the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Tropospheric aerosols over the western North Atlantic Ocean during the winter and summer campaigns of ACTIVATE 2020: Life cycle, transport, and distribution
Abstract. The Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE) is a six-year (2019–2024) NASA Earth-Venture Suborbital-3 (EVS-3) mission to robustly characterize aerosol-cloud-meteorology interactions over the western North Atlantic Ocean (WNAO) during winter and summer seasons, with a focus on marine boundary layer clouds. This characterization requires understanding the aerosol life cycle (sources and sinks), composition, transport pathways, and distribution in the WNAO region. We use the GEOS-Chem chemical transport model driven by the MERRA-2 reanalysis to simulate tropospheric aerosols that are evaluated against in situ and remote sensing measurements from Falcon and King Air aircraft, respectively, as well as ground-based and satellite observations over the WNAO during the winter (Feb. 14 – Mar. 12) and summer (Aug. 13 – Sep. 30) field deployments of ACTIVATE 2020. Transport of pollution in the boundary layer behind cold fronts is a major mechanism for the North American continental outflow to the WNAO during Feb.–Mar. 2020. While large-scale frontal lifting is a dominant mechanism in winter, convective lifting significantly increases the vertical extent of major continental outflow aerosols in summer. Turbulent mixing is found to be the dominant process responsible for the vertical transport of sea salt within and ventilation out of the boundary layer in winter. The simulated boundary layer aerosol composition and optical depth (AOD) in the ACTIVATE flight domain are dominated by sea salt, followed by organic aerosol and sulfate. Compared to winter, boundary layer sea salt concentrations increased in summer over the WNAO, especially from the ACTIVATE flight areas to Bermuda, because of enhanced surface winds and emissions. Dust concentrations also significantly increased in summer because of long-range transport from North Africa. Comparisons of model and aircraft submicron non-refractory aerosol species (measured by an HR-ToF-AMS) vertical profiles show that intensive measurements of sulfate, nitrate, ammonium, and organic aerosols in the lower troposphere over the WNAO in winter provide useful constraints on model aerosol wet removal by precipitation scavenging. Comparisons of model aerosol extinction (at 550 nm) with the King Air High Spectral Resolution Lidar-2 (HSRL-2) measurements (at 532 nm) and CALIOP/CALIPSO satellite retrievals (at 532 nm) indicate that the model generally captures the continental outflow of aerosols, the land-ocean aerosol extinction gradient, and the mixing of anthropogenic aerosols with sea salt. Large enhancements of aerosol extinction at ~1.5–6.0 km altitudes from long-range transport of the western U.S. fire smoke were observed by HSRL-2 and CALIOP during Aug.–Sep. 2020. Model simulations with biomass burning (BB) emissions injected up to the mid-troposphere (vs. within the BL) better reproduce these remote-sensing observations, Falcon aircraft organic aerosol vertical profiles, as well as AERONET AOD measurements over eastern U.S. coast and Tudor Hill, Bermuda. High aerosol (mostly coarse-mode sea salt) extinction near the top (~1.5–2.0 km) of the marine BL along with high relative humidity and cloud extinction were typically seen over the WNAO (< 35° N) in the CALIOP aerosol extinction profiles and GEOS-Chem simulations, suggesting strong hygroscopic growth of sea salt particles and sea salt seeding of marine boundary layer clouds. Contributions of different emission types (anthropogenic, BB, biogenic, marine, and dust) to the total AOD over the WNAO in the model are also quantified. Future modeling efforts should focus on improving parameterizations for aerosol wet scavenging and sea salt emissions, implementing realistic BB emission injection height, and applying high-resolution models that better resolve vertical transport.
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RC1: 'Comment on egusphere-2024-1127', Basudev Swain, 08 May 2024
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Review of "Tropospheric aerosols over the western North Atlantic Ocean during the winter and summer campaigns of ACTIVATE 2020: Life cycle, transport, and distribution" by Liu et al. (2024)
The manuscript provides valuable insights into the aerosol distributions and processes observed during the ACTIVATE 2020 campaign over the western North Atlantic Ocean. It effectively supports the campaign goal of studying aerosol-cloud-meteorology interactions by using data sets from all the different platforms, including GEOS-Chem model simulations, ground-based observations, and satellite data.
Moreover, the manuscript provides a comprehensive overview of the aerosol distribution, source, and transport coupled with meteorological parameters, along with an evaluation against ground-based and space-borne measurements. In addition, the authors perform sensitivity analyses with GEOS-Chem simulations, modifying parameters such as biomass burning height etc.
Overall, the manuscript demonstrates the satisfactory performance of the model against observations, especially in response to changing meteorological dynamics. It suggests potential improvements can be possible through finer resolution simulations.
I appreciate the authors for clearly defining the scientific objectives of the manuscript and for their valuable contributions to the understanding of aerosol dynamics, aerosol types, and associated meteorological drivers during the ACTIVATE campaign.
This manuscript falls well within the aim and scope of the ACP journal. I would recommend this manuscript for publication with minor corrections.
Total manuscript length is very high, authors need to work on reducing manuscript as well as abstract length. It would be helpful to future potential readers of this manuscript if the authors would consider these minor changes listed below:
General major questions:
1. The duration of the ACTIVATE campaign is six years (2019-2024), so why is this study focused only on the spring and summer of 2020?
2. In this manuscript, it has been mentioned several times that the vertical transport can be improved by applying high-resolution models/simulations. Thus, in this manuscript the study domain is very small (two box regions: the North ("N"; 36-39°N, 69-75°W) and the South ("S"; 32.5-36°N, 71-75.5°W)), so why this study has not used finer resolution nested grid (0.25*0.3125) simulations provided by GEOS-Chem model instead of global simulations (2*2.5)? This finer resolution simulation could be used as another sensitivity simulation.
3. This study is conducted during August Complex “Gigafire” took place in mid-August 2020 and the California Creek fire occurred in early September 2020, ranked among the top five in California wildfire history. Thus to see the GEOS-Chem model comparisons during other years of ACTIVATE campaign would be very interesting, and could bring some valuable knowledge.
4. I was unable to find the method used in this manuscript to spatio-temporally collocate the GEOS-Chem model with ground-based, satellite data sets for evaluation. As models provide spatio-temporally continuous data, while ground, airborne, and satellite data are very discontinuous. So, what is the collocation strategy between all the datasets?
Minor comments:
Abstract:
The abstract is very long, it needs to be shortened further to make it easier to follow. Furthermore, there is a single big sentence from line 30 to line 34 that can be reduced to one small sentence, and mentioning GEOS-Chem model driven by MERRA-2 does not bring any additional information to the abstract, I suggest to remove MERRA-2 from the abstract.
Similarly, from line 43 to 46, this large sentence must be reduced in length.
Introduction:
1. Please cite some references at line 65 and 66.
2. The introduction is very large at about 4 pages. I would strongly suggest to reduce it. From line 79 to 115 has the potential to be reduced as there is no need for such large discussions NAO+, NAO-, and synoptic scale impact of cyclones on wind pattern and consequent impact on aerosol transport in the introduction. Just mention the direction of the wind pattern created by NAO oscillation and cyclones, and the associated aerosol transport in 3-4 lines.
4. Furthermore, all these NAO and cyclonic transports have been well presented with figures in Section 4 "Meteorological Settings and Transport Pathways", so why discuss them twice? Once in the Introduction and again in Section 4.
After the end of the introduction, the second section starts as GEOS-Chem Model. I would suggest to have a section like Data and Methods. In this section all data sets from different platforms can be presented as sub-sections. Further, at the end of the Data and Methods section, write a paragraph about the collocation strategy used for GEOS-Chem model evaluation with ground-based and space-based datasets etc.
In Section 2 "Model Description", use only the information about the model simulations and the sensitivity simulations. Include the emission inventories used in an Appendix. This will help to reduce the length of the manuscript and will be easier for the readers.
At Section 4 “Meteorological Settings and Transport Pathways”, this section explains very well about the meteorological drivers for the aerosol transport and variability over WNAO region. So, please reduce the explanation in introduction about NAO+, NAO-, and synoptic impacts on aerosol distributions.
Furthermore, I would like to suggest to include figures 2 to 4 in the appendix, as the meteorological variability is not the result of this manuscript, but rather an established fact that is captured by the MERRA-2 and GEOS-Chem model. This will further help to shape the manuscript. Also keep figure 14 in the supplement, and also I was unable to read the color bar values, please improve it.
Section 5 "Simulated Aerosols over the WNAO and Model Evaluations" contains the most important result of this manuscript. This section is very well written and easy to understand.
It would be very helpful to segregate the section 7 “Summary and conclusions” into two sections, as this manuscript brings very valuable conclusion that supports the ACTIVATE campaign and future modelling aspects related to this campaign. However, the summary overshadows the conclusion of this manuscript.
Overall, although the manuscript is very long and took me a few days to read, I enjoyed reading it and appreciate the scientific motivation behind this manuscript to support ACTIVATE campaign.
Citation: https://doi.org/10.5194/egusphere-2024-1127-RC1
Data sets
Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE) ACTIVATE Science Team https://doi.org/10.5067/SUBORBITAL/ACTIVATE/DATA001
Model code and software
GEOS-Chem v11-01 for simulating tropospheric aerosols over the western North Atlantic Ocean H. Liu and B. Zhang https://zenodo.org/records/10982278
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