Sudden increases in the NO₂ column caused by thunderstorms: a case study in the northern subtropical region

Abstract. A long-term program for NO₂ column measurements started in 1993 at the subtropical Izaña Observatory (28° N, 16° W). Seasonal evolution shows a small day-to-day variability as compared with higher latitudes. Sharp increases in the column appear occasionally superimposed on the annual cycle. The origin of these spikes is explored by considering the possibility of tropospheric transport from polluted areas, stratospheric intrusions, meridional transport in the stratosphere and production by lightning, in a case study. From radiative transfer calculations and meteorological information available, it is shown that the NO₂ increase takes place in the upper troposphere with values of 300–400 pptv. Back-trajectories reveal that, for the case studied, the air masses came from an area of thunderstorms located upwind. After the analysis of the various possibilities, the NO₂ increase by lightning production appears to be the most feasible cause. Annual distribution of spikes displays a maximum in late winter and spring during the shift from midlatitude winter tropopause to summer tropopause.