NOx Source
Composition
Climate
Earth System
Lightning, Chemistry and the Impacts on Climate
Oliver WildDepartment of Environmental Science
Lancaster University
Royal Meteorological Society: The Electrifying Atmosphere, 12th Dec 2007
NOx Source
Composition
Climate
Earth System
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
NOx Source
Composition
Climate
Earth System
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. NOlivier Staiger
NOx Source
Composition
Climate
Earth System
Where is NO formed?
• 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
Vertical Distribution
NOx Source
Composition
Climate
Earth System
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: 2-40×1025 molecules NO• Global flash rate from OTD: 44 s-1
Storm extrapolation 5 (1-25) TgN/yr• Observational assessment of ΔNO (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: 5±3 TgN/yr (uncertain!)Detailed summary of methods in Schumann and Huntrieser, ACP, 2007
NOx Source
Composition
Climate
Earth System
Global NOx Sources
Lightning contribution~10% of current NOx source
~40% of preindustrial source
Present-day NOx Sources (TgN/yr)
Fossil Fuel 28
Biomass Fires 10
Soil 5.5
Lightning 5
Aircraft 0.7
Stratosphere 0.5
Total ~50
Global NO Emissions
Free Troposphere NO Emissions
Latitude
NOx Source
Composition
Climate
Earth System
Source Distribution
• 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
CTM with ECMWF met
Annual total NO source
kgN/km2/yr
NOx Source
Composition
Climate
Earth System
Source Distribution
CTM with ECMWF met
flashes/km2/yr
LIS flash frequency
Annual total NO source
kgN/km2/yr
NOx Source
Composition
Climate
Earth System
Tropospheric Fate of NO
Chemical transformation and deposition
Altitude NOx Lifetime O3 Prod. Eff.
8-12 km 10 days 50
4-8 km 5 days 15
0-4 km 1 day 5
Altitude Dependence
NO NO2 HNO3
PAN
RNO3, N2O5
Wet and dry deposition
HO2 OH
O3
hv
Lifetime 10-20 days
Lifetime 1-100 days Dry deposition
OH
R
hydrolysis
NOx Source
Composition
Climate
Earth System
Response to Lightning
• Impact on Global Tropospheric Chemistry
With Lightning
Without Lightning
Δ due to Lightning
O3 Burden (Tg) 309 262 15%
O3 Production (Tg/yr) 4950 4250 14%
O3 Deposition (Tg/yr) 945 875 7%
NOy Deposition (Tg/yr) 50 45 10%
CH4 Lifetime (yr) 8.7 10.3 -18%
NOx Source
Composition
Climate
Earth System
Effects of Lightning NO
• x
Change in O3 Chemistry
Change in CH4 Chemistry Percent Change in O3 Distribution
Lightning NO Source
Tg/day
Tg/day
Mg/day
%
15 km
10 km
5 km
2 km
0 km
Production
Loss
Loss
NOx Source
Composition
Climate
Earth System
Effects on NOy DepositionNOy DepositionLightning NO Source
kgN/km2/month kgN/km2/month
January January
July July
NOx Source
Composition
Climate
Earth System
Effects on Surface O3
January
July
Lightning NO Source
kgN/km2/month ppbv
January
July
Surface O3
NOx Source
Composition
Climate
Earth System
Effects on O3 DepositionO3 DepositionLightning NO Source
kgN/km2/month kg/km2/month
January January
July July
NOx Source
Composition
Climate
Earth System
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
• Δ Humidity reduces P(O3)
NO
O3Climate
External ForcingA temperature increase of 2°C may give extra 1.5 TgN/yr: more than increase in air traffic!
NOx Source
Composition
Climate
Earth System
Lightning and Climate
• Interactions through greenhouse gas CH4
– Equilibrium response: need to consider CH4 changes
– Lifetime drops from 10.3 to 8.7 years (ΔCH4: -500 ppb)
– Radiative forcing ~-0.2 Wm-2 (0.37 mW m-2 ppb-1 IPCC)
– Also reduces O3 RF by ~⅓
• Implications– Counteracts O3 warming
– No positive feedback cycle
• Net effect of lightning NO– Small radiative cooling!
NO
O3Climate
CH4
NOx Source
Composition
Climate
Earth System
Lightning and Climate
Earlier studies with a 10% change of lightning NO show an integrated net cooling (only aircraft NO causes a warming)
Integrated Radiative Forcing from NO Sources
Lightning
Aircraft
Tropics
Biomass
Fossil Fuel
Net Cooling
Net Warming
[Wild et al., 2001]
Responses to 0.5 TgN/yr
NOx Source
Composition
Climate
Earth System
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
UDA-ARS Air Quality Program, NCSU
Smaller impacts than from fossil fuel usage, but full interactions have not been quantified!
NOx Source
Composition
Climate
Earth System
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
NOx Source
Composition
Climate
Earth System
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]