Relationships between ozone and carbon monoxide at surface sites in the North Atlantic region

As part of the North Atlantic Regional Experiment (NARE), measurements of O-3 and CO at five surface sites were made from July 1991 to January 1995. The investigation of the variabilities and correlation of O-3 and CO presented here indicates that the seasonal cycles of the medians and the means of O-3 and CO are qualitatively similar to the cycles observed at other northern midlatitude sites. The signature of O-3 produced from anthropogenic precursors is dearest in the spring at the Azores and in the summer at Sable Island. The influence of the natural stratospheric O-3 source is apparent at Sable Island, particularly in the spring. At ah sites the variability of CO throughout the year is dominated by episodes of pollution transport. The slopes of the monthly O-3-CO correlations in the summer in Atlantic Canada and the spring in the Azores are quite uniform at 0.3 to 0.4. However, individual pollution transport events often have larger (less than or equal to 1.0) slopes, which indicate significantly different net O-3 production efficiencies between episodes. The average slope of O-3 versus CO at Sable Island in the winter for moderate pollution transport events (CO less than or equal to 180 ppbv) is -0.28, which indicates that the titration of ambient O-3 by emitted NO with little if any photochemical O-3 production dominates the O-3 chemistry over eastern North America in winter. Diurnal cycles driven by photochemistry are observed in the summer for both O-3 and CO at the Azores (net loss) and Sable Island (net production.) These observations are consistent with the work of Duderstadt et al. [this issue] who find positive net photochemical O-3 production at Sable Island, and with the modeling of Atherton et al. [1996] who find a region dominated by photochemical loss of O-3 and CO in the central Atlantic.