Measurements of trace gases with a MAX-DOAS system during a ship cruise from the Canaries to Ecuador (SO287) on board of RV Sonne

Ship-based trace gas measurements are of particular interest to the scientific community as they fill a gap in knowledge of trace gas concentrations in the marine boundary layer (MBL) on the open ocean. Remote sensing techniques, such as Multi AXis Differential Optical Absorption Spectroscopy (MAX-DOAS), offer the capability of probing air masses located further away from the ship, compared to in situ instruments. In this way, MAX-DOAS measurements taken during ship cruises can be used to examine larger volumes of air in the MBL where routinely measured data is sparse.

We present measurements of a MAX-DOAS system installed on the research vessel Sonne during the cruise SO287 from Las Palmas (Gran Canaria, Spain) to Guayaquil (Ecuador) from the 11th of December 2021 until the 11th of January 2022.    
Ship-based anthropogenic emissions have been identified as higher slant column densities (SCD) of nitrogen dioxide (NO₂) and sulphur dioxide (SO₂) while biogenic emissions are mainly found close to land, as indicated by higher SCDs of formaldehyde (HCHO). On the open ocean, the frequently detected abundance of iodine monoxide (IO) emphasizes that the MBL is mainly dominated by emissions from the ocean (algae) in the absence of anthropogenic emissions.               
In a second step, these SCD results have been analysed with the profiling algorithm BOREAS to further assess the vertical extent of trace gases in the MBL. Since trace gas concentrations are highest in the lowest kilometre, emission sources close to the shipping route and the ocean surface dominate the measurements taken during cruise SO287.