Simulative Study on the Sources and Sinks of OCS under lightning

Jie Zhu

Content

1. The importance of study on OCS, CS2, SO2/CH4

2. Why study on lightning

3. The similarity of natural lightning and spark discharge

4. Some experimental results

5. Mechanism of the reactions

6. Global calculation and discussion of practical analysis

About OCS

•The most abundant sulfur gas in the atmosphere

•Long-lived in the atmosphere (about 4 years)

•Contribute to form SO2 and SO3, and sulfate aerosol

About CS2

CS2 oxidation is a very important source of OCS

CS2+O OCS+S

CS2+OH OCS+HS

SO2 and sulfur cycle in the atmosphere

Global Sulfur Emissions

Why study on lightning?

Under UV light : CS2+OH

Lightning flashes are a very normal nature phenomena, they happens 100 times/sec in the earth

Global lighting flashes distribution

What is lightning?

Spark discharge?

Franklin’s famous kite experiment testifies lightning and spark discharge are actually the same thing

Conclusion: Using spark discharge to simulate the lighting and to study the reactions of OCS under lightning is reasonable

Franklin’s Kite Experiment

An easily operated experiment

•Gases studied are filled in the cell

•Spark discharge in the cell

•The reaction products are analyzed by FT-IR spectra technique

The system studied and products analysis• CS2+air system (pCS2=1.33103Pa, pair=1.01 105Pa)

Products: CO2, CO,OCS, N2O

• SO2+CH4+O2 system(pSO2=1.33103Pa, pCH4=1.33103Pa, pO

2 =1.01 105Pa)Products: CO, OCS

• OCS system OCS+O2(pOCS=1.33103Pa, pO2 =1.01 105Pa)

Products: CO2, CO, SO3

OCS+dry air(pOCS=1.33103Pa, pair =1.01 105Pa)

Products: CO2, CO, SO3, N2O OCS+ water saturated air(pOCS=1.33103Pa, pair =1.01 105P

a)

Products: CO2, CO, H2SO4, N2O

Reaction mechanism supposed

e*+O2 e+O+O

e* +H2O e+OH+H

CS2+air system:

e*+CS2e+CS+S

O + CS2 OCS+S

OH + CS2 OCS+S

CS+O2OCS+SO

Reaction mechanism supposed SO2+CH4+O2 system

e*+SO2e+SO+O

e*+CH4e+CH3(CH2 、 CH)+H

CH2+SO2OCS+H2O

CH+SO OCS+H

OCS system

e*+OCSe+CO+S

O+OCSSO+CO

OH+OCSCO2+SH

SO+O2 SO3

SO3+H2O HSO4

Practical Calculations Main points:

• The fraction of reaction of SO2 with CH4 to form OCS is relatively small because of very small reaction rate constant. (This will be discussed in the paper turned up).

• The lightning flash produces reactive species as e*, O and OH. As OH>>O>>e*, we consider OH radicals be the main species to react with OCS, CS2 et al.

• So the key reactions contributing to OCS changing in a lightning flash are: OH

+ CS2 OCS+S, k1, OH+OCSCO2+SH, k2

Practical Calculations

• In a lightning flash,

d[OCS]/dt=k1[CS2][OH]-k2[OCS][OH]

• Using the data of the rate constant, k1=1.85 10 –13cm3s-1, k2= 5.7 10 –14cm3s-1,and the global mean OCS and CS2 concentration to calculate, we get:

d[OCS]/dt < 0

That means lightning flashes destroy OCS in global scale.

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4. Chin M, Davis DD. Global sources and sinks of OCS and CS2, and their distribution. Global Biogeochemical Cycles, 1993,&:321-337

5. Atkinson R, et al. Rate constants for the reaction of OH radicals with OCS, CS2,and CH3SCH3 over the temperature range 299~430K ，Chem. Phys. Lett., 1978,54:14-18

6. Kuhn U, et at. Carbonyl sulfide exchange on an ecosystem scale: soil represents a dominant sink for atmospheric COS. Atmos. Environ.,1999,33: 995-1008