Rediscovering Atmospheric Surfactants

Abstract

In recent decades there has been surprisingly little research on surface-active compounds in the atmosphere. Why should we care, you ask? Are surfactants such a big deal? We know how anthropogenic surfactants in sea spay in some areas led to the death of long-established beach front pines, but is there a broader impact of atmospheric surfactants? Well, indeed there is, and it is the subject of continuing discussion and speculation. In the 1970s, a number of investigators studying organic micropollutants considered the existance of surface-active compounds in the atmosphere. Long-chain carboxylic acids were seen as a likely source of surface activity. There was also a growing interest in the potential of the sea surface microlayer to accumulate organic compounds and complexed metals, and to possibly transport the materials into the atmosphere during bubble bursting.[1,2] Appel et al. found some two thirds of the non-carbonate carbon in aerosols to be surface active.[3] By the early 1980s Seidl and Hanel[4] and Gill et al.[5] had established the presence of surface-active substances in both rainwater and aerosols, but only at some 50 picomoles per cubic metre, or 10% of total aerosol mass.This was so low that there was little chance that these compounds would play an important role in reducing droplet size or altering the rate of transfer of water to and from droplets. Despite this, Winfried Seidl[6] retained an interest in surfactants into the current millennium reasserting the point that soluble surfactants in rainwater behaved as if they were the expected long-chain carboxylic acids, recently identified using TOF-SIMS.[7] The early 21st century has seen a re-emergence of research on atmospheric surfactants along with a great deal of speculation about the potential role they might play. This interest has paralleled a rising number of papers on water-soluble organic materials and the awareness of the ubiquitous presence of humic-like substances (HULIS) in the atmosphere. Thus atmospheric surfactants have recently re-emerged with much of the enthusiasm they found among marine aerosol chemists a quarter-century ago. The current work on atmospheric surfactants now seems far-reaching and imaginative. Dobson et al.[8] have championed a role for such compounds in the origin of life. They propose an inverted micelle model for aerosol structure where the surfactants coat the aqueous core. The surfaceactive organic compounds have their hydrocarbon tails facing into the atmosphere and more polar ends in the water. In this way organic materials that accumulated at the surface of an early ocean would, once ejected as aerosols, have the high concentrations of materials necessary for the origin of life. Evidence of this process would have long been lost, but not the excitement such speculation has brought in allowing aerosol science to interact with exobiology. An equally broad vision comes from Cristina Facchini and her colleagues, who conceived that surface activity may play a role in controlling global climate. It had long been clear that surface tension was an important control on droplet size and aerosol activation,[9] but the re-interest in surfactants reminds us that the small changes in droplet populations they induce could alter cloud albedo and as well as the formation of precipitation;[10,11] yet another mechanism for natural and anthropogenic climate forcing. The sea surface microlayer is once again of interest. Caterina Oppo has, for some years, been studying the potential of the ocean surface to redistribute trace substances, through bubble bursting. This could be important in the mobilization of micropollutants at a global level. In an era where it is no longer generally believed that metals are strongly bound within the sea surface microlayer, attention has turned more towards the organic materials here as a mode of transfer to the atmosphere. Thus we have seen studies of the accumulation of polycyclic aromatic hydrocarbons, polychlorinated biphenyls, and tributyltin at the sea surface.[12] Surfactants can certainly solubilize materials, so they might also increase the bioavailability, especially that of relatively non-polar compounds. However atmospheric scientists are not always aware of the relevance of lipids as surfactants in biological systems. The pulmonary surfactant that coats the human lung is much affected by air pollutants. Ozone and trace metals can oxidize this material, reducing gas exchange.[13] Although particles are known to interact with pulmonary surfactants,[14] the effect of