Abstract
Abstract. During the Rain in (shallow) Cumulus over the Ocean (RICO) project simultaneous high rate sulfur dioxide (SO2) measurements and cloud condensation nuclei (CCN) spectra were made for the first time. For research flight 14 (14 January 2005) the convective boundary layer was impacted by precipitation and ship plumes for much of the midday period but not in the late afternoon. Number densities of accumulation mode aerosols (0.14 to 0.2 μm diameter) were a factor of two greater in the later period while CCN were 35% to 80% greater for aerosols that activate at supersaturations >0.1%. Linear correlations of SO2 and CCN were found for SO2 concentrations ranging from 20 to 600 parts-per-trillion (pptv). The greatest sensitivities were for SO2 and CCN that activate at supersaturations >0.1% for both clean and polluted air. In a region unaffected by pollution SO2 was linearly correlated only with CCN at >0.2% supersaturation. These correlations imply that the smallest CCN may be activated by SO2 through heterogeneous conversion. Evidence for entrainment of CCN from the cloud layer into the CBL was found.
Highlights
The Rain in Cumulus over the Ocean (RICO) project was an intensive study of shallow cumulus clouds within the trade wind (TW) inversion (Rauber et al, 2007b)
Correlations between SO2 and cloud condensation nuclei (CCN) at supersaturations >0.2 % in the convective boundary layer (CBL) were found which are consistent with SO2 being a primary source of the smallest CCN
In the absence of a pollution source of SO2 CCN were only correlated with SO2 at supersaturations above 0.2 % with strongest correlations ≥0.6 % supersaturation
Summary
RICO was to understand the conditions that exist at the onset of precipitation in the shallow cumulus clouds of the TW regime. Gerber et al (2008) point out these studies inferred the importance of subcloud aerosol measurements and near cloud base concentrations of droplets but neglected processes in the upper portions of the clouds including entrainment of environmental air above the convective boundary layer (CBL). Thornton et al.: Fast sulfur dioxide measurements flight track in the CBL near the surface. While RICO was considered “clean” for aerosol concentrations compared to continental or near shore conditions, the SO2 concentrations were much higher than those encountered in the central Pacific CBL (Thornton et al, 1999; Bandy et al, 1996). During flight legs at 800 and 1300 m above sea level (a.s.l.) devoted to in situ sampling of the shallow cumulus clouds within the TW inversion, numerous encounters of SO2 > 100 pptv within the clouds were observed. The descriptions of the SO2 in clouds are the subject of another paper
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