10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Marine Organics Alters Sea Spray Physicochemical and Radiative Properties
Aditya Vaishya, JURGITA OVADNEVAITE, Darius Ceburnis, Colin O’Dowd, National University of Ireland Galway, Ireland
Abstract Number: 994 Working Group: Aerosol Chemistry
Abstract Nature marine haze layers, predominantly driven by sea spray aerosol, effectively act as mirrors, reflecting back to space part of incoming solar radiation thus modulating global radiation budget. Modulations in the sea spray aerosol physical, chemical, and optical properties on account of enrichment by marine organics may influence the properties, extent, and lifetime of marine haze. Over the North Atlantic waters marine organics constitute significant fraction of the sub micron aerosol population [O’Dowd et al., 2004] during high biological activity (HBA) periods – typically occurring in the months of June, July, and August. Marine air masses, when traversed over the biologically rich North Atlantic waters, bring with them sea spray aerosols enriched with primary marine organics to the coastal measurement station Mace Head (MHD) [O’Connor et al., 2008], on the western periphery of Europe.
We present here the radiative and physicochemical properties of the sea spray aerosols, associated with the low biological activity (LBA) and HBA periods in the North Atlantic, advected to MHD during clean air mass conditions. Aerosol scattering coefficient show a LBA high and HBA low while the spectral scattering exponent shows opposite behaviour. The LBA-high and HBA-low in scattering is found to be driven by contribution of wind-speed generated sea spray aerosols to the marine boundary layer. LBA and HBA scattering values were found be dependent on the square of wind-speed, but LBA scattering values were ~ twice that of HBA scattering values. This apparent difference in LBA-HBA scattering is primarily due to changes in sea spray aerosol chemical composition on account of enrichment by marine organics during HBA periods. Under typical marine boundary layer high humidity fields, sea spray enriched with marine organics are optically less active and their parameterized hygroscopic growth shows a dual state of hygroscopicity flipping from high-hygroscopicity to low-hygroscopicity as the organic volume fraction increases from below ~ 0.55 to above ~ 0.55. The effect of this on Top of Atmosphere (TOA) direct radiative forcing (ΔF) is to reduce the cooling contribution of sea-spray by ~ 5.5 times compared to pure sea-salt spray. Further, using multi-instrument coherent measurement of aerosols properties we show that the LBA-HBA difference in aerosol scattering properties, are not just confined to a change in aerosol chemical composition but also due to change in aerosol dominant size mode, both sub micron and super micron. A shift in dominant aerosol mode to lower sizes, due to organic matter enrichment of sea spray, resulted in increase of aerosol sub micron scattering fraction by over 2.5 times. Combining the effects of changing chemistry and size modes, we show that cooling contribution of sea spray enriched in marine organics is suppressed over 30% under humidity fields associated with cloud base as compare to marine boundary layer humidity fields.
The results presented here highlight a significant coupling between the marine biosphere and the direct radiative budget through alteration of sea-spray chemical composition. The shift to smaller sizes is likely to significant increase the lifetime of the sea spray in the atmosphere, possibly extending the duration and impact of marine haze in the global climate system.
[1] O’Connor, T. C., S. G. Jennings, and C. D. O’Dowd (2008), Atmos. Res., 90, 338–355, doi:10.1016/j.atmosres.2008.08.014. [2] O’Dowd, C. D., M. C. Facchini, F. Cavalli, D. Ceburnis, M. Mircea, S. Decesari, S. Fuzzi, J. Y. Young, and J. P. Putaud (2004), Nature, 431, 676–80, doi:10.1038/nature02959.