Diurnal Variation of Non Methane Hydrocarbons and Carbon Monoxide in the Subantarctic Atmosphere

Non methane hydrocarbons and carbon monoxide have been monitored during the El CID campaign in the Kerguelen Archipelago (49° 21 S; 70° 13 E) from January 22 to February 22, 2002. Non methane hydrocarbons were monitored in situ for the first time in these remote areas by using an automated GC. Over the whole period about 1400 measurements of C₂-C₅ NMHCs have been realized with a step time of 30 minutes. It was observed for the C₂-C₅ alkenes (in the range of 10 to 100 pptv) a very regular diurnal trend with mixing ratios systematically peaking at 15-16 H local time, i.e. shifted by about 3 hours off the solar noon. Simultaneously, the CO levels remained always comprised between 30 and 50 ppbv, i.e. a figure characteristic of pure marine air masses at these latitudes, but CO presented in the same way diurnal fluctuations of few ppbv strongly correlated with those of alkenes. On the opposite this diurnal trend was hardly visible for alkanes. The occurrence of a remarkable correlation between UV irradiance and alkene mixing ratios in the marine air supports the evidence of a photolytic production of alkenes by degradation of dissolved organic carbon (DOC) as pointed out by several other authors. Therefore from a 1D model, the budget of NMHC and CO in the atmospheric column was examined by considering the usual chemical processes of alkene destruction in the atmospheric column, and exchanges at the top of the boundary layer. Assuming that the magnitude of the marine source magnitude was dependent on the UV irradiance, the diurnal cycles observed were perfectly reproduced for alkenes as well as for CO. Based on simultaneous measurements of dissolved alkenes in seawater this marine source appears to be consistent with strong alkenes (and probably CO) gradient within the seawater surface. Other synoptic observations including air masses back trajectories, and chlorophyll distribution in surface waters explain the variability of the alkenes and CO backgrounds consistent with the spatial variability of the marine photolytic source by degradation of DOC. Periods of high ventilation correspond also to significant changes in the alkanes/alkenes distribution in the atmosphere probably linked to turbulent mixing of surface water. Besides marine alkenes and CO similar features, our study finally support the existence of the measured significant levels (up to 100 pptv) of short lived NMHCs in the background sub-antarctic atmosphere due to their photolytic production by surface seawater; it also enables to conclude that different production mechanisms occur for alkanes involving possible biological processes in depth in the water column.