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 16^th May to 4^th 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 NO_x. An NO₂ peak differential slant column density of 1e16 molec/cm² could be attributed to the lightning activity in the morning of the 26^th May. The corresponding light path length was estimated from the simultaneously measured O₄ absorption. As lightning NO_x 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 NO_x chemistry during a thunderstorm.