1. Photochemical Formation of Sulfur-Containing Aerosols
Jay A. Kroll and Veronica Vaida
University of Colorado at Boulder
June 23rd, 2017

2. Sulfur in Earth’s Atmosphere
Sulfur compounds are found throughout Earth’s atmosphere.
SO2 is an important player.
Sources of SO2 are volcanic eruptions, burning of coal and other fossil fuels, oxidation of sulfur compounds emitted by ocean/plants
Alexander Belousov
Gino Santa Maria

3. Atmospheric Oxidation of SO2
Traditional SO2 Chemistry
SO2 + OH  HSO3
HSO3+ O2  HO2 +SO3
SO3 + H2O  H2SO4
H2O
H2SO4 + H2O  Aerosol Particles
SO2 lifetime in the troposphere is on the order of days

4. Why do we care about aerosol?
This is the issue I’m more interested in. Famous figure from the IPCC report. Atmospheric scientists who study aerosol wait with baited breath for this figure each time a new report is being released. The plot shows radiative forcing from anthropogenic sources. You’ll note that aerosol has a unique situation of being one of the few anthropogenic sources that has a net cooling effect on the planet. But there are huge error bars on it. There is still a ton of work to be done to shrink those error bars.
IPCC 5th Assessment Report

5. Solar Radiation Management
Stratospheric SO2 Injections
Mount Pinatubo Eruption (1991)
~20 Mt SO2 released into stratosphere
Max height ~30 km
Cooled the climate by ~1°F *
Aerosol formation peaked ~27 km
Most of the SO2 formed aerosol in ~1 month
It took ~2 years for the aerosol to be removed by sedimentation.
These sorts of large eruptions have inspired SRM schemes where we could inject SO2 into the stratosphere and cool the planet significantly.
*Shepherd, J., et al. Royal Society 2009
C. Bruhl, et al. Atmos. Chem. Phys. Discuss. 13, , 2013

6. SO2 UV Spectrum 1B1+1A2 3B1 Singlet state not photochemically active
C. Hermans, A.C. Vandaele, and S. Fally, J. Quant. Spectrosc. Radiat. Transfer 110, (2009)
A.C. Vandaele, C. Hermans, and S. Fally, J. Quant. Spectrosc. Radiat. Transfer 110, (2009)

7. SO2 Photochemistry
3B1
1B1+1A2
1A2
λ= nm

8. SO2 Photochemistry
3B1
1B1+1A2
1A2
λ= nm
Reaction?

9. 3SO2+H2O Theoretical Work
72.9 kcal/mol
SO2 (3B1) + H2O
0.0 kcal/mol
SO2 ∙ ∙ ∙ H2O TS
34.8 kcal/mol
5.6 kcal/mol
SO2 (1A1) + H2O
H2SO3
3H2SO3
71.8 kcal/mol
78.3 kcal/mol
CCSD/ G(3df,3pd) level of theory
Donaldson, Kroll, and Vaida, Scientific Reports, , 2016

10. Experimental Setup Low SO2 Experiment High SO2 Experiment
0.891 Torr SO2
7.010 Torr H2O
3.000 Torr SO2
7.010 Torr H2O
Brightline Pro Green Dot Alignment Laser
Silicon photodiode detector
Donaldson, Kroll, and Vaida, Scientific Reports, , 2016

11. Experimental Results
Donaldson, Kroll, and Vaida, Scientific Reports, , 2016

12. Experimental Results
Donaldson, Kroll, and Vaida, Scientific Reports, , 2016

13. Acknowledgements
Vaida Group Members– Allison Harris, Rebecca Rapf, Russell Perkins, Keaten Kappes
NSF CHE
NASA grant number NNX15APZOG
University of Colorado Center for the Study of Origins
Cooperative Institute for Research in Environmental Science (CIRES)