Atmospheric Chemistry of CO and NO

3 2 1

2

2 3

O +hυ O+O λ>315 nm O( D)+O λ<315 nm

O+O O quickly combines (no net change)

→

→

→

O(¹D) can have two routes:

1

2 3

1 .

2

(i) O( D)+M O+M O+O O

(ii) O( D)+H O 2OH

(hydroxyl radical)

→

→

→

This OH radical fundamental is responsible for the photo chemical/chemical reactions:

CO + OH

^.

→ CO

₂

+ H

^.

H

^.

+ O

₂

+ M →HO

₂^.

+ M

HO

₂^.

→ hydroperoxyl radical

2

. .

2 2

CO+OH ⎯⎯→

O

CO +HO

Or Simply combine:-

HO

₂^.

+ NO → NO

₂

+ OH

^.

OH^. radical regenerated.

This is one more route for NO oxidation to NO₂

Finally, OH and NO₂ may react to form nitric acid,

OH

^.

+ NO

₂

→ HNO

₃

All the reactions involved in the CO-NO_x chemistry are summarized in the following table:

Summary

• The basic reaction mechanism of the CO/NO

_x

system exhibit many of the key features of those involving much more complex organic molecuels.

• In particular, the role of OH as the oxidizing species and the NO to NO

₂

conversion by HO

₂

are central to virtually every atmospheric organic/NO

_x

mechanism.

• It is useful to proceed to a molecule that is somewhat more

complicated than CO to see how the similar NO

_x

mechanism develops.

• Formaldehyde (HCHO) is a primary pollutant and also an oxidation product of hydrocarbons.

• These are basically emitted from automobiles.

• Formaldehyde undergoes two routes for the primary reactions in the atmosphere: (i) Photolysis and (ii) Ordinary chemistry

Route (i):

2

. .

. .

2 2

. .

2 2

O .

2 2

HCHO + hυ H + CHO (Formyl radical)

H + O HO

HCO + O HO + CO

Overall: HCHO + hυ 2HO + CO H

+

C

O

→

→

→

⎯⎯→

→

Summary

• All reactions are because of photochemistry and reaction with OH^.

• HCHO/HC/CO → HO₂^.

• HO₂ to oxidize NO to NO_{2 .}

• NO_x removal from system is through OH^. to HNO₃

O2

. .

2 2

R

HCHO + OH HO + CO + H O oute 2:

⎯⎯→

All the reactions involved in the CO-NOx chemistry are

summarized in the following table:

Here the CO-OH reaction is omitted from the previous Table, as it is generally slower than those involving HCHO. Applying the PSSA, then:

CASE 1: (Figure 1)

[NO₂]₀=0.1, [NO]₀=0.01, [HCHO]₀=0.1.

Then during first two minutes:

[NO₂]=0.069, [NO]=0.0405, [O₃]=0.032

Over the 600 minute period, it can be observed that after the first 20 minutes NO₂ continually

decreases , since even though NO is continually being

converted to NO₂ by O₃, there is such an excess of NO₂ that the NO₂-OH reaction is removing NO₂ to HNO₃ at rate such that it dominates the NO₂

The behavior (concentration in ppm) of the system as a function of its initial conditions for HCHO and NO_x are calculated by using the models and explained in the figures

This figure shows the concentration versus time for the initial conditions: [NO₂]₀=0.1, [NO]₀=0.1, [HCHO]₀=1.0

This figure shows the concentration versus time for the initial conditions: [NO₂]₀=0.1, [NO]₀=1.0, [HCHO]₀=1.0

• The conversion of NO to NO

₂

and the formation of O

₃

are therefore driven by HCHO through its production of HO

₂^.

Thus, the theoretical maximum amount of O

₃

that could be produced in this system is:

[O

₃

]=[HCHO]

₀

+ [NO

₂

]

₀

• When all the NO

_x

is converted to HNO

₃

, the system ceases reacting. In a sense, a given system can be characterized by its ability to produce O

₃

.

• The effect of [HCHO]

₀

on NO

₂

dynamics is shown in the

following figure.

Oxidation of Methane

1

3 2

1 .

2

. .

4 3 2

4

. .

2

Then hydroxyl radicals react with CH and CO present in the atmosphere

O + hυ O +O( D) O( D) +H O 2OH

CH + OH CH +H O

(methyl radical) CO + OH CO + H

Then instantaneously th o .

e f

→

→

→

→

. .

2 2

. .

3 2 3 2

H + O +M HO +M

CH +O +M CH O +M

llowing r

(m

eaction takes p

ethylperoxy rad i

lac e

cal)

→

→

The peroxy radicals in turn, participate in a chain-propagating

sequence that converts NO to NO₂ and in the process, produces additional OH and peroxy radical species:

4

. .

4 3 2 2

. .

2 2

So that the CH and CO-OH reactions may be writte

CH + OH CH O +H O CO+OH

n si

HO

mply a

O

s:

+C

→

→

. .

3 2 2 3

. .

2 2

. .

3 2 2

CH O + NO NO +CH O

(methoxy radical)

HO NO NO +OH

CH O O HCHO + HO

(Hydroperoxyl radical)

→ + →

+ →

The major chain terminating steps include nitric acid and hydrogen peroxide formation,

3 2 2

.

2 3

. .

2 2 2 2 2

. .

3 2 2 3 2

The CH O radical can react with either NO or HO , the later reaction bei

OH + NO HNO

HO HO H O +O

C

ng

H O HO CH COOH+O :

→ + →

+ →

The following figure shows the atmospheric degradation path for methane.

Generalized –Chemistry for O 3 formation

2

2 3

3 2 2

. .

2

. .

2 2

. .

2 2

. .

2

2 2

NO + h NO + O O+O O

O NO NO O RH + OH R +H O

R O RO

RO NO RO NO RO O

(general HC)

(peroxy radical)

(It follows aldehyde chemistry) Eventually will lead to

RCHO

O +HO

C S



→

→

→

+ → +

→

+ →

→ + +

→

+

→

2 3

2 2

tabilize NO to HNO

Form H O (highly oxidizing agent)

→

Figure: Formation of ozone

RO^.

NO₂+hu

O₂

O

O₂ O₃

R^. RO₂^.

O₂

NO

NO

RH

Peroxyacetyl Nitrate (PAN):

Consider Acetaldehyde (CH3CHO) CH3CHO + h v CH3O2^.

+

HO2 ^. +CO …. and

. .

3 3 2

. .

3 2 3 2

.

3 2 2 3 2 2

CH CHO + OH CH CO H O

(Acetyl radical)

CH CO + O CH COO

(Acetyl peroxy radical)

CH COO + NO CH COO NO

(Peroxya

→ +

→

→

cetyl Nitrate)

Peroxybenzyol- Nitrate (PBN)

C O

OO NO₂

Pungent smell, Eye irritation,

Decomposes very quickly in the

atmosphere

Chemistry of Sulfur Dioxide

Gas-phase reaction

. .

2 2

. .

2 2 2 3

. .

2 2

3 2 2 4

(Rate limiting step, does not happen so ea OH + SO HOSO

HOSO O HO +SO HO +

sil

NO NO +OH SO + H O

y)

H SO

→

→

→

→

Aqueous-phase reaction

HSO₃^- SO₂ SO₄^2-

Through series of complex reactions, e.g. H₂O₂, O₃, O₂ → eventually oxidized to SO₄

Fe:

0.19 – 0.51 μg/m³

1.1 – 2.1 μg/m³ (Urban)

Fe₂O₃ + SO₂→ SO₄ (Gas-solid phase interaction)

Solid particle with Fe

Aqueous phase

Dissolved O₂ can oxidize the SO₂ in presence of Fe

Conc

Time

7 AM 8 AM _{12 Noon 2-3 PM} 6 PM

NO HC

NO₂ O₃

O₃

Isopleths In ppb

NO_x (ppb)

50 200

VOC (ppb)

400 300

200 150

200 1000

(VOC/NO_x)=15 (VOC/NO_x)=4

VOC limited

NO_x limited KNEE

Region

Low VOC/NOx ratio ? i.e. low O3 VOC limited High VOC/NOx ratio?

Low O3 consumed by organic radicals

Assignment/Example

Alkylperoxy nitrate decomposes in the following way

2 2

2

2

NO

^{1 2}

RO NO

RO ⎯ ⎯→

^k

 ⎯⎯

^K

+

2 2

2

RO

³

2 RO O

RO + ⎯ ⎯→

^k

+

2

2 ⁴

RONO

NO

RO + ⎯ ⎯→

^k

Assume that RO2NO2 are decomposing in a chamber and its decay is observed. We wish to estimate k1 from the experiment. Assume RO2 and NO2 are at pseudo-steady state and [RO2]=[NO2]. Show that first order rate constant for RO₂ NO₂ decay is related to fundamental rate

constants of the system in the following way