Ship aerosol emissions and marine fuel regulations: Impacts on
physicochemical properties, cloud activity and emission factors
Abstract
Marine regulations aim to reduce sulfur and nitrogen exhaust emissions
from maritime shipping. Here, two compliance pathways for reducing
sulfur dioxide emissions, fuel sulfur content reduction and exhaust wet
scrubbing, are studied for their effects on physicochemical properties
and cloud forming abilities of engine exhaust particles. A test-bed
diesel engine was utilized to study fresh exhaust emissions from
combustion of non-compliant, high sulfur content fuel with (WS) and
without (HiS) the usage of a wet scrubber as well as a regulatory
compliant, low sulfur content fuel (LoS). Particle number emissions are
decreased by ≈ 99% when switching to LoS due to absence of 20-30 nm
sulfate particles. While number emissions for WS are also decreased, a
shift in the sulfate mode towards larger sizes was found to increase
particle mass emission factors by at least 31%. Changes in the mixing
state induced by the compliance measures are reflected in the
hygroscopicity of the exhaust particles. Fuel sulfur reduction decreased
cloud condensation nuclei emissions by at least 97% due to emissions of
primarily hydrophobic soot particles. Wet scrubbing increased those
emissions, mainly driven by changes in particle size distributions. Our
results indicate that both compliance alternatives have no obvious
impact on the ice forming abilities of 200 nm exhaust particles. These
detailed results are relevant for atmospheric processes and might be
useful input parameters for cloud-resolving models to investigate ship
aerosol cloud interactions and to quantify the impact of shipping on the
radiative budget.