vii. tropospheric ozone catalytic cycles terminating the cycles initiating organic chemistry...

VII. Tropospheric Ozone

Catalytic CyclesTerminating the CyclesInitiating Organic ChemistryMeteorology

Ozone

Damages lung – hard on kids, the aged, and

anyone

with bad lungs

Concentrations

40-50 ppbv (continental background)

80 ppbv (EPA 8 hour limit)

120 ppbv (old EPA 1 hour limit)

EPA Map of Peak Ozone Concentration

0-60 ppb

61-79 ppb

80-99 ppb

100-110 ppb

111-124 ppb

125+ ppb

No Data

Source of Ground-Level O3

NO2 + h → NO + OO + O2 → O3 NO + O3 → NO2 + O2

NOx = NO + NO2

NOx + sunlight yields O3

These 3 reactions steady state w/limited [O3]

Add CO and HOx Chemistry

OH + CO → H + CO2

H + O2 → HO2

HO2 + NO → OH + NO2

NO2 + h → NO + O

O + O2 → O3

CO + 2 O2 → CO2 + O3

CO

CO2

OH

HO2NO

NO2

h

OO3 O2

Add Organic Compounds…

OH + CH4 → CH3 + HOH

CH3 + O2 → CH3OO

CH3OO + NO → CH3O + NO2

CH3O + O2 → CH2=O + HO2

HO2 + NO → OH + NO2

2{NO2 + h → NO + O}

2{O + O2 → O3}

CH4 + 4 O2 → CH2=O + 2 O3 + H2O

NOx Concentrations

Urban 5-20 ppbvRural 0.1 – 5 ppbvRemote 0.005 -0.1 ppbv

[HOO] ≈ Sum over [ROO] << [NOx] except for Remote

k(ROO + NO) ≈ k(HOO + NO) ≈ k(ROO + HOO)

So HOO and ROO mostly react with NO

Termination of Cycles

RO

RH + OH, O2 → ROONO

NO2

h

RONO2

Limits NOx Cycling and organic radical chem.

Termination of Cycles at High [NO2]

NO2

RCH3

RCH=O

OH

HO2NO

NO2

h

HONO2

Stops HOx Cycling

Termination of Cycles at low [NOx]

RO

ROO

NO

NO2

h

HOO

ROOH

2 HOO →HOOH + O2

organic radical chem.

HOx cycling

Contour of Maximum [O3]

VOC (ppmC)

NO

x (p

pb

)

[O3]max

160 ppb

120 ppb

80 ppb

NOx : VOC

≈ 1:7downtown

downwind

Fate of Organic Compounds- OH

vacuum cleaner of the atmosphere

OH + RH → R + HOH

OH + RCH=CH2 → RCHCH2OH or RCH(OH)CH2

OH Production photochemical (daylight)

O(1D) + HOH → 2 OH

HONO + h → OH + NO

HOO + NO → OH + NO2

HOO from CH2=O + h and organic

radicals

NO3 + RCH=CH2 → RCHCH2ONO2 or RCH(ONO2)CH2

NO3 Production

NO + O3 → NO2 + O2

NO2 + O3 → NO3 + O2 (slower than NO2 + h)

NO2 + NO3 → N2O5

NO3 + h → NO2 + O or NO + O2 ( = seconds)

[NO3] only significant in the dark !!

Fate of Organic Compounds- NO3

Fate of Organic Compounds- O3

O3

OO

O

OO O

CH2=O CH2

OO

+

++

-

++

-

OH (in variable yield)

Consider this mechanism: cm3 molecule-1 s-1 1) C3H8 + OH C3H7 + H2O k1 = 1.1 x 10-12 2) C3H7 + O2 C3H7O2 k2 = 1 x 10-12 3) C3H7O2 + NO C3H7O + NO2 k3 = 9 x 10-13 4) C3H7O + O2 C3H6O + HO2 k4 = 8.5 x 10-15 [C3H8] = 2 ppbv = 5 x 1010 molecules cm-3 [O2] = 0.21 atm = 5 x 1018 molecules cm-3 [NO] = 1 ppbv Calculate the steady state concentrations of: C3H7, C3H7O2, and C3H7O.

Activity

Meteorology and Air Pollution“Dilution is the Solution to Pollution”

Dispersing vs. Trapping pollutants

Planetary Boundary Layer- region influenced by surface (convection)- vertical mixing time scale of 1 hour- 1st 100 m – 2 km- diurnal (daily) cycle

Planetary Boundary Layer

0422 0758 1324 1700 2037

Ozone and Topography

Topography and Meteorology

Pacific Ocean (km) San Bernadino Mtns

Topography and Meteorology

Pacific Ocean (km) San Bernadino Mtns

Topography and Meteorology

Inversion Layer

• Warmer above than below, so air is trapped• time limited, specific geography

Key Points

• Sunlight, NOx, and Organics

• Cycles in HOx and NOx

• NOx is globally limiting reagent

• OH removes organics (O3, NO3)

• Kinetics calculations - steady state concentrations

- fate of species• Topography (meteorology)