Abstract
Abstract. Aerosols have a large potential to influence climate through their effects on the microphysics and optical properties of clouds and, hence, on the Earth's radiation budget. Aerosol–cloud interactions have been intensively studied in polluted air, but the possibility that the marine biosphere plays an important role in regulating cloud brightness in the pristine oceanic atmosphere remains largely unexplored. We used 9 yr of global satellite data and ocean climatologies to derive parameterizations of the temporal variability of (a) production fluxes of sulfur aerosols formed by the oxidation of the biogenic gas dimethylsulfide emitted from the sea surface; (b) production fluxes of secondary organic aerosols from biogenic organic volatiles; (c) emission fluxes of biogenic primary organic aerosols ejected by wind action on sea surface; and (d) emission fluxes of sea salt also lifted by the wind upon bubble bursting. Series of global monthly estimates of these fluxes were correlated to series of potential cloud condensation nuclei (CCN) numbers derived from satellite (MODIS). More detailed comparisons among weekly series of estimated fluxes and satellite-derived cloud droplet effective radius (re) data were conducted at locations spread among polluted and clean regions of the oceanic atmosphere. The outcome of the statistical analysis was that positive correlation to CCN numbers and negative correlation to re were common at mid and high latitude for sulfur and organic secondary aerosols, indicating both might be important in seeding cloud droplet activation. Conversely, primary aerosols (organic and sea salt) showed widespread positive correlations to CCN only at low latitudes. Correlations to re were more variable, non-significant or positive, suggesting that, despite contributing to large shares of the marine aerosol mass, primary aerosols are not widespread major drivers of the variability of cloud microphysics. Validation against ground measurements pointed out that the parameterizations used captured fairly well the variability of aerosol production fluxes in most cases, yet some caution is warranted because there is room for further improvement, particularly for primary organic aerosol. Uncertainties and synergies are discussed, and recommendations of research needs are given.
Highlights
Aerosols have a great impact on the Earth’s radiative budget by direct and indirect interactions with solar radiation
To investigate if there is a widespread seasonal regional relationship between secondary aerosol precursors or primary aerosols ejected from the ocean surface and the number concentration of particles potentially acting as cloud condensation nuclei (CCN), we constructed global maps of correlation coefficients of monthly data over the period 2001–2009 (Fig. 1)
At temperate to high latitudes, CCN numbers are higher in summer (Vallina et al, 2007), and this is the season when DMS occurs at its maximum and when high OH radical concentrations combine with high primary productivity rates to give rise to maximum secondary organic aerosol (SOA) fluxes
Summary
Aerosols have a great impact on the Earth’s radiative budget by direct and indirect interactions with solar radiation. When the CLAW hypothesis was published (Charlson et al, 1987), DMS was thought to be the main, if not the only, source of new CCN in the pristine ocean This scenario has been complicated with the discovery of the aforementioned wide range of volatiles and particles with potential to influence cloud condensation (O’Dowd et al, 1997; Andreae and Rosenfeld, 2008). We parameterize the emission fluxes of POA and sea salt from the surface ocean These aerosol sources are compared with the satellite-derived amount of potential CCN and size of cloud droplets on both a monthly and weekly bases over a 9-yr period. Temporal correlations at both the global scale and representative locations are analyzed as a means to assess the potential role of each marine aerosol source in driving the variability of cloud microphysics
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