1. METO 637
Lesson 16

2. Sulfur chemistry
The abundances, sources, budgets, and photochemistry of atmospheric sulfur compounds are poorly understood compared to carbon, nitrogen and oxygen species.
Sulfur can be converted to SO2, SO3, and H2SO4.
Hence it acts as an aerosol precursor
Sulfate aerosols are of climatological importance as they can effect the radiative balance of the atmosphere (primary effect).
They are also very efficient cloud condensation nuclei
This in turn may have an effect on cloud production, which is also of climatological significance (secondary effect).
H2SO4 can also lower the pH of rainwater.
Anthropogenic sources are comparable with natural sources – principally from the burning of fossil fuels.

3. VOLCANIC ACTIVITY
MOST VOLCANOES EJECT DUST ETC. INTO THE TROPOPSHERE WHERE IT IS QUICKLY RAINED OUT.
HOWEVER LARGE VOLCANOES CAN EJECT GASES, ESPECIALLY SULFUR DIOXIDE, INTO THE STRATOSPHERE.
IN THE STRATOSPHERE THE SULFUR DIOXIDE TRANSFORMS INTO AEROSOLS, WHICH REMAIN IN THE STRATOSPHERE FOR ONE TO TWO YEARS.
THIS WILL TEND TO COOL THE TROPOSPHERE - SCATTERS SOLAR RADIATION BACK TO SPACE.
ERUPTION OF MOUNT TAMBORA IN INDONESIA LED TO 'YEAR WITHOUT A SUMMER'
MOUNT PINATUBO, 1991, LOWERED TEMPERATURE BY 0.5 C

4. Radiative forcing

5. Sulfur chemistry Sources:
(1) Volcanic activity – 7x109 kg per year, principally as SO2
(2) Decay of biogenic matter produces 58x109 kg per year over land and 48x109 kg per year over the oceans, principally in the reduced forms of H2S, (CH3)2S (dimethyl sulfide, DMS) and (CH3)2S2 (dimethyl disulfide,)
(3) Sea spray – 44x109 kg per year
2 additional sulfur compounds found in the troposphere are COS (carbonyl sulfide) and CS2 (carbon disulfide).
The lifetime of CS2 is only a few weeks, converting to COS.
COS has a lifetime of more than a year, and is the most abundant. Largely comes from the oceans.

6. Sulfur chemistry
Oceanic source for COS estimated as 0.15x109 kg per year.
Most sulfur gases emitted into the troposphere have a short lifetime.
Hence stratospheric sulfur comes mainly from two sources – volcanic injection and COS.
The origins of CS2 and COS are really not known at this time. Not known how much is anthropogenic or natural.
COS is distributed fairly uniformly in the troposphere with a mixing ratio of about 0.5 ppb

8. Sulfur chemistry
DMS constitutes the most important biogenic flux of sulfur compounds.
Biogenic inputs dominate in the Southern hemisphere, but anthropogenic inputs dominate in the Northern hemisphere.
The manmade sources are rapidly oxidized and removed by rainout and deposition. So their impact tends to be regional.
The longer lived biogenic compounds have a larger global impact.

10. Sulfur chemistry
Oxidation of the inorganic reduced-sulfur oxidation is driven by reactions with OH:
OH + CS2 → COS + SH
OH + COS → CO2 + SH
OH + H2S → H2O + SH
The SH radical is then oxidized to SO2

11. Natural halogen containing species
Has long been recognized that the organic halides (e.g. methyl chloride and methyl bromide) can be found in he troposphere.
However there is increasing awareness that large amounts of inorganic chlorine and bromine might also be present and participate in tropospheric chemistry.
Vast quantities of inorganic halides are present as particles in the troposphere.
Wave action generates small airborne droplets of sea-water which can evaporate to leave suspended particles of sea salt.
However, can these particles be activated in such a way that the halogens play a role in tropospheric chemistry?

12. Natural halogen containing species
The unexpected answer was yes.
This is because chlorine is highly reactive toward many organic compounds. As with OH it can abstract hydrogen
Cl + CH4 → CH3 + HCl
This rate constant can be two orders of magnitude larger than the similar reaction with OH. Hence chlorine can be effective even if present at orders of magnitude lower concentrations.
Although atomic chlorine has not been detected molecular chlorine has.
BrO has also been detected in the troposphere, especially in the polar regions. Up to 30 pptv in the Arctic

13. Natural halogen containing species
There is increasing evidence that the source of the halogens is sea salt.
Sea salt particles have been found to be deficient in chlorine and bromine.
This suggests that reactions between sea salt and atmospheric gases (acids, oxides of nitrogen, etc). For example:
NaClcond + HNO3gas → HClgas + NaNO3cond
The HCl does not photolyze in the troposphere, and the reaction with OH is slow.
NaClcond + N2O5gas → ClNO2gas + NaNO3cond
NaClcond + ClONO2gas → Cl2gas + NaNO3cond

16. Yang et al.,’Tropospheric bromine chemistry and its impact on ozone – A model study’, J. Geophys. Res.,110,D23313, 2005

17. Bromine chemistry
The detection of BrO in the troposphere is of great interest. It has been suggested that BrO is ubiquitous in the free troposphere at 0.5 to 2 pptv.
In the troposphere the following reactions can occur:
Br + O3 → BrO + O2
BrO + HO2 → HOBr + O2
HOBr + hν → Br + OH
Essentially we destroy ozone without the need for atomic oxygen.
Ozone loss due to Bromine has been estimated to be up to 18%.

18. Bromine chemistry
Yang et al., found overall reductions in ozone of from 4-6% over most of the troposphere.
Larger reductions (up to 30%) at high latitudes.
Effect is due to two factors
(1) direct ozone loss by Br reactions
(2) NOx removal via bromine nitrate hydrolysis:
BrO + NO2 → BrONO2
BrONO2 + H2Oaq → HOBr + HNO3