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

5. 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

6. Global NOx Sources Global NO Emissions Lightning contribution ~10% of current NOx source ~40% of preindustrial source Free Troposphere NO Emissions Latitude

7. Source Distribution Annual total NO source kgN/km2/yr • 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

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

9. 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

10. Response to Lightning • Impact on Global Tropospheric Chemistry

11. Effects of Lightning NO Lightning NO Source Change in O3 Chemistry • x 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 Tg/day %

12. Effects on NOy Deposition Lightning NO Source NOy Deposition 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

14. Effects on O3 Deposition Lightning NO Source O3 Deposition January January July July kgN/km2/month kg/km2/month

15. NO Climate O3 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) A temperature increase of 2°C may give extra 1.5 TgN/yr: more than increase in air traffic! External Forcing

16. NO Climate O3 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! CH4

17. Lightning and Climate Integrated Radiative Forcing from NO Sources Earlier studies with a 10% change of lightning NO show an integrated net cooling (only aircraft NO causes a warming) Fossil Fuel Responses to 0.5 TgN/yr Biomass Net Warming Tropics Aircraft Lightning Net Cooling [Wild et al., 2001]

18. 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

19. 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

20. 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]