Airborne observations from 14 flights in marine stratus over the Gulf of Maine and Bay of Fundy in August and September of 1993 are examined for the relationships among the cloud droplet number concentrations (Nd), the out-of-cloud aerosol particle number concentrations (N-a), the major ion concentrations in the cloud water, and turbulence in cloud. There was a wide range of aerosol concentrations, but when low stratus and the main anthropogenic plume from eastern North America were in the same area the plume overrode the cloud. The N-d increased with increasing N-a and cloud water sulfate concentration (cwSO(4)(=)), but the relationships were very weak. The separation of the data between smooth and lightly turbulent air substantially improved the ability to explain the variance in the N-d by either of these two quantities. Also, the relative increase in N-d for increases in N-a and cwSO(4)(=) was greater for lightly turbulent air than for smooth air. The estimated minimum size of particles activated in these clouds ranged from 0.14 mu m to 0.31 mu m, corresponding to average supersaturations of <0.1%. The minimum size tended to be lower for lightly turbulent air and smaller N-a. The results for lightly turbulent air agree well with previously reported parameterizations of the impact of aerosols on N-d, but the results for smooth air do not agree. In general, more knowledge of the physical factors controlling the N-d in stratiform clouds, such as turbulence, is needed to improve not only our ability to represent N-d but also to increase our understanding of the impact of the aerosol particles on the N-d and climate.
Daily air measurements at one midcontinental and two coastal sites on the Atlantic Ocean and Pacific Ocean in Canada during April 1992 to June 1993 are used to show the seasonality of methanesulfonate (MSA), non-sea-salt (nss) SO4=, and SO2. At the Atlantic site, Kejimkujik National Park, Nova Scotia (44 degrees 22'N, 65 degrees 12'W), the concentration ranges were <0.002-0.13, 0.2-26, and <0.2-10 mu g m(-3) for MSA, nss SO4=, and SO,, respectively, and the annual means were 0.02, 2.2, and 1.3 mu g m(-3). MSA had a monthly median peak of 0.036 mu g m(-3) in June, versus an SO2 peak of 2.7 mu g m(-3) in February, and no discernible nss SO4= seasonal peak. The MSA/nss SO4= ratios were low compared to ratios in other parts of the world but exhibited a seasonality dominated by that in MSA. At the midcontinental site, Experimental Lake Area, Ontario (49 degrees 39'N, 93 degrees 43'W), the concentrations were usually lower than at the coastal sites. The ranges were <0.002-0.066, 0.04-12.5, and <0.16-16.6 mu g m(-3) for MSA, nss SO4=, and SO2, respectively, and the annual means were 0.01, 1.6, and 0.93 mu g m(-3). The monthly median seasonal peaks at this site were 0.01 mu g m(-3) for MSA in September, 2.1 mu g m for nss SO4= in March, and 2.8 mu g m(-3) for SO2 in January. The MSA/nss SO4= ratio at this site was similar to that found at Kejimkujik. At Saturna Island, British Columbia (48 degrees 47'N, 123 degrees 08'W), the ranges were 0.002-0.2, 0.14-6.4, and 0.12-21 mu g m(-3) for MSA, nss SO4=, and SO2, respectively, and the annual means were 0.06, 1.1, and 3.0 mu g m(-3). The monthly median peaks were 0.12 mu g m(-3) for MSA in August, 5 mu g m(-3) for SO2 in February, and a m nominal peak of 1.5 mu g m(-3) for nss SO4= in August. The MSA/nss SO4= ratio had a seasonal pattern dominated by that of MSA with a peak of 0.1 in August. These data should enhance the existing databases for testing global sulfur models.