Spring and summer night-time high ozone episodes in the upper Brahmaputra valley of North East India and their association with lightning

- Night-time O3 is higher on thunderstorm days as compared to that on clear days.
- Lightning induces simultaneous peak and dip in NOx and O3 followed by rise in O3.
- LNOx contributes ∼33% of the lightning induced O3.
- O3 production during lightning depends on the lightning intensity.
- Transportation from lightning active regions contributes to observed O3.

Simultaneous and round the clock measurement during thunderstorm and lightning has been used to study the impact of lightning generated NOx (LNOx) on O3 mixing ratio within the boundary layer during night-time. Lightning flash rate obtained from Lightning Image Sensor (LIS) on board TRMM over the Brahmaputra valley centred around Dibrugarh has been used to identify the events in which NOx was elevated with simultaneous dip in O3 for a short duration followed by subsequent elevation of O3 in spring and summer of 2012-2014. Seasonally averaged O3 attains higher values at night on thunderstorm days as compared to that on clear and cloudy days both in spring and summer i.e. as against a minimum night-time (1700-0500 h) values of ∼3 ppb on clear days, the mean night-time level on thunderstorm days is ∼18 ppb in spring and ∼12 ppb in summer. The estimated steady state O3 concentration is well correlated with the NO2/NO ratio and found to be ∼33% of the measured O3. One day isentropic concentration weighted trajectory (CWT) analysis carried out for the spring and summer seasons reveal that part of GWB and Bangladesh are the potential additional source of O3 in both the seasons. LNOx is found to contribute to ∼33% of the measured O3 during the study period. Enhancement of O3 is well correlated with the lightning radiance indicating direct photodissociation of O3 due to lightning. ΔO3 is independent of the location of the flash rather depends on flash intensity.