The nitrogen cycle

TOPICS FOR TODAY
The Nitrogen Cycle
Fixed Nitrogen in the Atmosphere
Sources of NOx
What about N2O?
Nitrogen Cycle: on the particle side
How might the nitrogen cycle be affected by climate change?

OXIDATION STATES OF NITROGEN
N has 5 electrons in valence shell ?9 oxidation states from –3 to +5
Decreasing oxidation number (reduction reactions)
Increasing oxidation number (oxidation reactions)
Nitrogen:
Nitrogen is a major component of the atmosphere, but an essential nutrient in short supply to living organisms.
free radical
free radical

THE NITROGEN CYCLE: MAJOR PROCESSES
ATMOSPHERE
N2
NO
HNO3
NH3/NH4+
NO3-
orgN
BIOSPHERE
LITHOSPHERE
combustion
lightning
oxidation
deposition
assimilation
decay
nitrification
denitri-
fication
biofixation
burial
weathering
free radical
"fixed" or "odd" N is less stable globally=> N2

BOX MODEL OF THE NITROGEN CYCLE
Inventories in Tg N
Flows in Tg N yr-1
[Jacob, 1999]

TOPICS FOR TODAY
The Nitrogen Cycle
Fixed Nitrogen in the Atmosphere
Sources of NOx
What about N2O?
Nitrogen Cycle: on the particle side
How might the nitrogen cycle be affected by climate change?

NOx: KEY TO MAINTAINING THE OXIDIZING POWER OF THE TROPOSPHERE
O3
O2
hν
O3
OH
HO2
hν, H2O
Deposition
NO
H2O2
CO, CH4
NO2
hν
STRATOSPHERE
TROPOSPHERE
8-18 km
SURFACE
ALSO key player in stratospheric O3 loss

NOy CYCLING
HO2
NO
NO2
hν
O3
O3
O2
Combustion
lightning
HNO3
OH, M
O3
NO3
N2O5
M
H2O
PAN
carbonyl
oxidation
T
τ~ 1 day
Example of PAN formation from acetaldehyde:
CH3CHO + OH ? CH3CO + H2O
CH3CO + O2 + M ? CH3C(O)OO + M
CH3C(O)OO+NO2 + M ? CH3C(O)OONO2 + M

PEROXYACETYLNITRATE (PAN) AS RESERVOIR FOR LONG-RANGE TRANSPORT OF NOx
[Jacob, 1999]

TOPICS FOR TODAY
The Nitrogen Cycle
Fixed Nitrogen in the Atmosphere
Sources of NOx
What about N2O?
Nitrogen Cycle: on the particle side
How might the nitrogen cycle be affected by climate change?

NOx EMISSIONS (Tg N yr-1) TO TROPOSPHERE
Zeldovich Mechanism: combustion and lightning
At high T (~2000K) oxygen thermolyzes:
O2 ?? O + O
O + N2 ?? NO + N
N + O2 ?? NO + O
[IPCC, 2007]

USING SATELLITE OBSERVATIONS OF NO2 TO MONITOR NOx EMISSIONS
SCIAMACHY data. May-Oct 2004
(R.V. Martin, Dalhousie U.)
detection
limit

LIGHTNING FLASHES SEEN FROM SPACE (2000)
DJF
JJA
Bottom-up Emission Estimate:
Emission = (# flashes) x (NOx molecules /flash)
IC or CG flash?
Length of flash?
HIGHLY
UNCERTAIN

TOP-DOWN ESTIMATES OF GLOBAL LIGHTNING NOx EMISSIONS
[Martin et al., 2007]
Using SCIAMACHY (NO2), OMI (O3), ACE-FTS (HNO3): Target locations/times where NO2 column is dominated by lightning source
Global source of 6 ± 2 TgN/yr from lightning
Obs (satellite)
Model (6 TgN/yr)
Model (4-8 TgN/yr)
Model (no lightning)

USING SATELLITE OBSERVATIONS TO ESTIMATE SOIL NOx EMISSIONS
Use GOME observations over Africa:
Soils: 3.3 TgN/year
Biomass Burning: 3.8 TgN/year
40% of surface NOx emissions!
Extrapolating to all the tropics:
7.3 TgN/year biogenic soil
(twice the IPCC value)
Biomass Burning
Soils
Fossil + biofuels
[Jaeglé et al., 2004]

GROWING CONTRIBUTION OF AGRICULTURE TO
N CYCLE
[IPCC, 2007]

TOPICS FOR TODAY
The Nitrogen Cycle
Fixed Nitrogen in the Atmosphere
Sources of NOx
What about N2O?
Nitrogen Cycle: on the particle side
How might the nitrogen cycle be affected by climate change?

N2O: LOW-YIELD PRODUCT OF BACTERIAL NITRIFICATION AND DENITRIFICATION
Important as
source of NOx radicals in stratosphere
greenhouse gas
[IPCC, 2007]

N2O EMISSIONS (Tg N yr-1) TO TROPOSPHERE
[IPCC, 2007]
Source is MOSTLY (~75%) natural

ADDING N2O TO THE NITROGEN BOX MODEL
Although a closed budget can be constructed, uncertainties in sources are large!
(N2O atm mass = 5.13 1018 kg x 3.1 10-7 x28/29 = 1535 Tg )

TOPICS FOR TODAY
The Nitrogen Cycle
Fixed Nitrogen in the Atmosphere
Sources of NOx
What about N2O?
Nitrogen Cycle: on the particle side
How might the nitrogen cycle be affected by climate change?

ANNUAL MEAN PM2.5 CONCENTRATIONS AT U.S. SITES
Air quality
standard
Dry mass concentrations

FORMATION OF SULFATE-NITRATE-AMMONIUM AEROSOLS
[Park et al., 2006]

GLOBAL SOURCES OF AMMONIA
[Park et al., 2004]
VERY UNCERTAIN!
Measurements are tough, so hard to verify regional estimates.

Efficient scavenging
of both HNO3(g) and nitrate aerosol
Efficient scavenging
of both NH3(g) and ammonium aerosol

TOPICS FOR TODAY
The Nitrogen Cycle
Fixed Nitrogen in the Atmosphere
Sources of NOx
What about N2O?
Nitrogen Cycle: on the particle side
How might the nitrogen cycle be affected by climate change?

PREDICTED CHANGES IN ANTHROPOGENIC NOX EMISSIONS
[IPCC 2007 (WG3)]
Emissions declining in NA, EU, growing in AS (transportation), but predicted to level off (may peak as early as 2015). What about natural sources?
Note: this include aviation NOx sources which are small but in UT and have grown from 0.55 to 0.7 Tg/yr from 1992-2002 (may double in next 20 years)

CHANGING LNOX?
Warmer climate = more thunderclouds = more lightning
Impact:
increasing UT ozone formation (positive forcing)
Increasing OH leads to small reductions in CH4 (negative forcing)
Models predict + 4-60 % LNOx per °K

THE NITROGEN CYCLE: MAJOR PROCESSES
ATMOSPHERE
N2
NO
HNO3
NH3/NH4+
NO3-
orgN
BIOSPHERE
LITHOSPHERE
combustion
lightning
oxidation
deposition
assimilation
decay
nitrification
denitri-
fication
biofixation
burial
weathering
"fixed" or "odd" N is less stable globally=> N2