Ozone production in the upper troposphere and the influence of aircraft during SONEX: Approach of NOx-saturated conditions

During October/November 1997, simultaneous observations of NO, HO2 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(O3), and to examine the relationship between P(O3) and the concentrations of NOx (= NO + NO2) and HOx (= OH + peroxy) radicals. A positive correlation is found between P(O3) and NOx over the entire data set, which reflects the association of elevated HOx with elevated NOx injected by deep convection and lightning. By filtering out this association we find that for NOx>70 pptv, P(O3) is nearly independent of NOx, showing the approach of NOx-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.