The Earth's Electrical Environment (1986)

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THE ROLE OF LIGHTNING IN THE CHEMISTRY OF THE ATMOSPHERE 74 tems, a value of 100 flashes/sec was assigned to F by Borucki and Chameides (1984) with an uncertainty factor of 25 percent. Substituting these parameters into Eq. (6.6), R was thus estimated to be about 4 × 1010 W with a possible range of (1.3 to 12) × 1010W. TABLE 6.1 Estimates of NO Yield from Lightning Discharge P(NO) (molecules/J) Investigator A. This work (9 ± 2) × 1016 Based on the calculations of Borucki and Chameides (1984) B. Theoretical calculation 3 × 1016 a Tuck (1976) (3-7.5) × 1016 Chameides et al. (1977) (4-6) × 1016 a Griffing (1977) 80 × 1016 b Hill et al. (1980) 16 × 1016 Hill et al., as corrected by Borucki and Chameides (1984) C. Laboratory spark experiment (8-17) × 1016 Chameides (1979) (6 ± 1) 1016 Chameides et al. (1977) (5 ± 2) × 1016 Levine et al. (1981) (2 ± 0.5) × 1016 Peyrous and Lapeyre (1982) D. Atmospheric measurement (20-30) × 1016 a Noxan (1976) (25-2500) × 1016 a Drapcho et al. (1983) a The NO yields obtained by these investigators were expressed as molecules/flash. These yields were converted to units of molecules/ Joule by assuming EF = 4 × 10 8 J/flash. b Derived from Hill et al. (1980) by dividing their NO yield (6 × 1025 molecules/flash) by their energy per flash [(1.5 × 104 J/m) (5 × 103m/flash)]. TABLE 6.2 Estimates of the Amount of Nitrogen Fixed by Lightning Investigator Nitrogen Fixed per Year(tg)a Tuck (1976) 4.2 Chameides et al. (1977) 30 to 40 Chameides (1979) 35 to 90 Dawson (1980) 3 Hill et al. (1980) 4.4 Levine et al. (1981) 1.8 Kowalczyck and Bauer (1982) 5.7 Ehhalt and Drummond (1982) 5 Peyrous and Lapeyre (1982) 9 Logan (1983) 8 Drapcho et al. (1983) 30 Present result [based on calculations of Borucki and Chameides (1984)] Best Estimate: 2.6 Range: 0.8 to 8 a 1 tg = 1012g = 106 metric tons. The Global NO Production Rate, φ(NO) The above estimates for R and P(NO) can be combined in Eq. (6.3) to yield a global NO production rate of ~2.5 tg of N/yr. However, it should be noted that this number is highly uncertain; Borucki and Chameides (1984) in a similar analysis arrived at a possible range in φ(NO) from 0.8 to 8 tg of N/yr. By far the largest source of uncertainty arises from the uncertainty in E F, the energy dissipated by a lightning flash. A comparison between the estimate for the global fixation rate calculated here and those of previous investigators is presented in Table 6.2. For the most part our result is consistent with, although somewhat smaller than, the other estimates. Biological processes fix atmospheric N2 at a rate of about 200 tg of N/yr and anthropogenic fixation (primarily due to the synthesis of fertilizers) occurs at a rate of about 60 tg of N/yr (Burns and Hardy, 1975). Thus it would appear that, at present, lightning is responsible for at most a few percent of the Earth's total nitrogen fixation. On the other hand, lightning appears to represent one of, if not the, major natural source of NO x to the atmosphere. The other natural sources of atmospheric NO x include stratospheric oxidation of N2O at a rate of 0.6 tg of N/yr (Levy et al., 1980); oxidation of NH3, which is not well known but could be important (Mc-Connell, 1973); and NO emissions from soils as a result of microbial activity at a rate of (1 to 10) tg of N/yr (Galbally and Roy, 1978; Lipschultz et al., 1981). As noted earlier, qualitative confirmation of the importance of lightning as a natural source of atmospheric NO x has been obtained from a variety of NO and NO2 measurements (Noxon, 1976, 1978; Drapcho et al., 1983; Davis and Chameides, 1984), which reveal anomalously high concentrations of NO and NO2 in air within and above clouds in remote regions of the globe. In regions strongly affected by anthropogenic activities, however, this natural NO source is swamped by NO production from the burning of fossil fuel and biomass