no x source composition climate earth system lightning, chemistry and the impacts on climate oliver...

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]