Two case studies are discussed that evaluate the effect of ocean emissions on aerosol-cloud interactions. A review of the first case study from the eastern Pacific Ocean shows that simultaneous aircraft and space-borne observations are valuable in detecting links between ocean biota emissions and marine aerosols, but that the effect of the former on cloud microphysics is less clear owing to interference from background anthropogenic pollution and the difficulty with field experiments in obtaining a wide range of aerosol conditions to robustly quantify ocean effects on aerosol-cloud interactions. To address these limitations, a second case was investigated using remote sensing data over the less polluted Southern Ocean region. The results indicate that cloud drop size is reduced more for a fixed increase in aerosol particles during periods of higher ocean chlorophyll A. Potential biases in the results owing to statistical issues in the data analysis are discussed. 1. Introduction Since oceans cover ~70% of the earth surface, they represent a massive source of gaseous and aerosol emissions that mix with ship and continental emissions to form a highly complex soup of marine aerosol particles. Aerosols directly interact with solar radiation via scattering and absorption of light, and they also serve as cloud condensation nuclei (CCN) and influence cloud properties and reflectivity. Attention to the importance of aerosols in cloud and rain formation can be traced back several decades ago to observations that maritime clouds exhibit lower droplet concentrations than similar clouds influenced by anthropogenic emissions over continental areas, and that the maritime clouds often rain in less than 30 minutes [1–3]. Since that time, research has pointed to two critical pieces of information linking aerosols to warm clouds: (i) more numerous subcloud aerosol particles result in more reflective clouds (all else being fixed) because of more abundant and smaller cloud droplets [4] and (ii) for more numerous and smaller cloud droplets, suppressed droplet collision-coalescence results in less precipitation [5]. But observational and modeling studies often provide conflicting results with regard to the magnitude and even the sign of aerosol effects on clouds and precipitation [6]. Furthermore, aerosol-cloud interactions represent the largest uncertainty in assessments of the total anthropogenic radiative forcing [7]. As shown in Figure 1 (see red arrows), aerosols are at the heart of the effect of ocean emissions on cloud properties. The sources and nature of marine
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