the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 19 Sep 2024
In situ vertical observations of the layered structure of air pollution in a continental high latitude urban boundary layer during winter
Abstract. Vertical in situ measurements of aerosols and trace gases were conducted in Fairbanks, Alaska, during winter 2022 as part of the Alaskan Layered Pollution and Chemical Analysis campaign (ALPACA). Using a tethered balloon, the study explores the dispersion of pollutants in the continental high latitude stable boundary layer (SBL). Analysis of 24 flights revealed a stratified SBL structure with different pollution layers in the lowest tens of meters of the atmosphere, offering unprecedented detail. Surface emissions generally accumulated in a surface mixing layer (ML) extending to an average of 51 meters, with a well-mixed sub-layer (MsL) reaching 22 meters. The height and concentrations within the ML were strongly influenced by a local wind driven by nearby topography under anticyclonic conditions. Counterintuitively, during strong radiative cooling, a drainage flow increased turbulence near the surface, altering the temperature profile and deepening the ML. Above the ML, pollution concentrations decreased but showed clear signs of freshly released anthropogenic emissions. Higher in the atmosphere, above elevated inversions, pollution levels were similar to previously reported Arctic haze concentrations, even though Fairbanks’ outflow concentrations below elevated inversions were up to six times higher, likely due to power plant emissions. In situ measurements indicated that gas and particle tracer ratios in elevated power plant plumes differed significantly from those near the surface. Overall, pollution layers were strongly correlated with the temperature stratification and emission heights, emphasizing the need for improved representation of temperature inversions and emission sources in air quality models to enhance pollution forecasts.
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RC1: 'Comment on egusphere-2024-2863', Anonymous Referee #1, 14 Oct 2024
Review of “In situ vertical observations of the layered structure of air pollution in a continental high latitude urban boundary layer during winter” by Pohorsky et al. (2024)
The authors present in-situ vertical measurements of aerosols and trace gases using tethered balloon system during wintertime January – February 2022 in Fairbanks, AK, USA. The manuscript is suitable for publication in EGUsphere, the conclusions are solid, valuable and clear, but only of local impact as authors point out themselves. Even though I am marking it as: accept with minor revisions, all below are mostly suggestions to improve the manuscript from the point of potential reader.
General comments:
While reading the manuscript it is obvious that a lot of work and subsequent analyses were done. However, the manuscript is extensive in length, and I would encourage the authors its shortening. It is not a critical point, but the authors in many places dive into unnecessary details with very low impact on conclusions and results, e.g. in the section 4.3 they present in detail method, models, analysis and the result of all that effort is that the models do not work (low correlations) and would probably perform better at higher winds. The authors made in-situ measurements and determined the mixing height, the models do not work very well in this particular case, it is ok, models are just models. It could be compressed into one paragraph and the interested reader you can direct to supplementary materials (SM), which are very extensive already. Similarly, the section 3, there is detailed discussion on radiative balance, all that would make very good sense and great value if the Helikite payload included 3D anemometer, and radiation sensors.
The authors could easily divide the manuscript into two separate manuscripts a) definition of multi-layered structure and b) its implications for air pollutants.
For any vertical profiling measurements, it is very important to persuade the reader your measurements are correct. Since there was a surface reference available, I would encourage the authors to provide a figure and a paragraph, where it is clear all profile measured variables when payload is on Helikite are reasonably correlating with the surface reference. The lab calibration of instruments or inter-comparison when the payload is measuring from the same inlet at surface is not enough, especially when the profiling is done in such an extreme environment.
The authors describe and define the complexity of Arctic boundary layer and its detailed structure. The definitions are mostly based on temperature measurements, however cross validated by pollutant concentrations in vertical column. All the complex layering in boundary layer and lower mixing height has strong implications for local air quality, however the manuscript even though having “air pollution” in the title is not using any of AQ nomenclature or comparison to EPA limits considering aerosol load – particulate mass concentrations PM1, PM2.5, etc., or gaseous pollutants.
Technical comments:
Page 8, line 204: Please add what inlet was used for surface measurements, what cut-off, what flows, heated/not heated, any losses accounted for? Inner diameter is more important for definition of laminarity in the sampling lines.
Page 10, line 276: just for consistency through the manuscript use …gradient (22.7C .100 m-1)
Figure 7. The appearance of geometric standard deviation of number size distribution used here is quite surprising, since there was not mentioned anything about the measured size distribution, the shape, modality, etc., yet. Why not use, at this point, the total particle concentration from the mSEMS, moreover the difference between POPS and SMPS would indicate presence of nanoparticles (the harmful ones) in each layer.
Figure 13. How do the surface measurements of PNSD fit into this figure? Also, are the surface measurements satisfactory indicator of local AQ?
Figure 14. The plum particle concentration enhancement (Nc of POPS) of 30/ccm is not very significant, suggesting that the edge of the plume was measured. It seems that there are high dynamics in the plume itself (condensation and coagulation) due to high temperature gradients. There is not mentioned the dominant size of the measured particles, 2 or 3 micron particles would have probably a good chance of deposition at low winds. Do the different power plants (diesel or coal) plumes have any specific signature in number size distribution?
Page 43 in Conclusions, have not found Fochesatto et al. in references.
Citation: https://doi.org/10.5194/egusphere-2024-2863-RC1 -
RC2: 'Comment on egusphere-2024-2863', Anonymous Referee #2, 30 Oct 2024
The paper address a very important and interesting topic connected to vertical profiles in the Arctic. It is quite long and any effort to shorten it should be done.
Here below major and minor comments.
Major comments:
Line 158: MAC of 7.5 m2 g-1 (at 550 nm). Please compare your data with ones of Savadkoohi et al. (2024; https://doi.org/10.1016/j.envint.2024.108553) and use the proper eBC nomenclature accordingly to the aforementioned paper.
Lines 266-268: “The 𝜀 threshold was set to 0.8 °C per layer Δz based on visual examination of the resulting simplified profiles that confirmed that the major temperature inflection points were correctly captured by the adapted algorithm” . Which is the physical meaning of this threshold?
Section 3 is quite long. Could you squeeze it or move some parts in supplementary material?
Section 4: MsL and ML: the cited Seibert et al. (2000) suggest to use gradient method to infer the mixing layer height. From your application you seem to use the point at which the derivative is zero and not at the minimum value. Please discuss this choice
Section 4 Line 410: weakly polluted background layer (WPBL): why not refers to it as residual layer? The explanation at the following 411-412 lines is just general: could you address a specific figure with measured WPBL and RL profiles?
Section 4.1 Lines 455-459: despite the huge description of the second case in Figure 7 I do not agree with the interpretation. Figure 7e clearly show multiple temperature inversion layers and accordingly to Figure 7f,h there are two separate layer. The MLH should be placed around 50 m in this case even because the geometric standard deviation (Fig. 7g) increases up to 2 in the second layer. It is a mistake to place MLH at 100 m
Section 4.3: I suggest to move all this section, Tables 3-4 and Figure 10 in supplementary material. It does not add significant information. The Pearson coefficients are so low that practically show the absence of any correlation
Figure 11: normalized vertical profiles are very clear but I suggest to add another panel, the one of the count median diameter of particles on the same size range used for the vertical profiles of geometric standard deviation. This enables to add the information of the average size together with this dispersion.
Lines 770-772: The eBC over Ny-Ålesund and Svalbard is taken from Mazzola et al. (2016). However the situation there is more complex. I suggest to complete the comparison of the obtained profiles with the ones reported by Ferrero et al. (2016; doi:10.5194/acp-16-12601-2016), by Marcowicz et al. (2017; https://doi.org/10.1016/j.atmosenv.2017.06.014) and by Cappelletti et al. (2022; https://doi.org/10.1016/j.atmosenv.2022.119373).
Section 6.1 and 6.2: these sections are quite long. Please consider the possibility to move them in supplementary material or to short them.
Minor comments:
Lines 119-120: Which is the expected impact of power plants in Fairbanks on your measurements?
Lines 401-402: “Note that in certain situations, no MsL is observed and the concentration gradient is strongly negative directly from the surface (dashed lines in Fig. 6)”: no red negative dashed line from the surface is present in Figure 6. Please add it as in the description at these lines.
Citation: https://doi.org/10.5194/egusphere-2024-2863-RC2 - AC1: 'Authors' responses to referee comments on egusphere-2024-2863', Roman Pohorsky, 11 Dec 2024
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