Global model simulations of the impact of ocean-going ships on aerosols, clouds, and the radiation budget

[摘要] International shipping contributes significantly to the fuel consumption ofall transport related activities. Specific emissions of pollutants such assulfur dioxide (SO₂) per kg of fuel emitted are higher than for roadtransport or aviation. Besides gaseous pollutants, ships also emit varioustypes of particulate matter. The aerosol impacts the Earth's radiationbudget directly by scattering and absorbing thesolar and thermal radiation andindirectly by changing cloud properties. Here we use ECHAM5/MESSy1-MADE, aglobal climate model with detailed aerosol and cloud microphysics to study the climate impacts of international shipping.The simulations show that emissions from ships significantly increasethe cloud droplet number concentration of low marine water cloudsby up to 5% to 30% depending on the shipemission inventory and the geographic region.Whereas the cloud liquid watercontent remains nearly unchanged in these simulations, effective radii ofcloud droplets decrease, leading to cloud optical thickness increaseof up to5–10%. The sensitivity of the results is estimated by using threedifferent emission inventories for present-day conditions. The sensitivityanalysis reveals that shipping contributes to 2.3% to 3.6% of thetotal sulfate burden and 0.4% to 1.4% to the total black carbon burdenin the year 2000on the global mean.In addition to changes in aerosol chemical composition,shipping increases the aerosol number concentration, e.g. up to 25% inthe size range of the accumulation mode (typically >0.1 μm) over theAtlantic. The total aerosol optical thickness over the Indian Ocean, theGulf of Mexico and the Northeastern Pacific increases by up to 8–10%depending on the emission inventory. Changes in aerosol optical thicknesscaused by shipping induced modification of aerosol particle numberconcentration and chemical composition lead to a changein theshortwaveradiation budget at the top of the atmosphere (ToA) underclear-sky conditionof about −0.014 W/m² to−0.038 W/m² for a global annual average.The corresponding all-sky direct aerosol forcing ranges between−0.011 W/m² and −0.013 W/m².The indirect aerosol effect of ships onclimate is found to be far larger than previously estimated. An indirectradiative effect of −0.19 W/m² to −0.60 W/m²(a change in the atmospheric shortwave radiative flux at ToA)is calculated here, contributing17% to 39% of the total indirect effect of anthropogenic aerosols.This contribution is high because ship emissions are released in regionswith frequent low marine clouds in an otherwise clean environment. Inaddition, the potential impact of particulate matter on the radiation budgetis larger over the dark ocean surface than over polluted regions over land.

[学科分类] 大气科学