lecture 12 aosc 434 air pollution russell r. dickerson
TRANSCRIPT
LECTURE 12
AOSC 434
AIR POLLUTION
RUSSELL R. DICKERSON
HYDROCARBRONS REACTIVITYFOR URBAN SMOG (OZONE) FORMATION
HYDROCARBON k(O) k(O₃) k(OH)
(All units: cm³s ¹)⁻
Methane, CH₄ 1.1x10⁻¹⁷ SLOW 7.9x10⁻¹⁵
Ethane, C₂H₆ 9.6x10⁻¹⁶ SLOW 2.7x10⁻¹³
Propane, C₃H₈ 1.5x10⁻¹⁴ SLOW 1.2x10⁻¹²
Butane, C₄H₁₀ 3.1x10⁻¹⁴ SLOW 2.3x10⁻¹²
Hexane, C₆H₁₄ 9.5x10⁻¹⁴ SLOW 5.7x10⁻¹²
2,3 Dimethyl butane (C₆H₁₄)
2.1x10⁻¹³ SLOW 6.3x10 ⁻ ¹²
Ethene, C₂H₄ 8.4x10⁻¹³ 1.8x10⁻¹⁸ 8.0x10⁻¹²
Propene, C₃H₆ 3.6x10⁻¹² 1.1x10⁻¹⁷ 2.5x10⁻¹¹
Benzene, C₆H₆ 1.6x10⁻¹⁴ SLOW 1.2x10⁻¹²
Toluene, C₇H₈ 5.9x10⁻¹⁴ SLOW 6.4x10⁻¹²
Faster rate constant implies more reactivity and more smog (O₃) produced. For detailed mechanism see “Development of Ozone Reactivity Scales for Volatile Organic Compounds” by W.P.L. Carter, EPA-91:epavoc, 1991.
Rates increase with increasing number of C atoms, with branching, and with sites of instauration (double bonds).
Emissions From Autos
HYDROCARBON %
ALKANES 53
ALKENES 16
ALKYL BENZNES 20
ACETYLENE 11
TOTAL 100%
Abstraction by O atoms
Example
Ozone oxidation of alkenes
Example: oxidation of propene (propylene).
OH attack on alkanes
OHHCOHC mnmn 1
2333 CHCHOHOCHCH
OOCHCHOCH
OOCHCHCOHCHCHOOOCHOCHCHCH
23
3232332
23233 CHCHOHCHCHOH
SINKS OF AIR POLLUTANTS
I. RAINOUT/WASHOUTOnly for soluble gases and particles
Lifetime the same as that for water 7 days
Lifetime increases with altitude
II. DRY DEPOSITIONOnly for “sticky” or reactive gases and particles
Rate determined by atmospheric turbulence, chemical and physical properties of both the atmospheric species and the surface, i.e. bare soil, vegetation etc.
III. REACTIONSTransformation to other species, usually by oxidation
OXIDIZING AGENTS SPECIES AFFECTED
a) OH CO + OH → CO₂ + H
NO₂ + OH + M → HNO₃
CH₄ + OH → H₂O + CH₃
SO₂ + OH + M → → H2SO4
CH₃CCl₃ + OH → H₂O + CH₂CCl₃
b) O₃ H₂C = CH₂ + O₃ → Prod
(all other alkenes too)
NO + O₃ → NO₂ + O₂
(Note, this is not a net sink for atmos. NOx
c) HO₂ O₃ + HO₂ → OH + 2O₂
d) O CH₃CH₂CH₃ + O → CH₃CH₂CH₂ + OH
(also other NMHC)
e) O(¹D) N₂O + O(¹D) → 2NO
→N₂ + O₂
H₂O + O(¹D) → 2OH
OTHER PROCESSES
f) hν H₂CO + hν → H₂ + CO
(with O₂) → 2HO₂ + CO
HONO + hν → OH + NO
CF₂Cl₂ + hν → CF₂Cl + Cl
(only in stratosphere.)
continue….
8
Useful technique for calculating fluxes or lifetimes.
•When the atmosphere shows horizontal uniformity, production and loss reduce to a 1 D problem.•This holds when vertical gradients are much greater than horizontal gradients and when the species X is in steady state.•Let z be altitude (m), F flux (g m-2s-1), [X] concentration (g/m3), k’ the pseudo first order rate constant (s-1) for loss of X, is lifetime of X.
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Example for fertilized soil NO emissions:
• We want to know the emission rate.• We have the NO profile at night; this only works at night. • NO goes from 20 g/m3 at the surface to essentially zero at 100 m with a scale height of 10 m.• The column content is therefore
10m*20x10-6g m-3 = 2x10-4 g m-2
• We know ozone is roughly constant at 50 ppb, therefore at RTP the lifetime is ~100 s. More generally, you can integrate with [O3](z) and k(z).• If is a constant then k’ is a constant:
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Example for crop soil NO emissions, continued:
11
Example: What is the lifetime of SO2 over the eastern US?
The flux is monitored.
Figure IIa
SO2 Emissions (tons/day)
0-20
20-75
75-150
150-300
300-500
Locations of flights made with aircraft (shown with black airplanes). Location of power plants emitting SO2 shown in pink circles (size of circle represents size of emissions for July 13, 2002).
Lifetime of SO2 over the eastern US. See Lee et al., (2011).
0
5
10
15
20
25
30
35
0 10 20 30 40 50 60
SO2 lifetime (hours)
Fre
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cy