Observation and interpretation of the seasonal cycles in the surface concentrations of ozone and carbon monoxide at Mace Head, Ireland from 1990 to 1994

In this study, three independent methods have been applied to the sorting of the daily and hourly mean concentrations of ozone and carbon monoxide measured at Mace Head, Ireland. From the entire 1990-1994 dataset, 61% of the hourly data points have been assigned to Northern Hemisphere midlatitude background air. The mean ozone and carbon monoxide concentrations in these air masses have been estimated as 35 +/- 4.3 and 125 +/- 27 ppb, respectively. We have characterised the seasonal variations of ozone and carbon monoxide in these air masses and found them to exhibit spring-time maxima and summer-time minima in monthly mean concentrations. Similar seasonal cycles have been found in the monthly mean concentrations of a number of trace gases in Northern Hemisphere mid-latitude background air at Mace Head, in addition to ozone and carbon monoxide. The occurrence of spring-time maxima is not unique to ozone and spring-time maxima are observed for many trace gases whether or not they have stratospheric sources. (C) 1997 Elsevier Science Ltd.