Department of Earth System Science

This paper begins a study on the role of lightning in maintaining the global distribution of nitrogen oxides (NO_x) in the troposphere. It presents the first global and seasonal distributions of lightning-produced NO_x (LNO_x) based on the observed distribution of electrical storms and the physical properties of lightning strokes. We derive a global rate for cloud-to-ground (CG) flashes of 20–30 flashes/s with a mean energy per flash of 6.7×10⁹ J. Intracloud (IC) flashes are more frequent, 50–70 flashes/s but have 10% of the energy of CG strokes and, consequently, produce significantly less NO_x. It appears to us that the majority of previous studies have mistakenly assumed that all lightning flashes produce the same amount of NO_x, thus overestimating the NO_x production by a factor of 3. On the other hand, we feel these same studies have underestimated the energy released in CG flashes, resulting in two negating assumptions. For CG energies we adopt a production rate of 10×10¹⁶ molecules NO/J based on the current literature. Using a method to simulate global lightning frequencies from satellite-observed cloud data, we have calculated the LNO_x on various spatial (regional, zonal, meridional, and global) and temporal scales (daily, monthly, seasonal, and interannual). Regionally, the production of LNO_x is concentrated over tropical continental regions, predominantly in the summer hemisphere. The annual mean production rate is calculated to be 12.2 Tg N/yr, and we believe it extremely unlikely that this number is less than 5 or more than 20 Tg N/yr. Although most of LNO_x, is produced in the lowest 5 km by CG lightning, convective mixing in the thunderstorms is likely to deposit large amounts of NO_x, in the upper troposphere where it is important in ozone production. On an annual basis, 64% of the LNO_x, is produced in the northern hemisphere, implying that the northern hemisphere should have natural ozone levels as much as 2 times greater than the southern hemisphere, even before anthropogenic influences. The amount of O₃ produced from this NO_x is expected to exceed the stratospheric source by a factor of 1.5, and thus the hemispheric asymmetry in LNO_x would lead to a significant excess of northern hemisphere O₃ even in the preindustrial troposphere. (The monthly climatologies for LNO_x on a 1°×1° latitude-longitude grid can be obtained by e-mail to cprice@flash.tau.ac.il).