Detecting lightning NOx from ship-borne MAX DOAS measurements

Transit cruises of German research vessels across the great oceans provide a unique platform for Multi AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements of atmospheric trace gases such as nitrogen dioxide (NO2), formaldehyde (HCHO) and sulphur dioxide (SO2) in pristine locations. Here, we present results of the Extra Atmospheric References for Satellites (EARS) campaign which took place from 16th May to 4th June 2023. During that period the research vessel Meteor was in transit from Walvis Bay (Namibia) to Las Palmas (Canary Islands) from 23 °S to 28 °N crossing the equator at around 10 °W. While crossing the Inner Tropical Convergence Zone (ITCZ), the vessel was sailing through a large convective system consisting of multiple cells and spreading over a few hundred kilometres. The combination of a remote location without nearby anthropogenic sources of NO2 and increased lightning activity allowed for the ground-based detection of lightning NOx. An NO2 peak differential slant column density of 1e16 molec/cm2 could be attributed to the lightning activity in the morning of the 26th May. The corresponding light path length was estimated from the simultaneously measured O4 absorption. As lightning NOx is usually investigated using satellite remote sensing or in-situ on air planes, these ground-based measurements provide a new complementary view on the atmospheric NOx chemistry during a thunderstorm.