On the production of active nitrogen by thunderstorms over New Mexico

Abstract

In July and August of 1989 the National Center for Atmospheric Research (NCAR) Sabreliner jet aircraft was used to probe electrically active and inactive convective storms over west central New Mexico to examine the production of odd nitrogen in the middle and upper troposphere by thunderstorms. In the anvil outflow or cloud top region of active and nonactive storms, the majority of flights showed that O3 was reduced relative to the extracloud air owing to transport of ozone‐poor air from lower altitudes. A similar result was found for active nitrogen (NOx) and total odd nitrogen (NOy) in nonelectrically active storms, but the reduction in NOy was also enhanced by removal of soluble constituents during convective transport. Examples of efficient removal from the gas phase are described. There was no evidence of O3 production by lightning discharges. Indeed, O3 was a good tracer over the lifetime (∼1 hour) of the storms. During the active‐to‐mature stage of air mass thunderstorms, large enhancements in active nitrogen were observed in the anvil altitude region (9–11.8 km) and, in one case, in the midlevel outflow (near 7 km) of a dissipating thunderstorm. Two thunderstorms allow good estimates of the NOx production by lightning within or transport to the upper altitude region (8–11.8 km). Thunderstorms of August 12 and August 19 yield amounts in the range of 253–296 kg(N) and 263–305 kg(N), respectively. If, as an exercise, these amounts are extrapolated to the global scale on the basis of the number of cloud‐to‐ground and intracloud lightning flashes counted or estimated for each storm and a global flash frequency of 100 s−1 the result is 2.4–2.7 and 2.0–2.2 Tg(N)/yr. Alternatively, an estimate for the two storms made on the basis of the average number of thunderstorms that occur per day globally (44,000) yields amounts in the range of 4.1–4.7 and 4.2–4.9 Tg(N)/yr, respectively. These estimates only apply to the production or transport of lightning‐generated NOx in or to the altitude region between 8 km and the top of the thunderstorm anvil (∼11.8 km in these studies). Since in some large‐scale models, lightning‐generated NOx is equally distributed by mass into each tropospheric layer, our estimates are roughly equivalent to those model runs that use a global source strength of about twice our estimate for the upper altitude region. In several flights where the region below the base of thunderstorms was examined, no large enhancements in odd nitrogen which could be clearly attributed to lightning were observed. Apparently, the aircraft was not in the right place at the right time. Thus no estimate of the NOx production by lightning that remains below ∼8 km could be made.