Advancing understanding of the marine biogeochemical influence on primary sea spray aerosol composition

Remote marine clouds, such as those over the Southern Ocean, develop in an extremely pristine environment, with very little influence of anthropogenic and continental sources of atmospheric aerosols and their precursors. Because they occur in regions with extremely low background aerosol concentrations, remote marine clouds are particularly sensitive to variations in the concentration and composition of aerosols that serve as cloud condensation nuclei (CCN), which can influence the number of cloud droplets formed within clouds, and their radiative properties.

One important aspect of marine aerosol composition is the organic mass fraction (OM) of the CCN-relevant submicron particles. Observational evidence indicates that the organic content of the fine marine aerosol is greatly elevated during the biologically active season near strong, localized mid-latitude phytoplankton blooms, e.g., near active blooms in the North Atlantic and the Southern Ocean. However, other observations indicate that the organic mass fraction is nearly constant in other locations, despite large variations in ocean chlorophyll, an indicator of phytoplankton biomass. A unified explanation of the underlying mechanism that drives variations in sea spray organic mass fraction across ocean ecosystems has been lacking, but primary emissions of oceanic dissolved organic matter in sea spray have been proposed as a mechanism explaining seasonal changes.

We have recently developed a novel framework for mechanistically simulating the fractionation of marine organic matter into sea spray, which potentially resolves some of the apparent discrepancies. The framework models aerosol organic enrichment as resulting from Langmuir adsorption of several classes surface-active macromolecules at the surface of bursting bubbles. Distributions of macromolecular classes are estimated using output from a global marine biogeochemistry model, and chemical properties are assigned using well-studied proxy molecules (Burrows et al., in discussion, ACPD, 2014; Elliott et al., ERL, 2014).

The proposed parameterization independently simulates relationships between chlorophyll-a and the sea spray organic mass fraction that are similar to observations in highly productive bloom regions, but which differ between seasons and ocean regions with different ecosystems. For example, at similar levels of ocean chlorophyll, the predicted variations in organic matter content are far smaller in the Arctic than in the North Atlantic. Also, maximal values of monthly mean chlorophyll and OM fraction coincide in nutrient-rich ocean biomes, but are offset in time in nutrient-poor (oligotrophic) regions. Insights such as these may help to resolve the puzzling inconsistencies between observations in different ocean basins.

We will also present results from a comparison of the simulated OM fraction with a bias-corrected satellite observational dataset. Our analysis shows that OM fraction is a significant predictor of cloud-top droplet number concentration over the Southern Ocean, improving the fraction of variance predicted beyond the amount attributable to sulfate aerosol (McCoy et al., in preparation).

Future work will focus on further evaluating and improving the parameterization based on laboratory and field experiments, as well as on further investigation of the atmospheric implications of the predicted sea spray aerosol chemistry. Experiments that may be useful in improving and testing aspects of our model will be discussed.

References cited:

Burrows, S. M., Ogunro, O., Frossard, A. A., Russell, L. M., Rasch, P. J., and Elliott, S.: A physically-based framework for modelling the organic fractionation of sea spray aerosol from bubble film Langmuir equilibria, Atmos. Chem. Phys. Discuss., 14, 5375-5443, doi:10.5194/acpd-14-5375-2014, 2014.

Elliott, S., Burrows, S. M., Deal, C., Liu, X., Long, M., Ogunro, O., Russell, L. M., and Wingenter O.. "Prospects for simulating macromolecular surfactant chemistry at the ocean–atmosphere boundary." Environmental Research Letters 9, no. 6 (2014): 064012.

McCoy, D., Burrows, S. M. and coauthors, paper in preparation.