Examining the vertical heterogeneity of aerosols over the Southern Great Plains
Abstract. Atmospheric aerosols affect the global energy budget by scattering and absorbing sunlight (direct effects) and by changing the microphysical structure, lifetime, and coverage of clouds (indirect effects). Both aerosol direct and indirect effects are affected by the vertical distribution of aerosols in the atmosphere, which is further influenced by a range of processes, such as aerosol dynamics, long-range transport, and entrainment. However, many observations of these processes are based on ground measurements, limiting our ability to understand the vertical distribution of aerosols and simulate their impact on clouds and climate. In this work, we examined the vertical heterogeneity of aerosols over the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) using data collected from the Holistic Interactions of Shallow Clouds, Aerosols and Land Ecosystems (HI-SCALE) campaign. The vertical profiles of meteorological and aerosol physiochemical properties up to 2500 m above the ground are examined based on the 38 flights conducted during the HI-SCALE campaign.
The aerosol properties over the SGP show strong vertical heterogeneity and seasonal variabilities. The aerosol concentrations at the surface are the highest due to strong sources of emission at ground level. The mode diameter of these aerosols during summer (~ 100 nm) is larger than that during spring (~ 30 nm), potentially as a result of enhanced condensational growth due to enriched volatile organic compounds in summer. The concentration of aerosols below 30 nm in the boundary layer (BL) (e.g., below 1000 m) during spring is higher than that during summer, a result of the stronger new particle formation (NPF) events and a reduced condensation sink in spring. In the BL, the size of the aerosols gradually increases with altitude due to condensational growth and cloud processing. However, the composition of the aerosols remained similar, with organics and sulfates representing 59.8 ± 2.2 % and 22.7 ± 2.1 % of the total mass in the BL. Through the vertical profiles of aerosol properties, we noticed a considerable number of NPF events (7 out of 38) in the upper BL, where the newly formed particles continue to grow as they are mixed down to the surface. There is also an indication that deep convection brings aerosols from the free troposphere (FT) to the surface, where they grow to contribute to the cloud condensation nuclei (CCN). Overall, the vertical heterogeneity of aerosols over the SGP is influenced by aerosol dynamics (new particle formation, growth, and cloud processing) and transport processes (long-range transport, entrainment, and convective downward transport). Case studies showing the influence of these factors are discussed.
Yang Wang et al.
Status: open (until 06 Jul 2023)
- RC1: 'Comment on egusphere-2023-830', Anonymous Referee #1, 01 Jun 2023 reply
Yang Wang et al.
Yang Wang et al.
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In the manuscript the authors study the vertical profile of aerosol particle properties over the Southern Great Plains (SGP) atmospheric observatory. The vertical profiles were measured using an instrumented Gulfstream I aircraft during two intensive campaigns in spring and summer 2016.
The observed vertical profiles are explained by the authors using aerosol dynamics and transport processes. The results add to the understading of aerosol properties at SGP and motivate further studies. The text is clear, well-structured and the topic is scientifically relevant. I can recommend the manuscript for publication in ACP after the below comments have been addressed.
Fig 1: It appears that the flights were done on average about two hours later during the spring campaign than during the summer campaign. Would the time difference contribute to a possible bias when comparing the time periods? Is it possible to check that the results don't change by comparing only overlapping times of day?
Fig 2 a and b: Add a scale bar indicating horizontal distance.
Fig 3: For evaluating the source area better it would be helpful to see the distribution of back trajectories associated with each cluster.
Fig 4: Add a size bin for 3-10 nm size range from the two onboard CPCs.
Fig 5: Add the vertical profile of 3-10 nm number concentration calculated from the CPCs. Also add a legend showing that red is summer and blue is spring (same for Fig. 6).
In addition to the vertical profiles of number concentrations and mass concentrations could you include a figure with average vertical profiles of the meteorological variables such as potential temperature as well as LWC. Alternatively add them as subplots to Fig. 5.
For the NPF case studies (Figs. 7-9) it would be useful to include horizontal track of the aircraft colored by the 3-10 nm number concentration. It was mentioned that several vertical profiles were made during each flight, it would be good to see all of them. Also inlcude the number size distribution as a function of time measured at the surface for further comparison.
Can you be sure that the observed heterogeneity in the vertical profiles is not actually due to horizontal heterogeneity? This is especially relevant in the case studies. If horizontal heterogeneity cannot be ruled out you should mention it in the text.
Fig 9: If the particles were formed in the upper parts of the well-mixed layer you would expect them to be more vertically mixed since it takes several hours for the particles to grow to such sizes. Do you see at the surface when the layer was mixed down?
Lines 339-441: Another explanation might be that the particles were formed in the residual layer and mixed down (Lampilahti et al., 2021). Precursor gases could be present in the residual layer as well (Beck et al., 2022).
Fig 10: S_tot and number size distribution show dicontinuities between 1300-1500 m and between 2000-2200 m. Gradient in potential temperature also slightly increases around or before these altitudes indicating weak inversions. Is it possible that the BL reached 2 km on a previous day and the sub-10 nm particles are in fact inside the residual layer? Cases in Figs. 9-10 might both be residual layer NPF events observed before (Fig 10) and after (Fig 9) entrainment into the well-mixed layer.
Lines 429-430: Show the ground-based number-size distribution measurements. Does it support the analysis of what happens between the morning and the afternoon flights in the well-mixed part of the BL.
Line 467: "FL" -> "FT"
Fig 13: Show the time evolution of the number concentrations in the relevant size ranges (N_<50 and N_>100) at the surface.
Line 525: Does the calculated particle exhange agree with the ground-based number concentration measurements during the convective event?
Lampilahti et al. (2021) https://doi.org/10.5194/acp-21-7901-2021
Beck et al. (2022) https://doi.org/10.5194/acp-22-8547-2022