On the role of lightning NOx 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 w estern Pacific and eastern Indian oceans between mid-November and mid-Decem ber shows a prominent ozone maximum reaching 50-80 ppbv between 5 and 10 km in the 20 degrees S-40 degrees S latitude band. This maximum contrasts wit h 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 oz one plume extending from Africa to the western Pacific across the Indian oc ean. These plumes occur several months after the peak in biomass burning in fluence and during a period of high lightning activity in the Southern Hemi sphere tropical belt. The composition and geographical extent of these plum es are similar to the ozone layers previously encountered during the biomas s burning season in this region. Our model results suggest that production of nitrogen oxides from lightning strokes sustains the NOx (= NO + NO2) lev els and the ozone photochemical production required in the upper tropospher e to form these persistent elevated ozone layers emanating from biomass bur ning regions.