the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Jun 2024
Potential impacts of marine fuel regulations on Arctic clouds and radiative feedbacks
Abstract. Increased surface warming over the Arctic, triggered by increased greenhouse gas concentrations and feedback processes in the climate system, has been causing a steady decline in sea-ice extent and thickness. With the retreating sea-ice, shipping activity will likely increase in the future driven by economic activity and the potential for realizing time and fuel savings from transiting shorter trade routes. Moreover, over the last decade, the global shipping sector has been subject to regulatory changes, that affect the physicochemical properties of exhaust particles. International regulations aiming to reduce SOx and particulate matter (PM) emissions, mandate ships to burn fuels with reduced sulfur content or alternatively, use wet scrubbing as exhaust after-treatment when using fuels with sulfur contents exceeding regulatory limits. Compliance measures affect the physicochemical properties of exhaust particles and their cloud condensation nuclei (CCN) activity in different ways, with the potential to have both direct and indirect impacts on atmospheric processes such as the formation and lifetime of clouds. Given the relatively pristine Arctic environment, ship exhaust particle emissions could be a large perturbation to natural baseline Arctic aerosol concentrations. Low-level stratiform mixed-phase clouds cover large areas of the Arctic region and play an important role in the regional energy budget. Results from laboratory marine engine measurements, which investigated the impact of fuel sulfur content (FSC) reduction and wet scrubbing on exhaust particle properties, motivate the use of large eddy simulations to further investigate how such particles may influence the micro- and macrophysical properties of a stratiform mixed-phase cloud case observed during the Arctic Summer Cloud Ocean Study campaign. Simulated enhancements of ship exhaust particles predominantly affected the liquid-phase properties of the cloud and led to a decrease in liquid surface precipitation, increased cloud albedo and increased longwave surface warming. The magnitude of the impact strongly depended on ship exhaust particle concentration, hygroscopicity, and size where the effect of particle size dominated the impact of hygroscopicity. While low FSC exhaust particles were mostly observed to affect cloud properties at exhaust particle concentrations of 1000 cm-3, exhaust wet scrubbing already led to significant changes at concentrations of 100 cm-3. Additional simulations with cloud ice water path increased from ≈5.5 g m-2 to ≈9.3 g m-2, show more muted responses to ship exhaust perturbations but revealed that exhaust perturbations may even lead to a slight radiative cooling effect depending on the microphysical state of the cloud. The regional impact of shipping activity on Arctic cloud properties may, therefore, strongly depend on ship fuel type, whether ships utilize wet scrubbers, and ambient thermodynamic conditions that determine prevailing cloud properties.
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- RC1: 'Comment on egusphere-2024-1891', Anonymous Referee #1, 19 Jul 2024
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RC2: 'Comment on egusphere-2024-1891', Anonymous Referee #2, 23 Jul 2024
The manuscript explores the impact of ship emissions to a campaign-based Arctic cloud scene through large-eddy simulations. To mimic various ship exhaust technologies and their emissions, the authors impose aerosol particle size distributions informed by previous laboratory work and find microphysical responses that slightly alter clouds’ condensate amount and emitted longwave radiation.
The overall manuscript is well written. I have a couple of concerns that the authors should address before publication.
Major concerns
The title suggests a wider picture on Arctic clouds, but the manuscript focusses on a particular case. It is unclear, whether these results are representative for the wider Arctic. The authors should (1) put the selected case into perspective by using cloud statistics from Arctic observations, and (2) discuss potential impacts to clouds that have not been touched on here. I wonder if there are other cases of stronger cloud-aerosol-precipitation interaction, where the various aerosol scenarios make a more meaningful difference. Perhaps it also necessary to change the title to: “Potential impacts of marine fuel regulations on an Arctic stratocumulus case and its radiative response”.
The description of simulations lacks important details: (1) Looking at Christiansen et al., (2020), there is a large-scale divergence imposed – is that the case here, too? (2) Furthermore, the authors show the vertical temperature profile and indicate that it’s kept constant – is that achieved through nudging or advective tendencies and is either technique applied throughout or only in the free troposphere? (3) The authors should also specify how they calculate turbulent surface fluxes. (4) How do the authors justify imposing ship-based aerosol uniformly rather than only within the marine boundary layer?
The authors show a significant difference in longwave emissions but none in the shortwave spectrum. The latter is puzzling to me, and the authors should elaborate on why that is – are there perhaps compensating effects of cloud fraction and cloud albedo?
Minor concerns
ll. 108-110 Would it be possible to provide satellite imagery of this case (or at least coordinates) so that readers can obtain a visual impression of the case?
ll. 136ff (and also Fig. 1) It took me a while to understand the scenarios and their number of modes (and would expect the same for other readers). Perhaps it would be simpler to display all initial aerosol size distributions in Fig 1. I’m also not sure I understand the value of Fig. 1b – perhaps its better suited in the appendix?
l. 191 The label “mix” is confusing and should instead be labelled as “no ship”.
l. 381 “smaller” is ambiguous here.
Citation: https://doi.org/10.5194/egusphere-2024-1891-RC2 -
AC1: 'Response to RC1 and RC2', Luis Santos, 10 Sep 2024
Thank you for your insightful and constructive comments. They have contributed to several clarifications and improvements in the revised manuscript.
Attached, you will find our responses along with a Tex-diff PDF highlighting the changes between the revised manuscript and the initial submission.
Status: closed
- RC1: 'Comment on egusphere-2024-1891', Anonymous Referee #1, 19 Jul 2024
-
RC2: 'Comment on egusphere-2024-1891', Anonymous Referee #2, 23 Jul 2024
The manuscript explores the impact of ship emissions to a campaign-based Arctic cloud scene through large-eddy simulations. To mimic various ship exhaust technologies and their emissions, the authors impose aerosol particle size distributions informed by previous laboratory work and find microphysical responses that slightly alter clouds’ condensate amount and emitted longwave radiation.
The overall manuscript is well written. I have a couple of concerns that the authors should address before publication.
Major concerns
The title suggests a wider picture on Arctic clouds, but the manuscript focusses on a particular case. It is unclear, whether these results are representative for the wider Arctic. The authors should (1) put the selected case into perspective by using cloud statistics from Arctic observations, and (2) discuss potential impacts to clouds that have not been touched on here. I wonder if there are other cases of stronger cloud-aerosol-precipitation interaction, where the various aerosol scenarios make a more meaningful difference. Perhaps it also necessary to change the title to: “Potential impacts of marine fuel regulations on an Arctic stratocumulus case and its radiative response”.
The description of simulations lacks important details: (1) Looking at Christiansen et al., (2020), there is a large-scale divergence imposed – is that the case here, too? (2) Furthermore, the authors show the vertical temperature profile and indicate that it’s kept constant – is that achieved through nudging or advective tendencies and is either technique applied throughout or only in the free troposphere? (3) The authors should also specify how they calculate turbulent surface fluxes. (4) How do the authors justify imposing ship-based aerosol uniformly rather than only within the marine boundary layer?
The authors show a significant difference in longwave emissions but none in the shortwave spectrum. The latter is puzzling to me, and the authors should elaborate on why that is – are there perhaps compensating effects of cloud fraction and cloud albedo?
Minor concerns
ll. 108-110 Would it be possible to provide satellite imagery of this case (or at least coordinates) so that readers can obtain a visual impression of the case?
ll. 136ff (and also Fig. 1) It took me a while to understand the scenarios and their number of modes (and would expect the same for other readers). Perhaps it would be simpler to display all initial aerosol size distributions in Fig 1. I’m also not sure I understand the value of Fig. 1b – perhaps its better suited in the appendix?
l. 191 The label “mix” is confusing and should instead be labelled as “no ship”.
l. 381 “smaller” is ambiguous here.
Citation: https://doi.org/10.5194/egusphere-2024-1891-RC2 -
AC1: 'Response to RC1 and RC2', Luis Santos, 10 Sep 2024
Thank you for your insightful and constructive comments. They have contributed to several clarifications and improvements in the revised manuscript.
Attached, you will find our responses along with a Tex-diff PDF highlighting the changes between the revised manuscript and the initial submission.
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