1. TROPOSPHERIC AEROSOLS
Part II: secondary aerosol

2. Aerosol properties Species Natural processes anthropogenic
Present burden vs pre-industrial
Elements of climate affecting emissions
Primary particles
Mineral dust
Wind erosion
Land use change, industrial dust
Incr.
Changing winds and precipitation
Sea salt
Wind
Changing winds
Biolog. Part.
Wind, biolog. processes
Agriculture
???
Carb. Part.
Vegetation fires
Fossil fuel & biomass burning
Changing precip.
Secondary
DMS
Phytoplankton degradation
More sulfate
SO2
Volc emissions
Fossil fuel comb.
NH3
Microbial activity
More ammonium nitrate
NOx
Lightning
Incr. nitrate
Change in convective activity
VOC
Vegetation
Industrial processes
Incr. Org. aerosol

3. Gas emissions leading to secondary aerosol
Dimethylsulfide (DMS)
SO2 emissions from volcanoes
Industrial SO2 emissions
Nitrogen oxides and ammonia
Volatile Organic compounds (VOC)

4. DMS, (CH3)2S, is the major one of biogenic gases emitted from sea
mean residence time is about 1-2 days - most of S from DMS is also re-deposited in the ocean
is produces during decomposition of dimethyl-sulfonpropionate (DMSP) from dying phytoplankton
DIN = Dissolved inorganic nitrogen
only small fraction lost into the atmosphere

5. Dimethylsulfide
Recent global estimates of DMS flux from the oceans range from 8 to 51 Tg S a-1
This is 50% of total natural S-emissions (presently nearly equivalent to anthropogenic emissions, 76 Tg S a-1)
- Differences in the transfer velocities in sea-to-air calculations
Uncertainties are due to:
- DMS seawater measurements (paucity of data in winter months and at high latitudes)

6. DMS and Climate
DMS is emitted by phytoplankton as a natural biproduct of metabolism
Possibly related to radiation protection
Gives sea water its characteristic smell
Forms much of the natural aerosol (sub-micron particles) in oceanic air
DMS is the major biogenic gas emitted from sea and the major source of S to the atmosphere. It contributes to the sulfur burden in both the MBL and FT.

7. The CLAW Hypothesis (Charlson, Lovelock, Andreae and Warren, 1987)
DMS from the ocean affects cloud properties and can feedback to the plankton community
This acts to regulate climate by increasing cloud albedo when sea-surface temperatures rise.
Shown here is a diagram of the so called CLAW hypothesis, named after the initials of the authors of the Charlson et al. Nature paper in The diagram is taken from that paper.
The first sentence summarises the hypothesis, i.e. that DMS from the ocean influences cloud properties which then feedback to the plankton community.
In the diagram we have ocean DMS emissions being oxidised in the atmosphere, which through the atmospheric chemistry can become sulphate aerosol. The aerosol acts as cloud condensation nucleii and can change the cloud properties. The feedback operates though the change in cloud albedo, changing the solar irradiance reaching the ocean beneath the cloud and changing the surface temperature.
This talk is about our attempt at the Hadley Centre to model this feedback loop using a numerical climate model.
Figure adapted from Charlson et al. (1987)
“Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate”
Nature, vol. 326, pp

8. DMS oxidation
The atmospheric oxidation pathways that lead from DMS to ionic species (essentially sulfate and methanesulfonic acid, MSA, CH3SO3H) are complex and still poorly understood
The first step to sulfate is SO2
SO2 is largely dominant vs MSA, except at high latitudes (reasons unclear)
MSA is unique for tracing marine biological activity, since it has no other source

9. About atmospheric SO2 SO2 has several sources:
either natural: marine MSA and volcanism
or anthropogenic: mining and fossil fuel burning
Its oxidation ways to SO4-- are still matter to investigation, in particular with the aid of S & O stable isotopes
This can occur either in the gaseous phase by OH radicals or in the liquid phase by O3 or H2O2 .
Generally gaseous phase process is dominant, except in regions of high sea salt concentrations

10. Effect of sea-salt chemistry on SO2 and SO42- concentrations
Percent (%) change in concentrations (yearly average)
Case A: SO2/SO42- concentration without sea-salt chemistry
Case B: With sea-salt chemistry
SO2 (decrease)
SO42- (small increase) 0%
50%
100%

11. Effect of sea-salt chemistry on gas-phase sulfate production rates
Percent (%) decrease (seasonal average):
Mar/Apr/May
Jun/Jul/Aug
Sep/Oct/Nov
Dec/Jan/Feb
50% 0%
100%

12. Aqueous versus Gas Phase Oxidation
Biological regulation of the climate?
(Charlson et al., 1987)
H2O2
Aqueous-phase
Gas-phase
CCN OH
DMS
SO2
H2SO4
New particle formation
NO3 OH
Light scattering
Aqueous-phase O3

13. SO2 emissions from volcanoes (1)
Volcanoes are a major natural source of atmospheric S-species
Injections are generally occurring in the free troposphere
Most active volcanoes are in the Northern Hemisphere (80%)
The strongest source region is the tropical belt, in particular Indonesia
Emissions are in the form of SO2, H2S and SO4--
Most eruptions do not have potential to impact global climate -
Do not penetrate the tropopause
Troposphere is turbulent, gives rise to the weather
Ash, soluble gases (e.g. H2O, HCl) quickly washed-out (minutes to days)

14. SO2 emissions from volcanoes (2)
560 volcanoes over the world are potential SO2 sources, but only a few have been measured
Volcanic activity is sporadic, with a few cataclysmic eruptions per century
Cataclysmic eruptions inject ash particles and gases (mainly SO2) into the stratosphere, where H2SO4 formed forms a veil (« Junge layer »)
Large eruptions can generate plumes that penetrate the stratosphere
Stratosphere more stable and dry than the troposphere
Particles and gases which enter this region not washed-out
Little vertical mixing, volcanic material remains at the level injected injected

15. Volcano locations

16. Continuously erupting volcanoes

17. Atmospheric impact of volcanoes
SO2 relatively insoluble, resists tropospheric washout
Injected into the stratosphere in large quantities (Pinatubo, 1991 ~20 Tg)
In stratosphere, SO2 oxidises to produce sulfuric acid aerosols (H2SO4)
Conversion of SO2 to H2SO4 slow (months), aerosol cloud replenished months after eruption

18. The total amount of volcanic tropospheric S-emissions is presently estimated at:
14 +/- 6 Tg a-1
Mean volcanic sulfur emissions are of comparable importance for the atmospheric sulfate burden as anthropogenic sources because they affect the sulfate concentrations in the middle and upper troposphere whereas anthropogenic emissions control sulfate in the boundary layer.
S-isotope measurements in central polar regions (i.e. in the free troposphere) seem to support the important role of volcanic sulfur

19. Volcanic aerosol and global atmospheric effects
Acid aerosols reside in the stratosphere for several years
Aerosol veils increase optical depth of the atmosphere
(inc. optical depth of 0.1% = 10% reduction sunlight reaching Earth surface). Spread around the globe by stratospheric winds
Injection of acid aerosols into stratosphere is the
fundamental process governing the atmospheric impact of volcanic eruptions

20. Atmospheric effects of volcanic eruptions
1. Tropospheric cooling due to increased albedo
Effects of aerosols can be direct or indirect
Albedo increased indirectly when aerosols fall out of the stratosphere
Nucleate clouds in troposphere - increase albedo
Recent major volcanic eruptions produced significant cooling anomalies ( oC) in the troposphere for periods of 1 to 3 years
Magnitude of volcanic effects masked by natural variations (e.g. El Nino)
2. Stratospheric warming
Acid aerosols absorb incoming solar radiation, heating the tropical stratosphere, e.g. Mt. Agung (1963), El Chichon (1982), and Pinatubo (1991) all caused warming of the lower stratosphere of ~2oC
3. Enhanced destruction of stratospheric ozone

21. Stratospheric warming
+3oC
0oC
El Chichon
Pinatubo
-3oC
Lower stratospheric temperature (global mean)
Localised heating in the stratosphere can influence how far volcanic aerosol veils spread, by influencing stratospheric wind patterns

22. Enhanced destruction of stratospheric ozone
Volcanoes do not inject chlorine into the stratosphere.
Aerosols improve efficiency with which CFC`s destroy ozone,
by activating anthropogenic bromine and chlorine, indirectly leading to
enhanced destruction of stratospheric ozone
Relatively short lived - aerosols last only 2-3 years in the stratosphere
Reduction in ozone following the June 1991 eruption of Pinatubo

23. Atmospheric “effectiveness”
Several factors combine to determine whether a volcanic eruption has the
potential to influence the global atmosphere
1. Eruption style
Energetic enough to inject aerosols into the stratosphere
Larger eruptions do not necessarily have greater effects
Increased SO2 results in larger particles, not more
Fall from the stratosphere faster, smaller optical depth per unit mass
volcanic effects on the atmosphere may be self-limiting
2. Magma chemistry
Importance of acid aerosols means that large eruptions of sulphur-poor
magma less significant than sulfur-rich magmas
e.g. Mt St Helens - sulfur poor - negligible global effects

24. Atmospheric“effectiveness”
3. Latitude
Proximity to the stratosphere: smaller eruptions at high latitude can inject as much SO2 into the stratosphere as larger eruptions at lower latitudes
Stratospheric dispersal: Aerosols from tropical eruptions have the potential to spread around the globe (e.g Pinatubo). Atmospheric influence of eruption outside the tropics is contained within the middle and polar latitudes of the hemisphere of origin

25. Volcanic eruptions and climate
Atmospheric processes are complex !
Understanding how an atmospheric perturbation influences climate and weather is still problematic, even for largest eruptions
However, understanding how volcanoes effect climate necessary to isolate other forcing processes
Comparison of chronology of known eruptions and climatic data shed light on the ways climate responds to large volcanic eruptions

26. Making the connection 1. The written record
Compare eruption chronologies with written records of unusual
climatic events
e.g. Benjamin Franklin (1784) ``During several months of the summer of the year 1783, when the effects of the Sun`s rays to heat the Earth should have been the greatest, there existed a constant fog over all of Europe, and great parts of North America.`` => Laki fissure eruption, Iceland
Disadvantages: record only a couple of thousand years, humans unreliable, eruption chronologies incomplete, geographical bias (e.g. no humans = no record)

27. Making the connection 2. Ice cores Acid aerosols fall on ice fields
Accumulation of ice preserves
information - acidity profile
Climatically significant eruptions
can be identified with great precision
Advantages: objective, precise, records
`climatically significant` eruptions only
Disadvantages: Which eruptions and why?
Only those with high sulfur contents.
Geographical bias.
HALF of known large eruptions not recorded in Greenland ice cores

28. Making the connection 3. Tree rings Proxy witnesses to eruptions
Temperate trees record passage of seasons in growth rings - dendochronology
Changes in ring spacing, frost damage correlate with known eruptions
Advantages: Trees, are old! Record extends back thousands of years. Objective, precise
Disadvantages: Tree growth sensitive to things apart from climate. Local environmental factors significant

29. Case study: Krakatau, 1883
20 km3 of pyroclastic material in a Plinian column 40 km high
Aerosol veil circumnavigated the globe in ~2 weeks
Initially confined to the tropics, later spread to higher latitudes in
both hemispheres
Caused spectacular sunsets worldwide
20% fall in radiant energy reaching Europe after the eruption
Average Northern Hemisphere cooling of 0.25oC, more pronounced at
higher latitudes (-1oC)

30. Case study: Tambora, 1815
50 km3 of pyroclasts, Plinian column 43 km high
Aerosol veil reached London in about 3 months
Many climatic effects attributed to Tambora
`the year without a summer`
inspired `Frankenstein`
Anomalously cold winter in North America and Europe
Widespread crop failures, famine

31. Global sulfur emissions

32. GLOBAL SULFUR EMISSION TO THE ATMOSPHERE (1990 annual mean)
Chin et al. [2000]

33. Industrial SO2 emissions
During the last decade, researchers from different countries have prepared separate country-level inventories of anthropogenic emissions (GEIA= Global Emission Inventory Activity). In regions were local inventories were not available, estimates based on fossil fuel consumptions and population were calculated.

34. Anthropogenic sulfur emissions
In 1985: about 81% of anthropogenic sulfur emissions were from fossil fuel combustion, 16 % from industrial processes, 3 % from large scale biomass burning and 1% from the combustion of biofuels, but these figures have to be revised for more recent years.
The total amount for 1985 is estimated at :
76 Tg S a-1, accurate to 20-30%

35. Future SO2 emissions in Asia are likely to be much lower than the latest IPCC forecasts

36. Sources of nitrogen oxides and ammonia
Fluxes in
TgN/year
Aircraft
0.5
NOx: ~32 TgN anthropogenic
~11 TgN natural

37. Nitrogen oxides They are important in atmospheric oxidant chemistry
They are precursors for nitric acid which is a contributor to atmospheric acidity and reacts with NH3 and alkaline particles

38. Global NOx emissions (Tg/yr)

39. A century of NOx emissions (van Aardenne et al., GBC, 15, 909, 2001)
1890: dominated by
tropical
biomass burning
1990: dominated by
northern hemisphere
industrialization

40. Global NOx from lightning

41. Ammonia NH3
Ammonia is the primary basic (i.e. not acidic) gas in the atmosphere, and after N2 and N2O, the most abundant nitrogen containing gas in the atmosphere
The significant sources of NH3 are animal wastes, ammonification of humus, emissions from soils, loss of fertilizer from soils and industrial sources – see next table
The ammonium ion, NH4+ is an important component of continental tropospheric aerosols (as is NO3-) forming NH4NO3
NH3 is highly water soluble and therefore has a residence time in the troposphere of around 10 days
Consequently, atmospheric concentrations of NH3 are quite variable, typically ranging from 0.1 to 10 ppb

42. Global NH3 emissions

43. Global NH3 sources

44. VOC = Volatile Organic Compounds
Natural biogenic and anthropogenic sources
-Anthropogenic: alkane, alkenes, aromatics and carbonyls
-Biogenic: isoprene, mono-and sesquiterpenes, a suite of O-containing compounds
They produce secondary organic particles
Based on emission inventories and laboratory data, the production of secondary organic particulate from VOC is estimated to:
30 to 270 Tg a-1

45. Spatial and temporal development of VOC emissions (Klimont et al
Spatial and temporal development of VOC emissions (Klimont et al., Atmos. Environ., 36, 1309, 2002)

46. Conclusion: Integrated observation and modeling programs like INDOEX, TRACE-P, and ACE-Asia improve our understanding of emissions …
Experimental
measurements
Theoretical
modeling

47. … but we desperately need more source testing in the developing world
Representativeness of entire
population of sources
Typical operating practices
Typical fuels and fuel characteristics
Relationship to similar sources in the
developed world
Daily and seasonal operating cycles

48. A Few Insights on Air Pollution and Climate from ACE-Asia
Barry J. Huebert
Department of Oceanography
University of Hawaii
The Real Authors:
Steve Howell, Byron Blomquist
Liangzhong Zhuang, Jackie Heath
Tim Bertram, Jena Kline
ACE-Asia Science Team
Supported by the US NSF
& 35 other agencies