Lightning, Chemistry and the Impacts on Climate

1
Lightning, Chemistry and the Impacts on Climate

• Oliver Wild
• Department of Environmental Science Lancaster
  University

Royal Meteorological Society The Electrifying
Atmosphere, 12th Dec 2007
2
Overview

• Formation of nitrogen oxides (NOx)
• How, where, how much?
• Effects on atmospheric composition
• Oxidation, lifetimes, deposition
• Implications for climate
• Greenhouse gas abundance
• Implications for the Earth System
• Role in global change

3
How is NO formed?

• Heating in lightning channel
• O2 O O (498 kJ.mol-1)
• N2 N N (941 kJ.mol-1)
• Plasma formation
• High levels of O, N, OH, NO
• Rapid cooling preserves NO
• NOx observed in outflow
• Also in lab (Cavendish, 1785)
• Minor products
• O3, N2O, HNO3, H2O2, CO
• Enhancements not observed
• Result Fixation of atmos. N

Olivier Staiger
4
Where is NO formed?
Vertical Distribution

• Model-based estimates
• Atmospheric observations
• Cloud-resolving model
• Estimate flash rate, yield
• Convective redistribution
• Features
• Detrainment in anvils
• Clearly observed
• Downdrafts to surface
• Assumed, not observed
• About 65 above 8km

Pickering et al., 1998
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How much NO is formed?

• Cannot be measured directly need to estimate
  using
• Flash extrapolation 5 (0.6-13) TgN/yr
• Base on flash energy, flash length or flash rate
• Typical flash 2-401025 molecules NO
• Global flash rate from OTD 44 s-1
• Storm extrapolation 5 (1-25) TgN/yr
• Observational assessment of ?NO (0.3-1.9 ppbv)
• Estimate number of storms (1800 concurrently)
• Estimate mean anvil outflow
• Global Models 5 (2-8) TgN/yr
• Base on NOx, O3 and NOy deposition
• Best estimate 53 TgN/yr (uncertain!)

Detailed summary of methods in Schumann and
Huntrieser, ACP, 2007
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Global NOx Sources
Global NO Emissions
Free Troposphere NO Emissions

• Lightning contribution
• 10 of current NOx source
• 40 of preindustrial source

Latitude
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Source Distribution
Annual total NO source kgN/km2/yr
CTM with ECMWF met

• Distribute based on lightning occurrence
• Flash observations real distribution
• Cloud top height
• Convective mass flux derived distribution
• Convective precipitation
• Results shown here use FRSGC/UCI Chemical
  Transport Model (CTM) with ECMWF met data and
  convective updraft mass flux

8
Source Distribution
Annual total NO source kgN/km2/yr
CTM with ECMWF met
flashes/km2/yr
LIS flash frequency
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Tropospheric Fate of NO

• Chemical transformation and deposition

HO2
OH
RNO3, N2O5
hydrolysis
OH
NO
NO2
HNO3
Lifetime 10-20 days
R
Wet and dry deposition
hv
PAN
O3
Lifetime 1-100 days Dry deposition
Altitude Dependence
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Response to Lightning

• Impact on Global Tropospheric Chemistry

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Effects of Lightning NO
Change in O3 Chemistry
Lightning NO Source
15 km
Production
10 km
5 km
Loss
2 km
0 km
Tg/day
Mg/day
Change in CH4 Chemistry
Percent Change in O3 Distribution
Loss

• x

Tg/day

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Effects on NOy Deposition
NOy Deposition
Lightning NO Source
January
January
July
July
kgN/km2/month
kgN/km2/month
13
Effects on Surface O3
Lightning NO Source
Surface O3
January
January
July
July
kgN/km2/month
ppbv
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Effects on O3 Deposition
O3 Deposition
Lightning NO Source
January
January
July
July
kgN/km2/month
kg/km2/month
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Lightning and Climate

• Interactions through greenhouse gas O3
• Contribution of lightning 45-50 Tg O3 in
  troposphere
• Radiative forcing 0.2 Wm-2 (42 mW m-2 DU-1,
  IPCC)
• Direct short-term warming from O3
• Implications
• Positive climate feedback
• Increased O3, warmer climate
• More convection and lightning?
• Sensitivity very uncertain
• Lightning source increase?
• Model estimates 15 K-1
• ? Humidity reduces P(O3)

A temperature increase of 2C may give extra 1.5
TgN/yr more than increase in air traffic!
External Forcing
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Lightning and Climate

• Interactions through greenhouse gas CH4
• Equilibrium response need to consider CH4
  changes
• Lifetime drops from 10.3 to 8.7 years (?CH4
  -500 ppb)
• Radiative forcing -0.2 Wm-2 (0.37 mW m-2 ppb-1
  IPCC)
• Also reduces O3 RF by ?
• Implications
• Counteracts O3 warming
• No positive feedback cycle
• Net effect of lightning NO
• Small radiative cooling!

CH4
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Lightning and Climate
Integrated Radiative Forcing from NO Sources
Fossil Fuel
Responses to 0.5 TgN/yr
Biomass
Net Warming
Tropics
Aircraft
Lightning
Net Cooling

• Earlier studies with a 10 change of lightning NO
  show an integrated net cooling (only aircraft NO
  causes a warming)

Wild et al., 2001
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Earth System Interactions

• Nitrogen fertilization
• Wet and dry deposition of NOy
• Provides nutrients to vegetation and marine
  ecosystems
• Vegetation damage
• O3 deposition causes leaf damage
• Implications
• Crop production
• Species distributions
• Uptake of CO2
• VOC emissions

Ozone damage to potato leaves
Smaller impacts than from fossil fuel usage, but
full interactions have not been quantified!
UDA-ARS Air Quality Program, NCSU
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Earth System Interactions

• Lightning ignition of wildfires
• Small effect in tropics due to moist conditions
• Accounts for 10-50 of fires over N. America
• Typically more than half of area burned
• Implications
• Potential feedbacks on climate
• Emissions of NOx, CO, VOC, CO2, aerosols
• Direct and indirect effects albedo changes
• Influence on vegetation patterns
• Effects on carbon cycling
• Sensitivity to climate change

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Conclusions

• Major environmental impacts
• Important role in tropospheric composition
• Climate O3, CH4 (net cooling)
• Vegetation O3 and NOy deposition
• Fire O3, NOy, aerosol, vegetation damage
• Big challenges remain
• Improved quantification of NO emissions
• Uncertainties in magnitude, location, response
• Better integration of observations and models
• Quantification of environmental impacts
• Role of lightning in global change
• Requires new generation of Earth System Models
  e.g., MetOffice HadGEM3, NERC QUEST ESM