the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 03 Jul 2024
Impact of mineral dust on the global nitrate aerosol direct and indirect radiative effect
Abstract. Nitrate (NO3-) aerosol is projected to increase dramatically in the coming decades and may become the dominant inorganic particle species. This is due to the continued strong decrease in SO2 emissions, which is not accompanied by a corresponding decrease in NOx and especially NH3 emissions. Thus, the radiative effect (RE) of NO3- aerosol may become more important than that of SO42- aerosol in the future. The physicochemical interactions of mineral dust particles with gas and aerosol tracers play an important role in influencing the overall RE of dust and non-dust aerosols but can be a major source of uncertainty due to their lack of representation in many global climate models. Therefore, this study investigates how and to what extent dust affects the current global NO3- aerosol radiative effect through both radiation (REari) and cloud interactions (REaci) at the top of the atmosphere (TOA). For this purpose, multi-year simulations nudged towards the observed atmospheric circulation were performed with the global atmospheric chemistry and climate model EMAC, while the thermodynamics of the interactions between inorganic aerosols and mineral dust were simulated with the thermodynamic equilibrium model ISORROPIA-lite. The emission flux of the mineral cations Na+, Ca2+, K+ and Mg2+ is calculated as a fraction of the total aeolian dust emission based on the unique chemical composition of the major deserts worldwide. Our results reveal positive and negative shortwave and longwave radiative effects in different regions of the world via aerosol-radiation interactions and cloud adjustments. Overall, the NO3- aerosol direct effect contributes a global cooling of -0.11 W/m2, driven by coarse-mode particle cooling at short wavelengths. Regarding the indirect effect, it is noteworthy that NO3- aerosol exerts a global mean warming of +0.17 W/m2. While the presence of NO3- aerosol enhances the ability of mineral dust particles to act as cloud condensation nuclei (CCN), it simultaneously inhibits the formation of cloud droplets from the smaller anthropogenic particles. This is due to the coagulation of fine anthropogenic CCN particles with the larger nitrate-coated mineral dust particles, which leads to a reduction in total aerosol number concentration. This mechanism results in an overall reduced cloud albedo effect and is thus attributed as warming.
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RC1: 'Comment on egusphere-2024-1579', Anonymous Referee #1, 01 Aug 2024
This manuscript presents a modeling effort to quantify the impact of mineral dust on radiative forcing effect of nitrate aerosol. Nitrate was believed to be a cooling aerosol although great uncertainties remain within the estimated radiative forcing. As the authors mentioned, nitrate is probably going to play a more important role due to the decrease in sulfate. In addition, mineral dust was known to affect the formation of nitrate aerosol through thermodynamic equilibrium and heterogeneous reactions. I strongly agree that a thorough investigation of the interaction between dust and nitrate and the associated radiative forcing effect will help to improve the understanding of climate change. Therefore, this study focused on an important and interesting topic and applied a proper modeling method. In general, the manuscript is well written with a clear description of the objective and well-organized discussions of the results. However, there are a few major issues regarding the modeling method which requires more details to help better justify and support the results and conclusion of this study. I would recommend a major revision to address these issues before a final decision could be made regarding the acceptance of the submission. Please find the major and minor comments below.
comment#1: A main issue is that it seems this study didn’t consider dust heterogeneous chemistry which may promote conversion from NOx to nitrate on the surface of dust particles. The “physicochemical interactions of mineral dust particles with gas and aerosol tracers” in this study refers to the thermodynamic equilibrium between gas-phase HNO3 and particle phase nitrate if I understand the manuscript correctly. I am not quite sure which one of these two process, heterogeneous chemistry or thermodynamic equilibrium, plays a major role in dominating the production of nitrate in the present of dust as there is no such demonstration in this work. Similarly, dust heterogeneous chemistry also promotes conversion from gas phase SO2 to sulfate which may further affect the thermodynamic equilibrium of HNO3. Therefore, as the topic focused on “impact of dust on the global nitrate”, I would recommend the authors to include a brief discussion to explain that omitting dust heterogeneous chemistry may or may not affect the conclusion of this study.
comment#2: The second main issue with the study is a lack of discussion about uncertainties of the REari and REaci estimations. Radiative forcing effect is a complex index calculated based on a series of model simulated variables, and it may subsequently inherit the associated uncertainties. For example, how were the model performances for simulating dust emission and size distribution, nitrate concentration, aerosol vertical distributions? These variables will significantly affect the estimations of REari and REaci, and a clear demonstration of modeling biases for these variables will help audiences to better understand the radiative effects quantified by the modeling system. Especially, many climate models represent formation of nitrate in a very simplified manner. Therefore, I am very interested to see how well the model used in this study can simulate mass concentration of nitrates as evaluated against observations.
line#133: Table 1 talks about details of simulation configuration, so it might be better to move it to section2.
line#133: It’s necessary to briefly describe the difference in ionic composition between Karydis et al. (2016) and global homogeneous setting.
line#171: Better use math symbol instead of letter for “x”
line#189: These “mineral ions” are treated as individual particles or as supplements of dust particle?
line#195: Does “chemically inert” mean the gas-phase HNO3 adsorbed onto the surface of dust particle will not partition into nitrate through equilibrium with NCVs?
line#197: Do you mean 94% of emitted dust mass is treated as “bulk dust” in the model? The particle size distributions of dust and other ions are the same?
line#209: Please include the function here to help illustrate the partition process and how it is represented in the model.
line#248: Freshly emitted species are usually NOx instead of HNO3.
line#279: Please specify how exactly nitrate formation was turned off. Was gas phase HNO3 still condense but no nitrate was produced, or HNO3 is the end product of nitrous oxide?
line#281: If HNO3 was forced to condense only on fine mode, will this lead to a lower level of sulfate formation since the model need to keep the equilibrium? Will there be any non-linear response of other aerosols such as sulfate by tuning off condensation? If we set up a 4th simulation by forcing HNO3 to condense only on coarse mode, can we estimate the radiative effect as: FfineNO3,ari = F1,ari – F4,ari ?
line#294: As there was no aerosol-cloud interaction, does it mean the simplest cloud scheme apply a prescribed aerosol configuration? Can it properly reproduce cloud over the study period?
line#317: Model configuration for cloud scheme is a little confusing here, line#313 mentions FN,ari is calculated using method in sec2.3.1 which applied the simplest cloud scheme as mentioned in line#294, but it seems in order to estimate aci, another set of cloud scheme was used.
line#330: A table showing REari reported in these references would be helpful to better demonstrate the comparison.
line#369: Please explain why there is a strong warming dot over Sahara in Fig.2(v)
line#403: It’s a very interesting point that nitrate REari seems insensitive to dust load but Table 2 suggested that interaction between nitrate and dust has a significant impact on coarse mode aerosols’ LW and SW forcing. It’s better to include more detailed discussions in this paragraph to explain why “nitrate-dust interactions are not linearly correlated”, and why “a given increase or decrease in dust emissions does not lead to an analogous change in nitrate aerosol level”.
line#489: What is the “advanced cloud scheme”?
Due to the comments mentioned above, I don’t think we are ready to further discuss the influence on cloud microphysics, although the manuscript showed some very interesting results. More comments regarding Section5 will be provided in the next review iteration if necessary.
Citation: https://doi.org/10.5194/egusphere-2024-1579-RC1 -
AC1: 'Reply on RC1', Vlassis Karydis, 15 Oct 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-1579/egusphere-2024-1579-AC1-supplement.pdf
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AC1: 'Reply on RC1', Vlassis Karydis, 15 Oct 2024
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RC2: 'Comment on egusphere-2024-1579', Anonymous Referee #2, 03 Sep 2024
The manuscript titled “Impact of mineral dust on the global nitrate aerosol direct and indirect radiative effect” by Milousis et al. investigated the radiative effects of nitrate on dust by using a climate model, including the aerosol-radiation interactions and aerosol-cloud interactions. Nitrate chemistry on dust is implemented in the EMAC model, simulations were conducted based on the base case and several sensitivity simulations. In general, the logic of the study is explicit and the organization of the manuscript structure is clear. However, the study lacks of necessary evaluation of the simulation results and thus the results could be subject to high uncertainties.
Major comments:
1. There is no comparison between model results and observations, e.g. mass concentrations of nitrate, dust (PM10), aerosol number concentrations. There are plenty of observational datasets or literature values available. The lacking of constraints from observational data will reduce the credibility of model simulations
2. Figure 2 & Line 371 – 377: In Figure 2v and 2vi, the TOA SW REari of coarse nitrate is much stronger than that of fine nitrate, it seems unreasonable as fine nitrate dominates the total nitrate, especially in East Asia.
3. Figure 4: the kappa values of fine aerosol over the continents are mostly lower than 0.04 (iii), which are incorrect. Even considering the mixing between dust and anthropogenic emissions, the hygroscopicity of aerosols couldn’t be so weak.
4. Line 195 – 197: Na+, K+, Ca2+ and Mg2+ constituted 100% of bulk dust? How are the anions treated?
5. Line 222 – 223: How is the delinquencies of salts treated in the model under different relative humidity?
6. Section 4: Why the radiative effects from Aerosol-Cloud Interactions are not separated for fine and coarse nitrate?
Citation: https://doi.org/10.5194/egusphere-2024-1579-RC2 -
AC2: 'Reply on RC2', Vlassis Karydis, 15 Oct 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-1579/egusphere-2024-1579-AC2-supplement.pdf
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AC2: 'Reply on RC2', Vlassis Karydis, 15 Oct 2024
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