Lightning and Atmospheric Chemistry

Transcription

1 Lightning and Atmospheric Chemistry 1785 Cavendish performed the first experiments with a spark discharge in glass tube. Discovered that oxidized nitrogen (NO x =NO + NO 2 ) compounds resulted from the burning of air von Liebig discovered nitric acid (HNO 3 ) in rain water and related it to Cavendish s experiments. NO x oxidized in drops. Nitric acid (HNO 3 ) provides an important source of nitrate for biosphere. Important in evolution of life Crutzen showed that tropospheric ozone (O 3 ) was strongly influenced by the amounts of atmospheric NO x. Ozone is poisonous to people, animals, plants, and is harmful to perishable materials such as rubber, plastics, etc s led to a dramatic increase in the study of lightning produced NO x since O 3 is a strong greenhouse gas.

2 Nitrogen: Nitrogen is a major component of the atmosphere, but an essential nutrient in short supply to living organisms. OXIDATION STATES OF NITROGEN N has 5 electrons in valence shell a9 oxidation states from 3 to +5 Increasing oxidation number (oxidation reactions) NH 3 Ammonia NH 4 + N 2 N 2 O Nitrous oxide NO Nitric oxide HONO Nitrous acid NO 2 - NO 2 Nitrogen dioxide HNO 3 Nitric acid NO 3 - Ammonium Nitrite Nitrate R 1 N(R 2 )R 3 Organic N free radical free radical N 2 O 5 Nitrogen pentoxide Decreasing oxidation number (reduction reactions)

3 NO x [NO+NO 2 ]: Why is NOx important? NOx indirectly affects our local air quality and global climate Has a strong influence on Ozone (O3) and hydroxyl radical (OH) concentration is a primary pollutant found in photochemical smog is a precursor for tropospheric ozone formation TROPOSPHERIC OZONE: is the third most important greenhouse gas impacts the Earth s radiation budget and can cause changes in atmospheric circulation patterns. is toxic to humans, plants and animals.

4 EULINOX Observational Evidence of LNOx

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7 Lightning-Produced NO x Theoretical estimates Laboratory estimates Field Measurements Tuck (1976) Chameides et al. (1977) Chameides (1979) Dawson (1980) Hill et al. (1980) Bhetanabhotla et al. (1985) Chameides et al. (1977) Levine et al. (1981) Peyrous and Lapeyre (1982) Borucki and Chameides (1984) Wang et al. (1998) Noxon (1976, 1978) Kowalczyk and Bauer (1982) Drapcho et al. (1983) Franzblau and Popp (1989) Huntrieser (1999) Tg N / yr

8 Lightning: An Important Source of NO x Current Annual NO x Source Fossil Fuel Burning Biomass Burning Lightning Soil Emissions Aviation N2O Degradation Total Tg N/yr (28) 4-24 (10) 1-20 (5-7) 1-16 (5.5) (0.7) (0.4) ~50 [Schumann and Huntrieser 2007]

9 Why such large uncertainties? To get a global number one has to answer 3 questions: What is the energy of a typical lightning discharge? How much NO x is produced per unit energy? How do we extrapolate to the globe?

10 1. Energy of a lightning discharge E 1 = L I(t) dt (t) r(t) 2 E 2 = V I(t) dt = V Q

11 Wang et al. (1998)

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13 How long is a typical lightning channel?

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15 Current (Amperes) E 2 = V I(t) dt = V Q I(t) = I o [exp -at -exp -bt +exp -ct ] I o = 35 ka I o = ka V ~ 3x10 8 Volts Time (microseconds) E = Joules <E> = 6.7 x 10 9 J (Price et al., 1997)

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18 2. How much NO x is produced per unit energy? T~30,000 K O 2 N 2 O N N NO NO 2 NO NO O NO NO 2 NO ~1mm ~5 cm

19 Temperature of Lightning

20 How much air is processed by lightning?

21 Size of lightning channel

22 Zel dovich Reactions O 2 O + O O + N 2 NO + N N + O 2 NO + O When the lightning channel cools below T ~ 2500 K NO x remains fixed or frozen in the atmosphere (fixed nitrogen) ~85% of NO x is in form of NO With volume mixing ratios of 1-4% P (NO) = molecules/joule

23 Wang et al. (1998)

24 Hot channel ~cm Region of coronae and streamers due to high electric fields surrounding channel ~ meters O + + N 2 NO + + N NO + + e - NO NO + + N N 2 O + N 2 O + + e - N 2 O Much larger volume of air

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29 3. Global Extrapolation

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31 Flashrate Parameterization CTH Z_IC Flashrate parameterization of Price and Rind [1992] F continental CTH 4.9 CC F marine CTH C Z_CG IC/CG Ratio parameterization of Price and Rind [1993] IC-CG Ratio CC 4.0

32 Using Satellite Observations of Clouds Using ISCCP clouds from Annual mean NO x production is 12 Tg N/yr Price et al. (1997)

33 Monthly NOx production (Tg) Global estimates of monthly NO production (Tg) J F M A M J J A S O N D Month

34 Using Modelled Clouds

35 How do we model lightning in GCMs? GCM simulations using lightning parameterizations Total Lightning: F ~ H 5 over land F ~ H 1.7 over oceans (Price & Rind, 1992) Fraction of CG vs. IC lightning: IC/CG ~ cold cloud thickness (Price and Rind, 1993)

36 Observations Model

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38 Levy et al. (1999)

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40 Obs (satellite) Model (6 TgN/yr) Model (4-8 TgN/yr) Model (no lightning) TOP-DOWN ESTIMATES OF GLOBAL LIGHTNING NOx EMISSIONS Using SCIAMACHY (NO 2 ), OMI (O 3 ), ACE-FTS (HNO 3 ): Target locations/times where NO 2 column is dominated by lightning source Global source of 6 ± 2 TgN/yr from lightning [Martin et al., 2007]

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42 Formation of Ozone (O 3 ) OH + CO H + CO 2 H + O 2 + M HO 2 + M Low NO x High NO x HO 2 + O 3 OH + 2O 2 Net: CO + O 3 CO 2 + 2O 2 Ozone destruction HO 2 + NO OH + NO 2 NO 2 + h NO + O O + O 2 + M O 3 + M Net: CO + 2O 2 CO 2 + O 3 Ozone production

43 Simplified Chemistry of Nitrogen Oxides Exploit Longer Lifetimes in Upper Troposphere hv NO NO 2 O 3, RO 2 Ozone (O 3 ) lifetime ~ month Upper Troposphere NOx lifetime ~ week HNO 3 lifetime ~ weeks NO / NO2 with altitude hv NO NO 2 O 3, RO 2 NOx lifetime < day Nitrogen Oxides (NO x ) HNO 3 Ozone (O 3 ) lifetime ~ days Boundary Layer

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49 July 21, UT at 400mb Total NO x Lightning NO x Maximum NO x production of 1 ppb (Flatoy and Hov, 1997)

50 July 21, UT Total Ozone Production Ozone Production Due to Lightning Ozone production : max of 1 ppb/hour

51 July 1998 mean at 400mb NO x from lightning Ozone from lightning

52 NASA Goddard Institute for Space Studies (GISS) General Circulation Model (GCM) 2xCO 2 climate - Control = ~ 4 o C global warming Price & Rind (1994) +30%

53 CHANGING LNO X? Warmer climate = more thunderclouds = more lightning Impact: (1) increasing UT ozone formation (positive forcing) (2) Increasing OH leads to small reductions in CH 4 (negative forcing) Models predict % LNO x per K

54 Summary and Conclusions Lightning is a major source of NO x in the troposphere and is likely the largest source in the upper troposphere. Lightning produces between 5-10 Tg N/yr. While NO x production by lightning is a minor contributor to surface O 3 concentrations, and is likely the largest contributor in the tropical upper troposphere. 75% of the lightning-no x and O 3 is produced in the tropics The highest global production of LNO x occurs during JJA and the lowest production occurs during DJF. Due to this natural imbalance, the northern hemisphere had a natural bias in tropospheric ozone, even in pre-industrial times. Future climate change may increase global lightning activity resulting in an increase in tropospheric O 3 (positive feedback).