Year-round measurements of nitrogen oxides and ozone in the tropical North Atlantic marine boundary layer

A highly sensitive chemiluminescence instrument has been deployed to measure nitric oxide (NO) and nitrogen dioxide (NO2) at the Cape Verde Atmospheric Observatory in the remote tropical North Atlantic marine boundary layer (MBL). Using two different methods, the instrument was assessed to have a detection limit of around 1.8 parts per trillion by volume (pptv) for NO and 5.5 pptv for NO2 for hour-long integration periods. The overall accuracy was estimated at similar to 18% for NO and 30% for NO2. Measurements of NO, NO2, and ozone (O-3) over a period of 12 months in 2007 show very low levels of NOx (typically <30 pptv) and net daytime ozone destruction on most days of the measurement period. Air originating over Africa exhibited the highest levels of NOx (similar to 35 pptv) and reduced daily O-3 destruction, with O-3 production observed on a few days. Air that had not originated over Africa showed lower NOx levels (similar to 25 pptv), with greater observed O-3 destruction. A dependence of the observed O-3 destruction on NO mixing ratios, averaged over all air masses, was observed and reproduced using a simple box model. The model results imply that the presence of between 17 and 34 pptv of NO (depending on the month) would be required to turn the tropical North Atlantic from an O-3 destroying to an O-3 producing regime.