Aerosol indirect effects from shipping emissions: sensitivity studies with the global aerosol-climate model ECHAM-HAM

Abstract

Abstract. In this study, we employ the global aerosol-climate model ECHAM-HAM to globally assess aerosol indirect effects (AIEs) resulting from shipping emissions of aerosols and aerosol precursor gases. We implement shipping emissions of sulphur dioxide (SO 2 ), black carbon (BC) and particulate organic matter (POM) for the year 2000 into the model and quantify the model's sensitivity towards uncertainties associated with the emission parameterisation as well as with the shipping emissions themselves. Sensitivity experiments are designed to investigate (i) the uncertainty in the size distribution of emitted particles, (ii) the uncertainty associated with the total amount of emissions, and (iii) the impact of reducing carbonaceous emissions from ships.We use the results from one sensitivity experiment for a detailed discussion of shipping-induced changes in the global aerosol system as well as the resulting impact on cloud properties. From all sensitivity experiments, we find AIEs from shipping emissions to range from −0.32 ± 0.01 W m −2 to −0.07 ± 0.01 W m −2 (global mean value and inter-annual variability as a standard deviation). The magnitude of the AIEs depends much more on the assumed emission size distribution and subsequent aerosol microphysical interactions than on the magnitude of the emissions themselves. It is important to note that although the strongest estimate of AIEs from shipping emissions in this study is relatively large, still much larger estimates have been reported in the literature before on the basis of modelling studies. We find that omitting just carbonaceous particle emissions from ships favours new particle formation in the boundary layer. These newly formed particles contribute just about as much to the CCN budget as the carbonaceous particles would, leaving the globally averaged AIEs nearly unaltered compared to a simulation including carbonaceous particle emissions from ships.

Real-case simulations of aerosol–cloud interactions in ship tracks over the Bay of Biscay

“…This is consistent with the equivalent increase of aerosol number concentration within the size range of activation in these simulations. This result is contradictory to global studies (Righi et al, 2011;Peters et al, 2012), where a high sensitivity of the aerosolcloud interactions to the aging of the prescribed emissions was found. The cause for these different sensitivities remains to be addressed.…”

“…Global general circulation model (GCM) simulations yield globally averaged ACIs due to ship emissions between −0.6 and −0.07 Wm −2 (Lauer et al, 2007;Righi et al, 2011;Peters et al, 2012;Partanen et al, 2013). Given the maximum simulated cooling effect, ACI induced by shipping emissions could significantly contribute to the current best estimate of globally averaged ACI (−0.45 Wm −2 , Myhre et al, 2013).…”

“…In the present simulations, the PM and SO 2 emission mass fluxes were emitted in one level ∼ 160 m above the surface (e.g. Peters et al, 2012) with a log-normal size distribution (σ = 1.59) and two different size specifications (Righi et al, 2011;Lund et al, 2012) shown in Table 1. Both size distributions, fresh and aged, are inferred from measurements and attempt to include coagulation effects of the aerosol as time progresses.…”

A pathway analysis of global aerosol processes

“…ECHAM-HAM has been used to study non-linearities in aerosol response due to emission changes (Stier et al, 2006a), aerosol effects in a transient climate (Stier et al, 2006b), aerosol activation and cloudprocessing (Roelofs et al, 2006), aerosol indirect effects (Lohmann et al, 2007), the impact of pollution mitigation on climate forcing (Kloster et al, 2008), the impact of volcanic eruptions on climate (Niemeier et al, 2009;Timmreck et al, 2010), the impact of aerosol nucleation on radiative forcing (Makkonen et al, 2009;Kazil et al, 2010) and brightening of surface radiation due to aerosols (Folini and Wild, 2011), climate forcing due to secondary organic aerosols (O'Donnell et al, 2011) and aerosol indirect effects due to shipping emissions (Peters et al, 2012) to name but a few studies. The general circulation model ECHAM was developed at the Max Planck Institute for Meteorology and evolved from the model at the European Centre for Medium-Range Weather Forecasting.…”

“…The early models represented only a single aerosol species such as sulfate or dust (Langner and Rodhe, 1991;Feichter et al, 1996;Tegen and Lacis, 1996;Roelofs et al, 1998;Lohmann et al, 1999;Rasch et al, 2000). Following that, models started to simulate a combination of aerosol species that were externally mixed, (Takemura et al, 2000;Chin et al, 2002).…”