Background NOx mixing ratios in air masses over the North Atlantic Ocean

A chemiluminescence analyzer was used to measure NO x mixing ratios at the west coast of Ireland. Two measurement modes allowed the determination of NO and NO x = NO + NO 2 . In a third mode using a molybdenum converter, higher signals were observed than was in the second mode indicating that nitrogen compounds other than NO + NO 2 are registered. They are denoted ‘excess NO x .’ The average NO 2 mixing ratio for a 3 week period was 101±87 pptv. In pure marine air masses identified by means of trajectory calculations, the NO 2 mixing ratios were lower and exhibited in addition a diurnal variation with nighttime values of 37±6 pptv and average values of 87±47 pptv. Possible origins of the diurnal variation are discussed. For such conditions, the NO mixing ratio generally was immeasurably small, certainly less than 10 pptv. The excess NO x is also higher during the day compared with nighttime values of about 70 pptv. Further studies are required to identify the compounds involved.