On the role of lightning NO_x in the formation of tropospheric ozone plumes: A global model perspective

A series of ozone transects measured each year from 1987 to 1990 over the western Pacific and eastern Indian oceans between mid-November and mid-December shows a prominent ozone maximum reaching 50-80 ppbv between 5 and 10 km in the 20° S-40° S latitude band. This maximum contrasts with ozone mixing ratios lower than 20 ppbv measured at the same altitudes in equatorial regions. Analyses with a global chemical transport model suggest that these elevated ozone values are part of a large-scale tropospheric ozone plume extending from Africa to the western Pacific across the Indian ocean. These plumes occur several months after the peak in biomass burning influence and during a period of high lightning activity in the Southern Hemisphere tropical belt. The composition and geographical extent of these plumes are similar to the ozone layers previously encountered during the biomass burning season in this region. Our model results suggest that production of nitrogen oxides from lightning strokes sustains the NO_x (=NO+NO₂) levels and the ozone photochemical production required in the upper troposphere to form these persistent elevated ozone layers emanating from biomass burning regions.