Introduction to Atmosphere – Ocean Interactions
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•
•
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Atmospheric composition
Aerosols & greenhouse gases
UV radiation and ozone
Some examples:
– Sulfur cycling
– Dust (iron)
– Acid rain
– Halogen chemistry & sea salt particles
Vertical levels of the atmosphere
Boundary Layer: ~ 1 km
100
0.001
0.002
0.005
0.01
0.02
Thermosphere
90
80
Mesopause
70
0.1
0.2
Mesosphere
60
0.5
1
50
Stratopause
2
40
5
Stratosphere
10
20
160
30
20
Tropopause
10
Troposphere
500
1000
Height (km)
Pressure (mb)
0.05
50
100
200
Troposphere: region of strong
vertical mixing
180
200
220
240
260
280
300
Stratosphere: region of weak
vertical mixing
Considerable differences in
size, chemical nature, sources
of the aerosols and gases
0
o
Temperature ( K)
Idealized vertical temperature profile according to the U.S. Standard Atmosphere
(1976). Also shown are the names commonly used for the various layers and pauses
in the atmosphere.
Figure by MIT OCW.
Transport across tropopause
via large scale ascent in the
tropics / descent in the poles,
cloud convection, or
tropopause folding
Atmosphere – Ocean Interactions in a Nutshell
Temperature - greenhouse effects
Ozone concentrations
Radiation budget
Climate relevant gases
CO2, O2, DMS, …
Cloud microphysics
Chemical reaction rates
Oxidizing capacity
Hydrological cycle
Aerosols & sea-salts
Wet and dry deposition
Iron, acid rain, NOx
Ecology
Sunlight is a powerful structuring agent
Direct deposition
Mixing and stratification
Photochemical reactions
Biological processes
Components of atmosphere
N2
O2
H20
Ar
C02
H2
CH4
N20
CO
SO2
H2S
O3
ppmv
780840
209460
0 - 40000
9340
350
0.53
1.7
0.3
0.04 – 0.2
10-4
10-4
.1
• ~ 78% nitrogen, 21% oxygen
• Water vapor is the most variable
component
• Trace components can have
enormous impacts on the radiation
budget and inputs to the ocean
• Lifetimes (residence times) vary from
107 – 109 years for N2 to days for ozone
at high altitudes
• Sources and sinks vary spatially and
temporally
Aerosols
• Aerosol = suspension of particles in a gas
• Liquid = cloud or mist
• Solid = dust or smoke
• The direct effect relates to the changes in net radiative
fluxes in the atmosphere caused by the modulation of
atmospheric scattering and absorption properties due to
changes in aerosol concentration and optical properties
• The indirect effect relates to the changes in net radiative
transfer in the atmosphere caused by the modulation of
cloud properties due to changes in the concentration of
cloud condensation nuclei, CCN
4 types of aerosol reactions
1) Homogeneous, homomolecular = the condensation of a
single gaseous component to form a new suspended
particle (H20 molecules forming droplets)
2) Homogeneous, heteromolecular = reaction of 2 or more
gases to form a new particle
NH3(g) + HNO3(g) ⇒ NH4NO3(s)
3) Heterogeneous, heteromolecular = reaction of gases
on a pre-existing particle
NaCl(s) + HNO3(g) ⇒ HCl(g) + NaNO3(s)
4) Chemical reactions within the aerosols themselves to
form particles of changed composition (oxidation of
SO2 to sulfate ions in clouds)
Absorption and scattering
• Different gases & aerosols absorb at different λs:
Direct absorption of UV by ozone (< 290 nm)
• Scattering is a function of particle size relative to the λ of
radiation:
Rayleigh scattering: particles are generally small
compared to the λs of light (gases)
Mie scattering: particles are generally equal to or greater
than the λs of light (aerosols & droplets)
CCNs are ~0.1 μm containing 108 H20 molecules per drop
Earth’s energy budget
Figure by MIT OCW.
Greenhouse gas warming
λελ Intermediate
Black Body Curves
o
o
6000 K
0.1 0.15 0.2
100
Absorption (%)
60
45
20
0
85
0.5
1
1.5
2
3
5
10
15
20
30
50
100
Ground
Level
85
100
0.3
255 K
CO
O2 O2 O2 O
2
O2
O2
O H2O
O2O2 2
H2O
CO
2
H2O H O
2 CO
2
2
CO
O2
O3
2
H O N O
2
2
H O
2 CH4
H O
2
N O
O3
H O
2
2
CO CO2
N2O
2
CH
• Absorb incoming photons
and dissipate energy as
heat
• Greenhouse gases have
overlapping infrared
bands
4
11 km
60
45
Species
removed
% trapped radiation
remaining
CH4
None
100
N2O
O3
97
CO2
88
Clouds
86
H2O
64
H2O, CO2, O3
50
H2O,O3,clouds
36
All
0
20
0
100
50
Absorption (%)
0
50
0
O2 and O3
50
0
CO2
50
0
H2O
50
0
0.1 0.15 0.2
0.3
0.5
1
1.5
2
3
5
10
15
20
30
50
100
Wavelength (µ)
Figure by MIT OCW.
Peixoto and Oort, 1992
Radiation at the ocean surface
From Palmer Station, 2000
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UV-C = λs < 280 nm
UV-B = λs 280 – 320 nm
UV-A = λs 320 – 400 nm
Visible = λs 400 – 700 nm
IR = λs 700 nm – 1 mm
The importance of UV radiation in the ocean
• Overall a very small % of total
solar flux but:
– Affects biology – protein &
nucleic acid damage,
inhibits photosynthesis,
bacterial production,
photoenzymatic repair, etc.
– Causes photochemical
reactions
• Inorganic – Nitrate/nitrite
photolysis, Fe(III) reduction to
Fe(II)
• Organic – DOM breakdown &
alteration
Flux regulated by
OZONE concentrations
Stratosphere ozone chemistry
• Stratosphere is dominated by oxygen atoms and ozone
related reactions
O2 + hν ( λ < 240 nm) ⇒ O(1D) + O(1D)
O(1D) + O2 + M ⇒ O3 + M
O3 + hν ( λ < 240 nm) ⇒ O(1D) + O2
O(1D) + O3 ⇒ 2O2
• UV light converted into heat
• Ozone can be destroyed by a number of free radical
catalysts: OH, NO, Cl, and Br
• OH and NO are predominately natural while Cl and Br
have anthropogenic origins from CFCs
Antarctic ozone hole
• Very low temps ( < 80 C) lead to
formation of polar
stratospheric clouds
• A vortex forms as air
cools and descends
in winter
• Strong westerly
circulation isolates
the region from
lower latitudes
Slow
radiatively
driven
circulation
Strong
westerly
wind in
mid-low
Stratosphere
Cold core of
Antarctic
vortex
Mixing
a u se
pop
o
r
T
9 km
ANTARCTIC CONTINENT
The winter vortex over Antarctica. The cold core is almost isolated from the rest of the
atmosphere, and acts as a reaction vessel in which the constituents may become chemically
'preconditioned' during the long polar night.
Figure by MIT OCW.
Antarctic ozone hole
ABUNDANCE
Inactive
Chlorine
Surface reaction
Active
Chlorine
Gas-phase Reaction
Inactive
Chlorine
ClONO2
ClONO2
(Cl2+ClO+(ClO)2)
HCl
Autumn
TEMPERATURE
• PSCs provide
surfaces for
chemical reactions
• Highly deficient in
oxides of nitrogen
• Very dry →
condensation of
water
• Catalytic destruction
of ozone
Cl2 + hν ⇒ Cl + Cl
Cl + O3 ⇒ ClO + O2
ClO + O ⇒ Cl + O2
HCl
Early
Winter
TIME
Late
Spring
Denitrification and Dehydration
Surface Processing
Cl-catalysed O3 Destruction
Inactive Chlorine Recovery
Formation,
Maximum Intensity
cooling,
descent
Breakup
Polar Vertex Evolution
TIME
Photochemistry and dynamics in the polar stratosphere.
Figure by MIT OCW.
Antarctic stratospheric ozone hole 2005
Figure courtesy NASA GSFC
1 Dobson unit = column ozone compressed to 10 um thick at STP
Chemistry in the troposphere
• Troposphere oxidation reactions are initiated by the
highly reactive hydroxyl radical (OH) during the day
O3 + hν ⇒ O2 + O(1D)
an electronically excited state of the oxygen atom, O(1D)
can then react with methane or water
O(1D) + H2O ⇒ OH + OH
O(1D) + CH4 ⇒ OH + CH3
• And nitrate radicals at night (nitrate is rapidly photolyzed)
NO2 + O3 ⇒ NO3 + O2
• Govern fates of almost all trace gases in the troposphere
Radiation Forcing
The Global Mean Radiative Forcing of the Climate System for the
year 2000, relative to 1750
Radiative Forcing (Watts per square metre)
Cooling
Warming
3
2
1
Halocarbons
N2O
CH4
CO2
Aerosols
Tropospheric
Ozone
Black carbon
from fossil fuel
burning
Mineral Dust
Aviation-induced
Contrails Cirrus
0
Stratospheric Ozone
1
2
Solar
Organic
Carbon Biomass
Sulphate from Burning
Fossil
Fuel Burning
Land-use
(Albedo) only
Aerosol Indirect Effect
High Medium Medium Low
Very
Low
Very
Low
Very Very
Low Low
Very
Low
Very Very Very
Low Low Low
Level of Scientific Understanding
Figure by MIT OCW.
• Our estimates
are ‘best’ for the
greenhouse
gases and
decrease in
certainty for
ozone
processes and
mineral and
sulfate aerosols
Now some examples of compounds that are
of great interest in the ocean….
Biogenic sulfur cycle
A negative
climate
feedback loop
to stabilize
temperature?
# concentration
of cloud droplets
Cloud
albedo
Change in cloud
optical thickness
Cloud condensation
nuclei (CCN)
Cloud nucleation
Sulfate
Aerosols
SO42-
Atmospheric acidity
& sulfur transport to
continents
Loss of solar
radiation
Oxidation
Surface
temp. of
Earth
DMSg
Solar
irradiance
at ocean sfc
Sea to air transport
DMSaq
Marine ecology and phytoplankton
abundance, speciation, and synthesis
rates of precursor, DMSP
CLAW hypothesis after Charlson et al., 1987
Sulfur flux
• DMS contributes > 90% of the oceanic sulfur flux and >
50% of the global biogenic flux
• Estimated anthropogenic sources of sulfur are
approximately three times larger than biogenic sources BUT
the lifetimes are much shorter
Upper Troposphere: Less than 20% of the
sulfate is from anthropogenic sources
Derived mostly from DMS
Deep
convective
events
Sulfur
SO
Dioxide2
Volcano
Oil refining
Coal and Oil
Combustion
Sea-spray
Sulfate
Aerosols
SO42-
Decay of vegetation
Phytoplankton DMS
DMS impacts atmospheric chemistry
Carbonyl sulfide
Hydrogen sulfide
COS
H2S
OH
OH
OH, hν
Carbon
disulfide
CS2
Wet or dry
deposition
OH
Sulfuric acid
OH
SO2
NO3
night
DMS
OH or IO (DMSO)
MSA
or
DMSO
H2SO4
H2O2
OH
day
Wet or dry
deposition
Methanesulfonate
Dimethylsulfoxide
• Acid rain - in the North Sea
region DMS can account for up
to 25% of the tropospheric
H2SO4 over Europe
• Is thought to be the source of
the stratospheric COS layer and
background atm. levels of MSA,
SO2, and nssSO42-
Iron Cycling – Mineral aerosols
• In the equatorial Pacific, the subarctic Pacific, and the
Southern Ocean nitrate and phosphate concentrations
are high year round and standing stocks of
phytoplankton are always low =
High Nitrate Low Chlorophyll (HNLC) regions
• Iron limitation?? (1988, Martin and Fitzwater)
• Fe(III) versus Fe(II)
Source
Flux
Fluvial particulate total iron
625 to 962
1.5
Fluvial dissolved iron
34 to 211
Glacial sediments
Atmospheric
16
Coastal erosion
8
Hydrothermal
14
Authigenic
5
Figure by MIT OCW.
Jickells et al., 2005
Input of new iron to the surface
waters is dominated by the
atmospheric deposition of
soluble iron in mineral aerosols
(wet & dry deposition)
Iron in the ocean
Image removed due to copyright restrictions.
• Sub-nanomolar concentrations in surface waters
• Fe is extremely insoluble in oxygenated seawater, so the
bio availability of Fe is controlled by presence of organic
ligands that enhance Fe solubility
Dust variability
Image removed due to copyright restrictions.
• Dust (iron) input to the ocean is highly variable:
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Spatially
Seasonally (rainfall & transport patterns) & glacial-interglacial
Episodic (wind speed)
Solubility
Dust transport examples
SeaWiFS image from Feb 28, 2000
Image Courtesy NASA GSFC
Rainfall sample from Sargasso Sea, July 2004
Nitrogen deposition
• Nitrous oxide from the ocean contributes ~4 Tg yr-1 to
the atmosphere (upwelling and deep convective areas)
O(1D) + N2O ⇒ NO + NO
• Anthropogenic sources of NOx (NOx = NO + NO2) are
fossil fuels, biomass burning, and fertilizer
• NOx chemistry in stratosphere:
NO + O3 ⇒ NO2 + O2
OH + NO2 + M ⇒ HNO3 + M
(M = species which dissipates energy)
• HNO3 nitric acid is highly soluble in droplets = wet
depositions
• Photolysis of NO2 is the only known way of producing
ozone in the troposphere
Acid Rain
GLOBAL PATTERN OF ACIDITY OF PRECIPITATION
4.6
5.0
4.4-5.6
4.5
5.5
5.0
6.0
4.5
4.0
4.0-5.0
5.0
4.5
4.5-5.0
6.0
5.0
5.9
5.0
6.3
5.5
5.5
5.2
4.7
4.7
5.1
4.7
4.1
3.8-5.4
Areas where effects have been reported
Isolated measurements in remote areas
6.3
Estimated Distribution of pH
Figure by MIT OCW.
• Natural precipitation is slightly acidic (~ 5.6)
• HNO3 & H2SO4 decrease pH & deliver N and S to the
surface ocean
Halogen chemistry
• Organic biogenic oceanic sources: methyl halides
CH3Cl, CH3Br, CH3I
• CH3Cl, CH3Br have lifetimes > 1 year wrt OH so they can
reach the stratosphere and function as catalytic species
in O3 chemistry
• Inorganic sources: sea salt which is ‘activated’
NaCls + ClONO2g ⇒ Cl2g + NaNO3s
Cl2 is readily photolyzed in the troposphere
• Sea-salt particles are an important removal pathway
(reaction surface) of nitrogen oxides in the marine
troposphere:
HNO3(g) + NaCl(s,aq) --> NaNO3(s,aq) + HCl(g)
This affects the NOx partitioning and the ozone chemistry
Summary
• Atmospheric gases and aerosols impact the
global radiation and heat budget
• Trace constituents dominate these effects
• The reaction pathways and feedback
mechanisms are often complex, interrelated,
and cyclical making climate change estimates
difficult
• Many biogeochemical cycles deliver key
substances to the ocean (iron, nitrogen, sulfur)
and vice versa