GLOMAP

Global Model of Aerosol Processes

8 Jan 2010: SOLAS project concludes with some interesting results

Our contribution to the Surface Ocean-Lower Atmosphere Study (SOLAS) project has concluded. The research was funded by NERC for 3 years.

We have used GLOMAP to understand the factors that control marine aerosol and quantified how changes in aerosol could affect climate. Our most important findings are listed below. We have published 8 papers.

1. The seasonal cycle of cloud condensation nuclei (CCN) in the marine boundary layer (MBL) can be explained in terms of emissions of dimethylsulphide (DMS). A global aerosol microphysics model has been used to confirm this link quantitatively for the first time (Korhonen et al., 2008a).
2. The sensitivity of CCN to changes in DMS emission is lower than previously derived from observations. The model shows that CCN enter the MBL from the free troposphere, where new sulphate particles are formed. However, the long range transport of aerosol through the global free troposphere means that local DMS emissions have only a small impact on CCN concentrations (Korhonen et al., 2008a, Woodhouse et al., 2010). Previous observational studies were based on satellite-derived CCN, which is flawed because CCN are mostly too small to be detectable. (Although funded by a different NERC grant, related work has quantified the global sources of CCN at the surface. Over marine regions we estimate that 55% of CCN come from nucleation, mostly from the free troposphere (Merikanto et al., 2009). This large nucleated fraction has many implications for long term changes in marine aerosol.)
3. There is a globally significant source of marine organic aerosol of 8Tg/a, which we estimated for the first time. This emission is comparable in magnitude to the fossil fuel organic carbon source and increases the global organic carbon burden by 20% (Spracklen et al., 2008).
4. Changes in southern hemisphere winds over the last 20 years (driven mainly by stratospheric ozone depletion and intensification of the jet) have caused increases in sea spray, more aerosol, increases in cloud drop concentrations and a summertime forcing of minus 0.7 W/m2, comparable, but opposite, to the greenhouse gas forcing over the same period.

Other findings are:

1. Localised changes in DMS emission have a very widespread effect on CCN (over thousands of kilometres), thereby explaining why previous ship observations found very little correlation between DMS and aerosol (Woodhouse et al., 2008). This work also showed that proposed iron-DMS-aerosol geoengineering is unlikely to work.
2. Predicting the transport of aerosol to the Arctic is severely hampered by our poor understanding of aerosol wet removal in clouds (Korhonen et al., 2008b).
3. Nucleation of new aerosol in the summer Arctic is a potentially important source of CCN which has previously been ignored (Korhonen et al., 2008b).
4. BrO contributes 13% of the annual DMS oxidation sink and 20% over the Southern Ocean (Breider et al., 2009). We have identified a new feedback between DMS and its oxidation by BrO because DMS affects aerosol acidity and BrO release from sea spray is pH dependent.
5. Previous model predictions of global cloud drop number based on aerosol-drop relations over the Atlantic are unreliable over many other marine regions. The error (compared to a microphysics model) is 25–75% in the Southern Ocean, the Arctic and regions of persistent marine stratocumulus.

Publications

Korhonen, H; Carslaw, KS; Spracklen, DV; Mann, GW; Woodhouse, MT (2008a) Influence of oceanic dimethyl sulfide emissions on cloud condensation nuclei concentrations and seasonality over the remote Southern Hemisphere oceans: A global model study, J GEOPHYS RES-ATMOS, 113(D15), . doi:10.1029/2007JD009718
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Korhonen, H; Carslaw, KS; Spracklen, DV; Ridley, DA; Strom, J (2008b) A global model study of processes controlling aerosol size distributions in the Arctic spring and summer, J GEOPHYS RES-ATMOS, 113(D8). doi:10.1029/2007JD009114

Spracklen, DV; Arnold, SR; Sciare, J; Carslaw, KS; Pio, C (2008) Globally significant oceanic source of organic carbon aerosol, GEOPHYS RES LETT, 35(12), . doi:10.1029/2008GL033359
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Woodhouse, MT; Mann, GW; Carslaw, KS; Boucher, O (2008) New Directions: The impact of oceanic iron fertilisation on cloud condensation nuclei, ATMOS ENVIRON, 42(22), pp5728–5730. doi:10.1016/j.atmosenv.2008.05.005
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Spracklen, DV; Pringle, KJ; Carslaw, KS; Mann, GW; Manktelow, P; Heintzenberg, J (2007) Evaluation of a global aerosol microphysics model against size-resolved particle statistics in the marine atmosphere, ATMOS CHEM PHYS, 7(8), pp2073–2090.

Pringle, KJ; Carslaw, KS; Spracklen, DV; Mann, GM; Chipperfield, MP (2009) The relationship between aerosol and cloud drop number concentrations in a global aerosol microphysics model, ATMOS CHEM PHYS, 9(12), pp4131–4144.
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Korhonen, HK, Carslaw, KS, Forster, PM et al., Aerosol climate feedback due to decadal increases in southern hemisphere wind speeds, in press Geophys. Res. Lett., 2009.

Breider, TJ, Chipperfield, MP, Richards, NAD and Carslaw, KS, The impact of BrO on dimethylsulfide in the remote narine boundary layer, in press, Geophys. Res. Lett., 2009.

Woodhouse, MT, Carslaw, KS, Mann, GW, Vallina, S, Vogt, M, Halloran, P, and Boucher, O, Low sensitivity of cloud condensation nuclei to changes in the sea-air flux of dimethyl-sulphide, to be submitted to Atmos. Chem. Phys., 2010.

08/01/10 14:00