TROPOSPHERIC AEROSOLS

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TROPOSPHERIC AEROSOLS

• Part II secondary aerosol

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Aerosol properties
 
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Gas emissions leading to secondary aerosol

• Dimethylsulfide (DMS)
• SO2 emissions from volcanoes
• Industrial SO2 emissions
• Nitrogen oxides and ammonia
• Volatile Organic compounds (VOC)

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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

• only small fraction lost into the atmosphere

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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)

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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.

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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.

Figure adapted from Charlson et al. (1987)
Oceanic phytoplankton, atmospheric sulphur,
cloud albedo and climate Nature, vol. 326, pp.
655-661
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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

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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

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

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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 )

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Volcano locations
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Continuously erupting volcanoes
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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

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• 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

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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
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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 (0.4-0.7oC) 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

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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
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Enhanced destruction of stratospheric ozone
Volcanoes do not inject chlorine into the
stratosphere. Aerosols improve efficiency with
which CFCs 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
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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 Importanc
e 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
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Atmosphericeffectiveness
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
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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

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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 Suns 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. gt 1783 -
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)
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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
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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 Disadvantag
es Tree growth sensitive to things apart from
climate. Local environmental factors significant
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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)
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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 1816 - the year without a
summer inspired Frankenstein Anomalously cold
winter in North America and Europe Widespread
crop failures, famine
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Global sulfur emissions
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GLOBAL SULFUR EMISSION TO THE ATMOSPHERE (1990
annual mean)
Chin et al. 2000
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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.

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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
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Future SO2 emissions in Asia are likely to be
much lower than the latest IPCC forecasts
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Sources of nitrogen oxidesand ammonia
Fluxes in TgN/year
Aircraft
0.5
NOx 32 TgN anthropogenic 11 TgN
natural
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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

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Global NOx emissions (Tg/yr)
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A century of NOx emissions(van Aardenne et al.,
GBC, 15, 909, 2001)
1990 dominated by northern hemisphere industriali
zation
1890 dominated by tropical biomass burning
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Global NOx from lightning
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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

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Global NH3 emissions
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Global NH3 sources
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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

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Spatial and temporal development of VOC
emissions(Klimont et al., Atmos. Environ., 36,
1309, 2002)
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Conclusion Integrated observation and modeling
programs like INDOEX, TRACE-P, and ACE-Asia
improve our understanding of emissions
Experimental measurements
Theoretical modeling
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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
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A Few Insights on Air Pollution and Climate from
ACE-Asia
Barry J. Huebert Department of
Oceanography University of Hawaii huebert_at_hawaii.e
du 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
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