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.

9

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:

10

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

qu

en

cy