Light in the Ocean
.. and its influence on photochemistry
Light travels faster than sound.
This is why some
people appear
bright - until you
hear them speak.
Disclaimer – quote probably not attributed to Einstein
Light is a form of electromagnetic radiation – with
wave-like properties
wavelength (λ) = C/ ν
where λ is the wavelength in meters, C is the speed of light in
a vacuum (3 x 108 m s-1) and ν is the wave frequency (# of
wave crests (cycles) per second)
Electromagnetic energy travels in distinct packets called
photons. The energy in each photon is given by:
E = h
where h is Planck’s constant (6.63 x 10-34 J s) and ν is the
frequency (s-1).
Since ν = C/ λ we can relate energy to wavelength:
E = hC/ λ
Thus, the energy in a photon is inversely proportional to the
wavelength (longer λ = less energy; shorter λ = more energy)
Units used during measurements of light
Einstein = 6.02 x 1023 photons i.e. 1 mole of photons
It is convenient to work with Ein since photochemistry is a
quantum process (if the quantum yield is 1, then one mole of
photons causes 1 mole of molecules to react)
Light photon flux – often given as μEin cm-2 s-1.
Photosynthetically active radiation (PAR) is often given in these
units; A bright summer day has a solar photon flux of about 2500
μEin cm-2 s-1 (= 2500 μmol photons cm-2 s-1)
The rate of light Energy delivery is often given in Watts
1 Watt (W) = 1 Joule second-1
Light energy flux is given in W m-2
Wavelength-specific energy is
often specified. That is, the energy
at a certain λ (or from a range of λ
i.e. PAR (400-700 nm))
Energy per mole of photons at specific wavelengths
Energy
per photon at specific wavelengths
8E-19
Joules per photon
7E-19
UV-R
Visible
6E-19
5E-19
4E-19
InfraRed
3E-19
Photosynthetically-Active
Radiation (PAR) is ~400-700 nM
2E-19
1E-19
0
280
400 nm
380
480
580
680
780
wavelength (nm)
Short wavelength light (i.e. UV) has higher energy per photon!
UV-R
Visible light
Most incident
solar energy is
in the visible
band!
No UV-C
reaches
Earth’s
surface
From Whitehead et al, 2000
Light penetration into the ocean
Light energy is absorbed in seawater such that total light
energy (irradiance) at a given wavelength decreases
exponentially with depth into the water
Total light energy
Iz = irradiance at depth z
Io = irradiance at surface
Kd = attenuation coefficient (m-1)
Depth
(z)
Kd is the fraction
absorbed per meter
Iz = Io e-K
d
z
There will be a different
Kd for each wavelength!
What happens to this light energy?
Absorbed Energy
Iz/Io (fraction of surface irradiance)
Large Kd
rapid
extinction
& shallow
penetration
λ1
λ2
λ3
Depth
Small Kd - slow
extinction with
depth & deep
penetration
Light absorption (i.e. Kd,λ) will be affected
by several factors – more later….
Within the visible bands,
red wavelengths are
absorbed rapidly with
depth. Blue wavelengths
generally penetrate the
deepest.
The penetration of visible
light (PAR) depends on the
characteristics of the
water, including
phytoplankton abundance.
Spectral irradiance
at depth in the ocean
is measured by
spectral radiometers
Spectral radiometer data for optically-clear water from the
central Gulf of Mexico
0
Low UV wavelengths
are attenuated rapidly
with depth. Greater
than 90% of UV-B (<
320 nm) is absorbed
above 15-20 m.
5
305 nm
10
320 nm
340 nm
15
380
nm
20
25
PAR
30
0
20
40
60
Total energy in
400-700 nm band
The 1% PAR depth at
this site was ~120 m
80
100
% of surface irradiance
120
UV absorption properties vary among water masses
305
This is equal to the 10% irradiance depth
From Whitehead et al, 2000
1% light depth for any wavelength is given by 4.6/Kd
That is Iz/Io = e-Kz or ln(Iz/Io) = -Kdz
ln(0.01) = -Kdz
or
4.6/Kd= z1%
DOM is the main chromophore (absorber of light) in the
ocean. More correctly, it is specific constituents of the DOM that
are the chromophores.
Together these organic chromophores constitute the
Colored Dissolved Organic Matter (CDOM)
(also called chromophoric DOM)
CDOM (at high concentration) can give water a yellow color
(Gelbstoffe) and a high optical absorbance, particularly in the
UV part of the spectrum.
Tea colored, black-water rivers are very high in CDOM!
Absorption coefficient (m-1)
Total
0.8
0.6
Light absorption
by seawater is
mainly by
DOM!
DOM
Absorption spectra
for whole water,
and the DOM,
particulate matter,
and pure water
fractions for a
coastal seawater
sample from the
mid Atlantic Bight.
0.4
0.2
particles
Water
0.0
300
350
400
450
Wavelength (nm)
500
550
From DeGrandpre
et al, 1996
Differences in light absorption/attenuation in
different water masses is governed mainly by:
 Particles – organic and inorganic
 DOM - quantity and quality
Influenced by
primary production
and proximity to
rivers and
sediments
The optical absorbance of water is
usually directly related to the DOC
(and DOM) concentration – but this Abs
350 nm
relationship varies from one water
mass to another.
River
water
Shelf
water
Ocean
water
With exposure to UV-R, CDOM becomes bleached
and it losses its absorbance, thereby changing the
A350 vs. DOC relationship
DOC conc.
Seawater CDOM absorption coefficient
for 370 nm light as a function of Chl a
concentration in those same waters
Chlorophyll a (mg m-3)
This study found that seawater
absorption coefficients in these
hyper-oligotrophic waters were
lower than published values for
pure water!
Photochemistry
Photochemistry affects:
 Photosynthesis & Bacterial growth (photobiology)
 Biological reactivity of DOM (both increasing and
decreasing its lability)
 Molecular weight distribution of DOM
 Mineralization (loss) of DOM
 Production of CO2(aq) from DOM
 Metal cycling and availability – via
photoreduction etc.
 Pollutant degradation
When photon (light) energy is absorbed by molecules, a
variety of things can happen.
• Electrons transiently jump to higher orbitals, then
spontaneously fall to their original position (times scales of
nanoseconds). This results in fluorescence with emission being
longer than the  of the photon absorbed (the excitation
photon)
• Molecule becomes “excited” and more reactive A --> A*
• Molecule becomes oxidized (loses electron to a receptor)
• Molecule becomes reduced (steals electron from a donor)
Primary vs. Secondary Photochemistry
For a primary process, compound A absorbs light energy
directly and is converted into terminal products:
hν
AB+C
For a secondary reaction, A absorbs light energy and
becomes excited – but it then transfers the energy to a
receiving molecule B, forming excited-state B*. This can go
on and on …
In this case, A
hν
A  A* + B  B* + A’  chain reactions
Where A* and B* are excited state species
Example: DMS does not absorb light directly so no
primary photolysis. DMS oxidation in the light,
occurs via a photosensitizer (e.g. DOM or NO3-).
functions as a
photosensitizer - it
absorbs light energy
and then causes
something else to
react.
Molecular oxygen is a major reactant in photochemical
reactions (though it doesn’t absorb light directly).
If O2 reacts with excited molecule, highly reactive singlet O2
(1O2) can form
If a photoactive molecule absorbs a photon and donates e- to O2,
it yields superoxide anion (O2-) a reduced form of oxygen that is
highly reactive.
Superoxide can be converted to hydrogen peroxide (H2O2), either
chemically or enzymatically (superoxide dismutase does this).
H2O2 is also a strong oxidant and reactive form of oxygen.
H2O2 can undergo direct photolysis (with UV-R) or can react with
Fe(II) to form OH radicals (OH), one of the most potent oxidants
known.
All these reactive oxygen species are formed in
seawater via photochemical reactions!
Inorganic constituents in seawater are not generally photoreactive.
Several notable exceptions include:
Nitrate
319 - 333 nm
NO3- + H2O + light  NO2- + •OH + OHNitrite
325 - 380 nm
NO2- + H2O + light  NO + •OH + OH-
OH radicals
are about
the most
potent
oxidants
known!
The specific photo-reactivity (per mole) of nitrite is much greater than
for nitrate
Polar seas have
Concentrations in
Antarctic surface waters
Nitrate
15-30 µM
Nitrite
0.1 – 0.2 µM
Nitrate and nitrite can range from 1-15 µM in temperate
waters, mainly in winter and spring. Even higher
concentrations can be found in coastal waters & river plumes
very high nitrate
concentrations!
This has
implications for
photochemisty &
biology
Transition metals such as Fe, Mn & Cu have primary photochemistry
hν
More labile and biologicallyavailable
Fe(III)  Fe(II)
Percent contribution of different wavelength bands of solar radiation to
photoreduction of Fe(III) colloids in Antarctic waters
Surface
irradiance
UV-A (320-400 nm)
Depth integrated in water
column
>60%
55%
Visible (400-700 nm)
30%
40%
UV-B (290-320 nm)
3.5 – 6.5%
1.8 – 3.0%
From Rijkenberg et al. 2005. GRL
Photoreduction (direct & photosensitized) is important in maintaining metals in surface
waters and keeping some of the metal pool available to phytoplankton
Influence of photochemistry on organic compounds
• Mainly driven by UV-R
• DOM (i.e. CDOM) is the main absorber of UV-R in seawater
• UV-R absorption by CDOM causes alteration of DOM
hν
CDOM
Altered
CDOM
+ photoproducts
CO2
CO
After
Kieber
Mopper
Miller
Moran etc
Photodegradation also
bleaches CDOM,
decreasing its absorption
and its photoreactivity
COS
H 2 O2
Low molecular weight
organic compounds e.g.
formaldehyde, glyoxylate,
etc. (i.e. labile to bacteria!)
Moran and Zepp, 1997, L&O
From Mopper and Kieber, 2000
Photooxidation as a major
sink for refractory DOM
in the sea
Photochemical Blast Zone - some DOM oxidized
Upwelling of
refractory,
old DOM
NADW formation.
Labile DOM is
utilized in
relatively short
time - leaving old
refractory carbon
to make another
circuit
Deep water transit (= 1000 y)
Little alteration of old, refractory carbon
If ultra-refractory DOM has average age of 6000 years, and if ocean circulation time is
1000 y, then on average 16.7% of this old carbon will be lost each circulation cycle.
Summary of Important photolabile compounds in seawater
Compounds which are
photolabile
Products
Nitrate (NO3-)
•OH, NO2-
Nitrite (NO2-)
•OH, NO?
Fe3+
Fe2+
DOM-humic substances
Anthropogenic pollutants
H2O2, Low molecular weight
acids and aldehydes, CO,
CO2, COS
Modified pollutants
DMS (dimethylsulfide)
DMSO and other products
Optical Buoy - In situ incubations in natural light field
0
Quartz tubes
5
305 nm
10
320 nm
340 nm
Incubated
water
experiences
natural light
field
15
380
nm
20
25
PAR
30
0
20
40
60
80
100
% of surface irradiance
120
Comparison of Photochemical and Biological DMS loss processes
– Ross Sea Polynya, Terra Nova Bay - January 13, 2005
-1
DMS consumption rate constant (d )
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0
In situ irradiation
5
Photolysis
10
Depth (m)
MLD
15
Bio consumption
from CTD samples
20
25
30
35
Conditions:
Photolysis
Bio consumption
Shallow mixing,
calm winds,
cloudy am,
sunny pm.
How are the biological processes affected by
exposure depth (UV radiation)?
-1
-1
Bacterial production (nM Leu d ) DMSPd consumption (nM d )
Irradiation depth (m)
0.0
0.5
1.0
1.5
DMS consumption (nM d-1)
0 2 4 6 8 10 12 14 16 18
2.0
0
0
0
0
5
5
5
10
10
10
15
15
15
20
20
25
25
30
30
20
25
30
In situ irradiation
Array
Dark
2
4
6
8
10 12
Mixing Depth governs exposure
of surface plankton to PAR and
UV-R
Hypothetical UV-R
penetration
Deep Mixing
Mixing depth also
affects surface nutrient
regime and distribution
of key phytoplankton
e.g. N-fixers
Shallow Mixing
0
MLD = 50m
MLD = 25m
25
Depth (m)
UV Shade
50
75
100
Deep mixing
gives lower
dosage to surface
plankton - allows
recovery/repair
from UV damage
t
Shallow mixing
results in higher UV
dosage for surface
plankton, with less
recovery time
t
Using Solar UV to disinfect
drinking water
National Geographic,
April, 2010
Our changing atmosphere –
stratospheric ozone depletion and the increase
of UV-radiation at the Earth’s surface
Stratospheric ozone depletion
is causing UV-R to going up
everywhere on Earth
• Highest total solar energy and highest total
UV energy are at the equator.
• Largest seasonal variability in UV-R occurs
at high latitudes- like the Arctic and Antarctic.
The ozone hole
over Antarctica
enhances flux of
UV-B at the sea
surface
Spectral shift in energy under ozone hole conditions
Ozone in the atmosphere
is measured in Dobson
Units (DU).
Data from Palmer Station, 1993
Marine organisms are very
sensitive to UV-B radiation!
Dave Kieber
How does UV-R affect biology (and hence
chemistry) in the surface ocean?
 Inhibition of photosynthesis
 Inhibition of bacterial production and growth
 Selection force for UV-resistant organisms, and those
able to adapt by production of UV screens (i.e.
Mycosporine amino acids)
 Possible mutagen driving evolution?
 Factor affecting viability of eggs and larvae of
macroorganisms that reside in surface waters?
 Possible synergism with pollutants (e.g. PAH’s)
End
Metals (e.g. Fe, Mn) held in organic complexes including
• Humics
• EDTA
• Siderophores
are photolabile – resulting in photoreduction of metal and oxidation
of the organic molecule.
This type of photosensitized metal photoreduction is more
important than primary photo-reduction of the metals.
Photoreduction (direct & photosensitized) is important in
maintaining metals in surface waters and keeping some of the
metal pool available to phytoplankton
The absorption of light is a quantum process
Φ = quantum yield = # of reactions / # of photons absorbed
The rate of a photochemical
reaction of A is given by:
dA

  AI A
dt
Quantum yield
Only when light is absorbed
can photochemical reactions
occur – if compounds are
transparent to light, then no
photo reactions occur
Specific absorption of actinic
radiation – light absorbed per
unit volume of water per unit
time
Actinic = chemically-active
radiation
If the concentration of the actual
chromophore is not known, the
apparent quantum yield (ΦA )
is reported. The ΦA for seawater
reactions is usually << 1 (few
reactions per photon absorbed)
The quantum yield for marine photochemical reactions is often much
less than 1. For example, the quantum yields of H2O2 photoproduction
range from 0.00003 to 0.001.
Quantum yields
also tend to
decrease at
longer
wavelengths
(less energy per
photon).
Figure from Moran and Zepp, 1997
Open
symbols are
seawater
Absorption of light in the sea
> 95% of light energy reaching the
ocean surface enters the water if 
> 20o. 5-7% of light that enters
ocean is lost due to backscatter
(water leaving radiance)
reflection

Absorption
5-7 %
backscatter
About 50% of absorbed radiation
is infrared which heats the water.
Total attenuation of light in aquatic systems
K=
Total
attenuation
aw
Absorbance
by water
molecules
+
ap
+
ao
Absorbance Absorbance
by particles by organics
+
ai +
Absorbance
by inorganic
molecules
Sw +
Scattering by
water
Sp
Scattering by
particles
Absorbance means light energy is absorbed by chemicals in the
system. Molecules that absorb light are called chromophores
Practical considerations for UV-research
Optical properties of various lab materials
 Optical quartz – transparent to virtually all wavelengths
 Borosilicate glass (e.g. Pyrex, Kimax, Duran) - (variable –
can cut off < 340 nm)
 Teflon (FEP) – Transparent to most UV- but some
scattering
 Polycarbonate (cuts off < 340 nm)
 Whirlpak polyethylene bags - Transparent to UV-R –
convenient to use, but must check for contamination
 Acrylics – different optical properties depending on type
(see next slide)
Selective filtration of light wavelengths
Plexiglas-G – cuts
off < 370 nm
Mylar-D – cuts off <
315 nm (i.e. UV-B)
Mylar D and UF-3 acrylic % Transmittance
100
90
% transmittance
UF-3 (or UV-O)
acrylic – cuts off <
400 nm (i.e. all UVR)
80
70
60
50
40
30
Mylar-D
20
UF-3 Acrylic
10
0
270 300 330 360 390 420 450 480 510 540 570 600
Wavelength (nM)
The ozone hole over Antarctica increased through
2005 – letting in more ultraviolet B radiation
2004 Total Ozone Mapping Satelite (TOMS) ftp://jwocky.gsfc.nasa.gov/pub/eptoms/images/spole/Y2004/
Stratospheric ozone depletion
affects the total flux of ultraviolet
radiation (UVR) to the Earth’s
surface, but has a bigger impact on
UVB fluxes.
 Natural ozone in the stratosphere
absorbs UV-B radiation (290-320 nm)
 The protective layer is being destroyed by
man-made chlorofluorocarbons (CFC’s)
 Natural sources of halocarbons (i.e.
methyl bromide) also play a role in ozone
destruction
Action Spectra - For photochemical reactions, the rate of
reaction is a function of wavelength
Sunlight normalized
action spectrum
Action spectrum
Rate of
DMS
photolysis
Rate of
DMS
photolysis
per
photon
290
In
daylight
Wavelength (nm)
400
290
More
photons in
UV-A
Peak activity
near 350 nm
Wavelength (nm)
(the type of dependency is governed by the light absorbance
of the material or system)
400
Bouillon & Miller, GRL 2004
DMS photolysis is
mainly driven by UV
wavelengths
Apparent quantum
yield
DMS loss (%)
100
10 m
150m
80
60
65% of photolysis caused by UV-A
40
35% by UV-B
20
Toole et al., GRL 2004
0
Full 305
320
345
380
395 Dark
Optical Cut-off Filter (nm)
(Toole et al., GRL 2004)
About 35% of the
DMS photolysis
attributed to NO3-
Nitrite (M)
0
1
2
3
4
5
100
DMS loss (%)
High DMS photolysis rates
in Southern Ocean waters
are partially due to natural
high nitrate concentrations
A
80
+Nitrate
60
40
+Nitrite
20
0
The remaining 65% of
photolysis was due to
highly reactive CDOM
0
100 200 300 400 500
Nitrate (M)
2.0
B
k (h-1)
1.6
1.2
Natural
NO3conc.
0.8
0.4
Added NO3-
0.0
0
100
200
300
400
Nitrate (M)
500
Influence of NO3- on
DMS photochemistry
also documented in the
sub-Arctic Pacific
during an Fe-fertilized
phytoplankton bloom
NO3- drawdown in patch 
Decreasing DMS
photolysis rate
constant with time
Bouillon and Miller,
(GRL, 2004)
Photolysis rate given
here as apparent
quantum yield (AQY) at
330 nm.
NO3drawdown in
bloom
Photolysis dominated DMS loss processes at the ice
edge station, during pre-bloom conditions. Data from
Toole et al. 2004 GRL
Rate constants for depth-integrated water column DMS loss
processes at station M (Nov 11 &13, 2003).
Process
rate constant (d-1)
Sea-air exchange
0.023
Biological consumption
0.083 - 0.20
Photolysis
0.5 - 0.71
Summary of DMS photolysis in high
latitude, nitrate-rich waters
• Toole et al., 2004 results – high photolysis rates in pre-bloom
Southern Ocean waters compared to other ocean locations–
partly due to nitrate, but also high CDOM reactivity
• Photolysis was major loss process in ice edge waters where bio
and ventilation loss was low
• Mechanism still unknown – Bouillon (OH radicals vs. Toole
(not just OH)
• DMSO yield from DMS photolysis ranged from 33-45% in
pre-bloom waters
Strong
absorption
Weak
absorption
Strong
absorption
Surface irradiance
Downwelling spectral irradiance
Irradiance at
depths indicated
Strong
absorption
means rapid
attenuation
with depth
UVR
PAR
Adapted from: Smith et al. 1992.
Ozone depletion: UV radiation and
plankton biology in Antarctic
waters. Science 255: 952
PAR
UV energy, as % of total,
decreases with greater sun
angle
Larger zenith angle means
longer path through
atmosphere, and hence
more UV absorption
79o
Infrared
Earth
atmosphere
From Whitehead et al, 2000
0o
zenith
angle
The quartz tube parade
Quartz tube retrieval – oh,
so gentle
Bacterial production and biological DMS turnover are strongly
inhibited under surface irradiance conditions
Leucine incorporation (nM h-1)
0.1
0.2
0.4
0.5
Dark incubated
sample
5
10
0.3
Full spectrum
light at depth
15
20
Leucine incorporation is an index of
bacterial protein production
0.0
0
Incubation depth (m)
Incubation depth (m)
0.0
0
Biological DMS loss rate constant (d-1)
5
10
0.1 0.2
0.3 0.4
0.5
0.6
Dark incubated
sample
Full spectrum
light at depth
15
20
In situ quartz tube deployment - April 23, 2000
Central Gulf of Mexico
In permitting the ozone layer to be destroyed and the
intensity of UV at the Earth's surface to increase, we are
posing challenges of unknown but worrisome severity to
the fabric of life on our planet. We are ignorant about the
complex mutual dependencies of the beings on Earth ad
what the sequential consequences will be if we wipe out
some especially vulnerable microbes on which larger
organisms depend. We are tugging at a planet wide tapestry
and do not know whether one thread only will come out in
our hands, or whether the whole tapestry will unravel
before us.
Carl Sagan, Billions and Billions, 1997
The quality of light is important!
Shorter wavelengths have higher energy per photon. These
photons can cause reactions that would not occur (or would
occur at slower rates) with longer wavelength photons.
This is why the UV-B part of the spectrum is important!
UV-B drives many photochemical reactions
UV-B also directly affects biological systems
 DNA absorbs UV-B directly, causing thymine-thymine dimers
and other DNA damage. Harmful effects of UV are only
significant if rate of damage exceeds rate of repair (see work of J.
Cullen and co-workers).
 UV-A (320-400 nm) may also be harmful because there are
more photons of this relatively high energy radiation
Chlorofluorocarbon gases increased dramatically in the atmosphere but now
appear to be leveling off. Many CFC’s are long lived in the atmosphere and
are potent greenhouse gases
CFC-11 is
just one type
CFC
http://lifesci.ucsb.edu/~biolum/chem/
Great animation on light generation and
fluorescence
http://lifesci.ucsb.edu/~biolum/chem/
Important photoproducts in seawater
Compounds which are
photochemically produced
Source
H2O2
DOM+O2
CO (carbon monoxide) and CO2 DOM – humic substances
COS (Carbonyl sulfide)
Organic sulfur compounds
Low molecular weight acids and DOM
aldehydes (pyruvate,
formaldehyde, acetaldehyde etc)
Reduced metals
DMSO
Fe3+ and Mn4+ organic
complexes, especially humic
complexes
DMS (dimethylsulfide)
A 37% increase in DMS
photolysis is predicted
under ozone hole
conditions
Calculated spectral photolysis rate of dimethylsulfide in Antarctic
seawater under ozone hole and non–ozone hole conditions.
Ozone hole
The Antarctic ozone hole - 1995
Dobson units
Dobson units (a measure of atmos.
Source: NOAA TOVS
satellite ozone)
column
Gulf of
Mexico
Specific rate of light
absorption of chromophoric
DOM (Ka) plotted as a
function of wavelength for
estuarine and oceanic water.
Note the differences in scales!
Mobile Bay
From Mopper and Kieber, 2000
CFC’s catalytically destroy stratospheric ozone
 Each molecule of CFC can destroy many
molecules of ozone
 This destruction is especially severe at the
poles such that ozone holes develop, letting in
much more UV-B than normal.
Ozone hole over a city for
first time (from news story
on MSNBC)
Chile’s Punta Arenas
is first urban area
to fall under hole
For two days last month
the ozone hole, seen in
purple, extended over
Punta Arenas, Chile,
located at the
southwestern tip of
South America.
In general, ozone depletion is greater at higher latitudes. Thus, the decrease
near Seattle will be greater than near Los Angeles, while Miami will see the
smallest depletion of the three US cities. However, southern cities also have
much higher incidence of UVB light; even with less depletion, the net increase
in UVB can be greater.
http://www.epa.gov/docs/ozone/science/marcomp.html
High photolysis rates of DMS in surface layers may be offset by
lower rates of biological consumption
DMS photoxidation rate constant - 35S-DMS in
quartz tubes incubated in situ - Station 4
DMS Total loss rate constant - and Bacterial
production
DMS loss rate constant (per day)
0.2
0.4
0.6
0.8
0.0
1.0
0
0
-5
-5
-10
Bacterial
production
scaled to dark
DMS loss
-15
-20
-25
light
dark
Leucine
Leu dark
Depth (m)
Depth (m)
0.0
Fraction of DMS converted per day
0.2
0.4
0.6
0.8
-10
-15
-20
-25
light
dark
1.0
Other Photoreactive inorganic species:
Hydrogen peroxide
H2O2 + UV-R  •OH
Adapted from: Smith et al. 1992.
Ozone depletion: UV radiation and
plankton biology in Antarctic
waters. Science 255: 952
Electromagnetic spectrum
Wavelength (λ)
Gamma
X- rays
Ultraviolet
UV-C
UV-B
Visible
Infrared
Radio waves
UV-A
Energy per photon
All are forms of radiation
Jargon: rays = photons = waves
Components of Solar radiation
UV-C
190-290 nm
none reaches Earth surface
UV-B
290-320 nm
energetic photons, but few reach surface
UV-A
320-400 nm
less energy per photon, but more photons
PAR
400-700 nm
photosynthetically active radiation
IR
700- >1000 nm Near Infrared (heat) (far IR is up to 300,000 nm)