in mass concentration units of
µg m-3
• Different size fractions behave rather differently
• Topic of intense research
• More (much more) later!
Atmospheric Structure (review)
Time and space scales for
atmospheric processes
• Domains of influence (overlapping spatial
scales)
–
–
–
–
–
Microscale
(Urban or Local scale)
Mesoscale (10’s to 100’s km)
Synoptic scale (100’s to few 1000’s km)
Global scale
• Transport or mixing times in the troposphere
– Zonal (E-W) – weeks
– Hemispheric (N-S cross equatorial) – 6-12 months
– Vertical – days (BL) to months (FT) to years (cross
tropopause)
• Chemical lifetimes – seconds to centuries
Spatial and Temporal Scales
Spatial Scales (Section 1.7)
Chemical Lifetimes
• Chemical Reaction A + B → C + D;
rate constant k
• [A] = concentration of species A, etc.
• -d[A]/dt = k [A] [B]
• Assume [B] = constant (or nearly so) and that other
reactions involving A are negligible, solve for [A]
• -d[A]/[A] = k [B] dt; integrate to obtain
• ln [A] (evaluated at [A] and [A]0) = k [B] t
• [A] = [A]0 e-k [B] t or [A]/[A]0 = e-k [B] t
Chemical Lifetime (cont)
• By definition, the lifetime of a species is the time
it takes for its concentration to decrease to 1/e of
its initial concentration
• [A]/[A]0 = e -1 or k [B] t = 1, t = τCHEM
• τCHEM = 1/(k [B])
• This is the lifetime of A wrt constant [B]
• If τCHEM << τTRANS (τMIX), then the species is
“chemically controlled”
• If τCHEM >> τTRANS, then the species is
“dynamically (or transport) controlled”
Chemical Composition of the
Atmosphere
• Major constituents are
– Outgassed from the mantle
– Radioactive decay products
– Photochemically produced
– Biologically produced
• Trace gases are generally most reactive –
we will focus on these
Table is from ~1986 - CO2 is now ~390 ppm, and CH4 is now ~1.9 ppm, etc
Clean and Polluted Concentration Levels of
Chemically Active Trace Gases
Boundary layer concentrations in ppbv
Species
SO2
CO
NO
NO2
O3
HNO3
NH3
HCHO
NMHC
Remote
0.020
120(N), 60(S)
0.02 – 0.04
0.02 – 0.04
5-10 (or less)
0 – 0.1
0 – 0.5
0 - .5
2 – 20
Rural
0.2 – 2
120-400
0.2 – 2
0.2 -10
20 – 80
0–5
0–5
0.1 – 5
50 – 100
Polluted
5-50
1000- 10,000
10-100
10 – 500
0 – 300
1- 30
1 – 10
20 - 50
100 - 1000