Potential for photochemical ozone formation in the troposphere over the North Atlantic as derived from aircraft observations during ACSOE

In this paper, ozone (O 3 ), water vapor (H 2 O), carbon monoxide (CO), and peroxide concentrations and photolysis rates measured in the troposphere over the North Atlantic during two Atmospheric Chemistry Studies in the Oceanic Environment (ACSOE) aircraft field campaigns are used to calculate the concentration of nitric oxide (NO) required for net photochemical O 3 production (nP O3 ) to be positive (NO comp ). NO comp tended to show a decrease with altitude, although it was sometimes found to be low in the marine boundary layer (MBL) where H 2 O concentrations were high and O 3 concentrations were low. nP O3 was calculated for the spring when NO data were available and was found to be mostly negative and generally increased from about −0.5 to −0.2 ppbv hr −1 in the MBL to +0.04 ppbv hr −1 at about 7–8 km altitude. The results suggest that much of the lower and middle troposphere over the eastern North Atlantic during spring is in a state of slow net photochemical O 3 destruction. However, in the upper troposphere, the system changes to one of net photochemical production, which results from the drier environment and higher NO concentrations. Furthermore, examples of net O 3 production were also observed in the lower and middle troposphere associated with either in situ sources of NO or long‐range transport of pollution. The paper also illustrates the sensitivity of this O 3 production/loss state to H 2 O and NO concentrations, photolysis rates, and temperatures.