Ozone production in the upper troposphere and the influence of aircraft during SONEX: approach of NO_x‐saturated conditions

During October/November 1997, simultaneous observations of NO, HO 2 and other species were obtained as part of the SONEX campaign in the upper troposphere. We use these observations, over the North Atlantic (40–60°N), to derive ozone production rates, P(O 3 ), and to examine the relationship between P(O 3 ) and the concentrations of NO x (= NO + NO 2 ) and HO x (= OH + peroxy) radicals. A positive correlation is found between P(O 3 ) and NO x over the entire data set, which reflects the association of elevated HO x with elevated NO x injected by deep convection and lightning. By filtering out this association we find that for NO x >70 pptv, P(O 3 ) is nearly independent of NO x , showing the approach of NO x ‐saturated conditions. Predicted doubling of aircraft emissions in the future will result in less than doubling of the aircraft contribution to ozone over the North Atlantic in the fall. Greater sensitivity to aircraft emissions would be expected in the summer.