The background level of the summer tropospheric aerosol, sulphur dioxide and ozone over Greenland and the North Atlantic Ocean

Abstract

A description is given of airborne measurements over Greenland and the surrounding seas of the concentration and size of Aitken nuclei and large nuclei, the concentration of cloud nuclei at a supersaturation of 1%, the concentration of ice nuclei activated at −20°C, the chemical nature of the tropospheric aerosol, the concentration of sulphur dioxide and ozone. The Aitken nuclei concentrations were generally log-normally distributed in the range of less than a 100 to 10 4 cm −3 with strong peaks of nuclei concentrations appearing in widely distributed thin layers superimposed on the background concentrations below 500 cm −3. The geometric mean value was 960 cm −3. The particle size distributions of the Aitken nuclei fluctuated widely, showing spectra of mono-, bi- and trimodal shapes. The particle size spectra are grouped by similarities in shape and their frequencies of occurrence are reported. The size spectra strongly indicate that formation of fresh nuclei by gas phase reactions is a common phenomenon in summer tropospheric air over Greenland and adjacent seas. The large nuclei concentrations were generally log-normally distributed in the region 0.3–300 cm −3 correlating in most cases with the Aitken nuclei concentrations. The geometric mean was 9 cm −3. The size distribution of the large nuclei covers the range 0.3–3 μm dia. and follows a power law above 0.4 μm. Cloud nuclei concentrations are reported for the North Sea and the North Atlantic only, the concentrations were normally low, nearly 50% of the measured concentrations were below 12 cm −3. Peak concentrations of up to 500 cm −3 were measured during a cold front passage. Ice nuclei concentrations were in the range of 0.01–0.1 1 −1. Samples taken for analysis of particulate sulphur compounds gave concentrations in the range 30–110·10 −9 g S m −3 over the nearby seas and 35–220·10 −9 g S m −3 over the ice cap. Measurements of sulphur dioxide gave concentrations in the range 50–725·10 −9 g SO 2 m −3 over the nearby sea areas and 60–1950·10 −9 g SO 2 m −3 in samples taken over the ice cap. A correlation coefficient between particulate sulphur and sulphur dioxide of 0.92 was found for samples taken over the sea areas, whereas a negative correlation between samples from the ice cap areas is dubious. Neutron activation analyses were performed for elements with a potential contribution from ground surface, volcanoes and anthropogenic activities. The volumes of the samples (50–100 m 3) were, however, too low to give reliable results. The mean concentration of ozone measured over the ice cap was 34 ppb, higher with a 90% level of confidence than the mean concentration of 30 ppb, measured over the nearby seas. No correlation between ozone and altitude was found at a 95% level of significance. Measurements of Aitken nuclei concentrations taken on the surface of the ice cap and showing only small variations around 150 cm −3, were strikingly different from the airborne measurements. Air mass trajectories, computed for selected events, show that the air sampled had within its recent history passed over Northern Canada. Greenland or the arctic seas.