Oxidation photochemistry in the Southern Atlantic boundary layer: Unexpected deviations of photochemical steady state

Ozone (O3) is a photochemical oxidant and a green house gas. As the main precursor of the hydroxyl radical (OH) it strongly affects the oxidation power of the atmosphere. The remote MBL is considered an important region in terms of chemical O3 loss; however surface atmospheric observations are sparse and the photochemical processes are not well understood. To investigate the photochemistry of O3 and its precursors in the clean background conditions of the Southern Atlantic Ocean, accurate and precise measurements of NO, NO2, O3, JNO2, J(O1D), HO2, OH, ROx and a range of meteorological parameters were carried out. The concentrations of NO and NO2 measured on board the French research vessel, Marion-Dufresne, crossing the Southern Atlantic (28°S-57°S, 46°W-34°E) in March 2007 are among the lowest amounts yet observed. The data is evaluated for consistency with photochemical steady state (PSS) conditions and the calculations indicate deviations from PSS (Φ>1). The deviations observed under low NOx, conditions (5 to 25 pptv) demonstrate a remarkable and unexpected trend in the Leighton ratio (a ratio used to characterize PSS) dependant on the NO2 mixing ratio and JNO2 intensity. Further, a direct comparison of the measured species with model output of an atmospheric chemistry general circulation model (EMAC) is performed along the exact cruise track. To further investigate the consistency of the measurements and our current understanding of atmospheric radical chemistry a steady-state box model is constrained using data obtained during the cruise. The comparison between measured and model calculated ROx concentrations in the low NOx, regime shows that conventional theory fails to reproduce the observations. The possible causes and implications are discussed.