Vienna, 6.05.2020

New pathways of the reaction of OH radicals with dimethyl sulfide

based on CH3SCH2OO isomerization

HOOCH2SCHO + OHSO2,etc.

95%

OH

65%HO2

NO/RO2CH3SCH2O

CH3SCH2OOH

- HCHOCH3S

OH OH

OH + S

•O2CH2SCH2OOH

MSA,SO2,etc.OH

S OO

isomeriizzaat

ionpaathway

T. Berndt,1 W. Scholz,2 B. Mentler,2 L. Fischer,2 E. H. Hoffmann,1 A. Tilgner,1

N. Hyttinen,3 N. L. Prisle,3 A. Hansel,2 H. Herrmann1

1 Leibniz Institute for Tropospheric Research (TROPOS), 04318 Leipzig, Germany.2 University of Innsbruck, 6020 Innsbruck, Austria.3 University of Oulu, 90014 Oulu, Finland.

Motivation

DMS- Largest natural sulfur source to the Earth´s atmosphere- Estimated emission rate: (10 - 35) ´ 106 metric tons of sulfur per year over the oceans

Lana et al., Global Biochem. Cy.(2011)- Gas-phase oxidation mainly initiated by the reaction with OH radicals

-> formation of sulfuric acid (H2SO4) and methane sulfonic acid (MSA)- Concentration over the oceans: a few 109 molecules cm-3

- Also emission from soil/vegetation over the continents- Biomass burning in Australia: DMS: 9 x 1011 molecules cm-3 (max.)

DMDS: 3 x 1012 molecules cm-3 (max.)Meinardi et al., GRL(2003)

Reduced sulfur compounds- H2S, OCS, CS2, CH3SH, CH3SCH3, CH3SSCH3

DMS degradation - knowledge in literature (beginning of 2019)

Barnes et al.,Chem. Rev.(2006)

But: results from QC calculations point to rapid CH3SCH2OO isomerization

CH3SCH2O2 (+ O2) ® O2CH2SCH2OOH (1)

O2CH2SCH2OOH ® {HOOCH2SCHOOH} ® HOOCH2SCHO + OH (2)

k1(293 K) = 2.1 s-1; k2(293 K) = 73 s-1

Wu et al.,JPC A(2015)

Experiment

Free jet flow system- 1 bar purified air- Residence time: 3.0 - 7.9 s- “early stage” of a reaction- RO2 radical formation/isomerization- Controlled bimolecular RO2 steps- benefit: „wall-free“ conditions

CI-APi-TOF mass spectrometry- Boulder-Typ inlet system- Detection limit: 103 - 104 molecules cm-3

- Different ionisation schemes:Iodide (I-) / CH3COO- -> clustering with S-speciesRNH3

+ -> clustering with SO3 (indirect OH)(CH3C(O)CH3)H+ -> proton transfer reaction

NH4+-CI3-TOF (Innsbruck)

Observed products from CH3SCH2OO isomerization

OH via O3 + TME OH via i-C3H7ONO photolysis

- I--CI-APi-TOF analysis - reacted DMS: 4.5´107 molecules cm-3 and

reacted DMS-d6: about 2.8´107 molecules cm-3

-> k(H-shift) / k(D-shift) about 15

- I--CI-APi-TOF analysis - reacted DMS: 2.5´108 molecules cm-3

- C2H6O4S: HOOCH2SCH2OOH formedvia HO2 + O2CH2SCH2OOH ?

CH3SCH2O2 (+ O2) ® O2CH2SCH2OOH (1)

O2CH2SCH2OOH ® {HOOCH2SCHOOH} ® HOOCH2SCHO + OH (2)Wu et al.,JPC A(2015)

230 240 250 260

0.0

0.5

1.0

DMS-d6

DMS

(O2CH

2SCH 2OO

H)I-

(HOO

CH2SC

HO)I-

signa

l (cp

s)

mass / charge (Th)220 230 240 250

0

2

4

6

(C2H 6O 4S)

I-

(O2CH

2SCH 2OO

H)I-

(HOO

CH2SC

HO)I-

signa

l (cp

s)mass / charge (Th)

CH3SCH2OO isomerization products (OH via O3 + TME)

0 2x107 4x107 6x1070.0

0.0005

0.0010

HOOCH2SCHO

O2CH2SCH2OOH

reacted DMS (molecules cm-3)

norm

alize

d sig

nal

0.0 5.0x108 1.0x109 1.5x1090

5x10-5

1x10-4

C2H6O4SO2CH2SCH2OOH

HOOCH2SCHO

norm

alize

d sig

nal

reacted DMS (molecules cm-3)

NH4+-CI3-TOF (Innsbruck) (CH3C(O)CH3)H+-CI-APi-TOF

0.0 5.0x108 1.0x109 1.5x1090.0

0.004

0.008HOOCH2SCHO

reacted DMS (molecules cm-3)

norm

alize

d sig

nal

0 50 100 150

0.0

0.0005

0.0010

(O2CH2SCH2OOH)I -

(HOOCH2SCHO)I -

TME

and

DMS

off

C 3H 8 off

C 3H 8 on

DMS

onTM

E on

O3 on

norm

alize

d sig

nal

measurement cycle

I--CI-APi-TOF

→reacted DMS: 4.5´107 molecules cm-3

CH3SCH2OO isomerization rate

Competition kinetics:- Signals of O2CH2SCH2OOH and HOOCH2SCHO = f(NO) -> k1 / k3 if k2 >(or better: >>) k1

But: NO addition changes HOx system-> NO + HO2 -> OH + NO2

-> measurement of the integral OH conc. via SO3 formation from OH + SO2- low SO2 addition: r(OH+SO2) / r(OH+DMS) = 0.05- SO3 formation allows to account for rising OH conc.

-> k1 / k3 = (2.1 ± 0.4)´1010 molecule cm-3 (HOOCH2SCHO)(1.5 ± 0.3)´1010 molecule cm-3 (O2CH2SCH2OOH)

-> k1 = 0.23 ± 0.12 s-1 at 295 ± 2 K (k3 from literature)

0 1x1010 2x1010 3x1010 4x1010 5x10100

1

2

3

4

HOOCH 2SCHO

O2CH2SCH2OOH

[NO] (molecules cm-3)

prod

uct /

reac

ted

DMS

(arb

itrar

y)

A)

B)0.0001

0.001

0.01HO2

SO3

HOOCH2SCHO

O2CH2SCH 2OOH

norm

alize

d sig

nal

CH3SCH2O2 (+ O2) ® O2CH2SCH2OOH (1)O2CH2SCH2OOH ® {HOOCH2SCHOOH} ® HOOCH2SCHO + OH (2)CH3SCH2O2 + NO ® products (3)

Experiment- OH via i-C3H7ONO photolysis

OH radical recycling

-> constant OH formation rate via i-C3H7ONO photolysis-> constant OH consumption rate via OH + DMS/organic

-> measurement of the integral OH conc. via SO3 formation from OH + SO2

-> enhanced integral OH signal for OH + DMS-> OH recycling

CH3SCH2O2 (+ O2) ® O2CH2SCH2OOH (1)O2CH2SCH2OOH ® {HOOCH2SCHOOH} ® HOOCH2SCHO + OH (2)

0 50 100 150 200 2500.0

0.002

0.004

0.006

0.008no hydrocarbon

OH + DMS

OH +

α-p

inene

OH +

dec

alin

OH +

TM

E OH + DMS

photolytic OH formation and SO2 on

norm

alize

d SO

3 sign

al(in

tegr

al OH

sign

al)

measurement cycle

no DMS

Updated scheme

Berndt et al., JPC Lett.(2019)

DMSOH +

CH3SCH2O2CH3S(OH)CH3O2

X-XO

HO2 -O2

CH3SCH2OOH

CH3SCH2O

CH3S

-HCHO

O2,O3

products(MSA, SO2, etc.)

O2

products

isomerizationO2

O2CH2SCH2OOHisomerization

HOOCH2SCHO

-H2O

-HO2

CH3S(O)CH3

O2

CH3S(OH)CH3

DMSO

-O2

O2

addition channel 35%

abstraction channel 65%

-OH

products(SO2, etc.)

OH

-> Supported by measurements from field campaigns of U.S. and U.K. groupssee: Veres et al.,

P.N.A.S.(2020)

Summary

• Rapid two-step CH3SCH2OO isomerization process forming finallyHOOCH2SCHO

• Isomerization rate outruns “traditional” bimolecular CH3SCH2OO reactions with HO2, NO, RO2

• Still open question: Formation of SO2 and MSA• A next example for a rapid unimolecular RO2 pathway

important for atmospheric conditions

-> Improved techniques allow a more direct insight into a reaction!

Thanks!

K. Pielok

A. Rohmer

Thank you for your attention!

N. Hyttinen

W. ScholzB. MentlerL. Fischer

ACD modeling group (TROPOS)