Vertical distribution of pollution trapped by wintertime surface inversions in Fairbanks, Alaska, during the ALPACA experiment

Stutz, Jochen; Kuhn, Jonas; Cooperdock, Sol; Johnson, Sarah; Guo, Fangzhou; Flynn, James H.; Cesler-Maloney, Meeta; Simpson, William R.

doi:10.5194/egusphere-2026-816

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https://doi.org/10.5194/egusphere-2026-816

Preprints

23 Mar 2026

| 23 Mar 2026

Vertical distribution of pollution trapped by wintertime surface inversions in Fairbanks, Alaska, during the ALPACA experiment

Jochen Stutz, Jonas Kuhn, Sol Cooperdock, Sarah Johnson, Fangzhou Guo, James H. Flynn, Meeta Cesler-Maloney, and William R. Simpson

Abstract. Air pollution in cold wintertime urban areas is a ubiquitous problem. Surface temperature inversions under low wind conditions and surface emissions lead to the formation of a shallow polluted surface layer (PSL). The presence of this PSL is well documented, but its height, the rates of exchange with the background atmosphere, and the interplay between vertical mixing and chemistry remain poorly quantified. Here we provide quantitative insights into this coupled chemistry-transport system using observations and modeling of vertical pollutant profiles.

Long-path Differential Optical Absorption Spectroscopy of vertical trace gas profiles during the 2022 Alaskan Layered Pollution and Chemical Analysis (ALPACA) experiment in Fairbanks, Alaska, quantitatively track the frequent establishment of PSLs with heights of 20–40m, which sometimes persist for several days. PSL trace gas mixing ratios often plateau at levels of up to 35 ppb SO₂, 60 ppb NO₂, 2.5 ppb HONO, 7 ppb HCHO, and low ozone as a result of surface emissions, gas-phase chemistry, and ineffective mixing with the background atmosphere. Parameterizing vertical mixing with observed surface temperature gradients and wind speed in the PACT-1D chemistry and transport model leads to excellent agreement of modeled and measured trace gas profiles. The PACT-1D transport scheme determines the residence time of pollutants within the PSL to be 1–4 h, confirming the strong influence of atmospheric mixing on the composition of sustained PSL events. Altitude-dependent ozone and NOx chemistry highlight the strong coupling between mixing and chemistry, which must be considered to quantify pollutant concentrations accurately in shallow PSLs.

Received: 10 Feb 2026 – Discussion started: 23 Mar 2026

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12 Oct 2026

Vertical distribution of pollution trapped by wintertime surface inversions in Fairbanks, Alaska, during the ALPACA experiment

Jochen Stutz, Jonas Kuhn, Sol Cooperdock, Sarah Johnson, Fangzhou Guo, James H. Flynn, Meeta Cesler-Maloney, and William R. Simpson

Atmos. Chem. Phys., 26, 14345–14365, https://doi.org/10.5194/acp-26-14345-2026,https://doi.org/10.5194/acp-26-14345-2026, 2026

Short summary

Jochen Stutz, Jonas Kuhn, Sol Cooperdock, Sarah Johnson, Fangzhou Guo, James H. Flynn, Meeta Cesler-Maloney, and William R. Simpson

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2026-816', Anonymous Referee #1, 17 Apr 2026
General comments
This manuscript presents valuable LP-DOAS observations during ALPACA. Based on ground measurement and remote sensing, results well characterized the overall field setting of ALPACA campaign, and gives valuable analysis results about the winter pollution near surface. However, the main quantitative interpretation is still not sufficiently supported. I think major revision is required for reconsideration.

Specific comment
Chapter 2
The retrieval assumes that concentration varies only with altitude, but the upper light paths are long slant paths extending roughly 4 km toward across non-uniform terrain and likely non-uniform source intensity. I think how horizontal gradients might alias into the retrieved vertical gradients should be discussed. A simple 2-D synthetic test or at least a representativeness discussion would greatly strengthen the interpretation. Additionally, the four reflector arrays are observed sequentially rather than simultaneously, therefore the rapid transition may induce apparent vertical structure or timing lags. How can these uncertainties be evaluated?

For NOx measurement, chemiluminescence analyzers commonly use molybdenum converter, which can convert other NOz species (e.g. HONO, HNO3, ...); Especially in high NOx region with the existence of oxidant, its influence can be enlarged that be expected. I think the model can represent this measurement uncertainties and its impact on analysis if there is a proper chemical mechanisms.

Chapter 3
Could the authors explain why NO2 was selected as the primary tracer for identifying PSL structure, rather than SO2 or a combined multi-species framework? Because NO2 is not significantly stable in aspects of chemical cycle.

Chapter 4
Lots of descriptions are related to the model setup. I think 4.1-4.3 should be moved to supplementary information. Or I recommend constructing methodology chapter for the basic information about observations and models before result and discussion.

Also considering tthe observed condition (~200 ppbv of NOx), scaling NO emissions by 2.3 and NO2 emissions by 3 is a major adjustment. Please show unscaled vs scaled simulations to clarify whether the main conclusions are robust to the exact scaling factors. Especially the difference of scailing in NO and NO2 can change the NO/NO2 ratio, which strongly impact to O3-NOx cycling.

Chapter 5
The residence-time analysis is interesting. I recommend additional sensitivity tests to another condition (e.g. deposition). These points matter because the residence-time estimate is one of the paper’s most policy-relevant conclusions.

Supplementary
I'm not sure about the temperature dependency of absorption cross section. The in-situ condition is not similar to room temperature, which can induce strong uncertainties in HONO quantification. Please clarify about this limitation.
Citation: https://doi.org/10.5194/egusphere-2026-816-RC1
- AC1: 'Reply to Review 1 of EGUSPHERE-2026-816', Jochen Stutz, 10 Jul 2026
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-816/egusphere-2026-816-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2026-816-AC1
RC2:
'Comment on egusphere-2026-816', Anonymous Referee #2, 17 Apr 2026

The article “Vertical distribution of pollution trapped by wintertime surface inversions in Fairbanks, Alaska, during the ALPACA experiment” by Stutz et al., provides a quantitative characterization of pollutant profiles and their vertical distribution using transport models and experimental chemistry. It proposes an interpretation of various coupled transport–chemistry mechanisms to investigate wintertime air pollution. Understanding pollution processes in polluted Fairbanks city requires integrating data from emissions, transport models, and chemical processes.
The article shows excellent agreement between experimental observations and model results, providing a strong understanding of atmospheric parameters during the ALPACA campaign.
Some minor specific comments and suggestions should be addressed before publication.
Please explore the discussion on the possible interference, well documented in the literature, on the data obtained from monitors, especially NOx monitor. How these extreme conditions would affect the measurements? How the interfering species will act in the extreme RH and Temperature conditions?
Even if the following paper, Kuhn et al., 2026, will add details on the implication for multiphase chemistry, please add in this article a brief input about those implications.
Line 152: C(z) = Ca + Cb × (1/1 + exp(−(z − H)/M)) probably should be C(z) = Ca + Cb × 1/(1 + exp(−(z − H)/M))
Figure 2: Please open a bracket for ppb on the y-axis for HCHO
Figure 4: y-axis „surface ...”

Citation: https://doi.org/10.5194/egusphere-2026-816-RC2
- AC2: 'Reply to Review 2 of EGUSPHERE-2026-816', Jochen Stutz, 10 Jul 2026
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-816/egusphere-2026-816-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2026-816-AC2

Peer review completion

AR – Author's response | RR – Referee report | ED – Editor decision | EF – Editorial file upload

AR by Jochen Stutz on behalf of the Authors (10 Jul 2026) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (13 Jul 2026) by Chiara Giorio

RR by Anonymous Referee #2 (25 Jul 2026)

RR by Anonymous Referee #3 (04 Aug 2026)

Suggestions for revision or reasons for rejection

The use of a multi-path LP-DOAS system to achieve continuous vertical profiling of trace gases at 20–200 m represents a significant methodological improvement over conventional balloon-borne snapshot soundings or single-level fixed-point monitoring. The long-term time-series profiles of trace gases within the Fairbanks winter persistent stable boundary layer (PSL) help fill an important observational gap in the vertical distribution of pollutants under stable stratification. I therefore recommend the manuscript for minor revision. However, I have several specific points that require clarification or further discussion:

1. Signal intensities and SNR considerations (Figure 1): What are the actual signal intensities at the 3963 m and 1154 m paths, respectively? Given that the optical path lengths range from 1154 m to 3963 m, the instrument signal-to-noise ratio (SNR) likely varies substantially among the different distances. How is the reliability of the retrieved data assessed across these varying SNR conditions, and what criteria were used to ensure the robustness of the derived products?

2. Interpretation of O₃ variations (Figure 2): Can the observed ozone changes in Figure 2 be unambiguously attributed to stratospheric intrusion events, or are there other plausible boundary-layer processes that might explain the variability?

3. Near-surface enhancement of HCHO and HONO (Figure 2): The concentrations of HCHO and HONO in the lowest layer (12–17 m) are markedly higher than those at all other heights. Could the authors elaborate on the underlying mechanisms responsible for this pronounced near-surface enhancement?

4. Role of molecular mass in vertical dispersion (Figure 4): The discussion on wind speed effects in Figure 4 is interesting. I would like to ask whether the vertical dispersion range of gases is expected to be dependent on molecular mass, and if so, whether such a mass-dependent effect might influence the observed vertical gradients.

5. Horizontal representativeness of the vertical profile (Figure 6): The measurements in Figure 6 span heights from 12 to 191 m and are reported as column concentrations. Where is the horizontal location of the retrieved profile relative to the light paths? More specifically, how is a given vertical level linked to its corresponding horizontal position along the multi-path configuration?

6. Selective presentation of time-series (Figure 9): Time-series plots are shown for O₃ and HONO in Figure 9, but NO₂ is not included. Is there a specific reason for omitting the NO₂ time series? Including it, or explaining its absence, would improve the completeness of the analysis.

7. Uncertainty in vertical profile retrieval: The LP-DOAS system uses only four light paths, yet the manuscript presents vertical profile results with relatively high spatial resolution. A more detailed quantitative assessment of the retrieval uncertainty—particularly regarding the spatial resolution and precision achievable with such a limited number of optical paths—is essential. If such an assessment is not feasible, the authors should at least explicitly acknowledge the limitations in terms of vertical resolution and accuracy in the conclusions section.

Hide

RR by Anonymous Referee #4 (04 Aug 2026)

Suggestions for revision or reasons for rejection

This manuscript describes the long-path DOAS measurements taken during the ALPACA campaign in Fairbanks, Alaska in the winter of 2022, when shallow polluted boundary layers often persist for several days due to stagnant air conditions from temperature inversions. Using a new fitting algorithm, they used LP-DOAS measurements at 4 heights to construct a vertical profile of NO2, O3, SO2, and HCHO within the polluted layer, and then constructed a PACT-1D transport scheme to characterize the movement of pollutants throughout the layer.

It’s a good study, well defined and described, and combines observations with theoretical modeling to understand the vertical structure of polluted layers in Fairbanks, which can be one of the most polluted places in the US in winter. The analysis of residence times within the PSL is particularly relevant. I would recommend publication after the authors address a few issues, listed below.

Figure 1: It isn’t clear where the CTC is located. Is it at the orange triangle? How far is that from the parking garage? Can you put this information in the caption?

Line 179: Do the 3m and 158m in situ O3 measurements agree with each other or with the retrieved values? I would expect the latter, but the phrasing of this sentence makes it unclear.

Line 194: “Further formation of NO2 from continued emissions of NO, which can lead to large spikes in NOx, does not contribute to NO2 due to a lack of ozone” is a confusing sentence. Do you mean that continued emissions of NO stop forming NO2 at some point once the ozone is titrated? Maybe rephrase.

Figure 3: It’s hard to see which day is which when the ticks only mark every 5 days. Can you include more ticks, or put a box around the time periods that you discuss in further detail in the text?

Line 229: It seems odd to include the no-PSL case in the lowest dT bin. Doesn’t this throw off the statistics of the plot? Could you only include PSL cases in this statistical analysis? Likewise with the wind plots

Line 266: What is the ADEC?

Line 273: What is the correction suggested by Brett et al 2025? How close is it to the 2.3 and 3 factors that you used? How sensitive are your results to this selection of the scaling factor?

Section 5.1: Although a good illustrative case study, I’m not sure this section is adding anything to your analysis and could be removed or moved to the SI. Figure 9 is also quite hard to read due to the thin line traces.

Hide

ED: Reconsider after major revisions (06 Aug 2026) by Chiara Giorio

AR by Jochen Stutz on behalf of the Authors (31 Aug 2026) Author's response Author's tracked changes Manuscript

ED: Publish as is (05 Sep 2026) by Chiara Giorio

AR by Jochen Stutz on behalf of the Authors (14 Sep 2026) Author's response Manuscript

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2026-816', Anonymous Referee #1, 17 Apr 2026
General comments
This manuscript presents valuable LP-DOAS observations during ALPACA. Based on ground measurement and remote sensing, results well characterized the overall field setting of ALPACA campaign, and gives valuable analysis results about the winter pollution near surface. However, the main quantitative interpretation is still not sufficiently supported. I think major revision is required for reconsideration.

Specific comment
Chapter 2
The retrieval assumes that concentration varies only with altitude, but the upper light paths are long slant paths extending roughly 4 km toward across non-uniform terrain and likely non-uniform source intensity. I think how horizontal gradients might alias into the retrieved vertical gradients should be discussed. A simple 2-D synthetic test or at least a representativeness discussion would greatly strengthen the interpretation. Additionally, the four reflector arrays are observed sequentially rather than simultaneously, therefore the rapid transition may induce apparent vertical structure or timing lags. How can these uncertainties be evaluated?

For NOx measurement, chemiluminescence analyzers commonly use molybdenum converter, which can convert other NOz species (e.g. HONO, HNO3, ...); Especially in high NOx region with the existence of oxidant, its influence can be enlarged that be expected. I think the model can represent this measurement uncertainties and its impact on analysis if there is a proper chemical mechanisms.

Chapter 3
Could the authors explain why NO2 was selected as the primary tracer for identifying PSL structure, rather than SO2 or a combined multi-species framework? Because NO2 is not significantly stable in aspects of chemical cycle.

Chapter 4
Lots of descriptions are related to the model setup. I think 4.1-4.3 should be moved to supplementary information. Or I recommend constructing methodology chapter for the basic information about observations and models before result and discussion.

Also considering tthe observed condition (~200 ppbv of NOx), scaling NO emissions by 2.3 and NO2 emissions by 3 is a major adjustment. Please show unscaled vs scaled simulations to clarify whether the main conclusions are robust to the exact scaling factors. Especially the difference of scailing in NO and NO2 can change the NO/NO2 ratio, which strongly impact to O3-NOx cycling.

Chapter 5
The residence-time analysis is interesting. I recommend additional sensitivity tests to another condition (e.g. deposition). These points matter because the residence-time estimate is one of the paper’s most policy-relevant conclusions.

Supplementary
I'm not sure about the temperature dependency of absorption cross section. The in-situ condition is not similar to room temperature, which can induce strong uncertainties in HONO quantification. Please clarify about this limitation.
Citation: https://doi.org/10.5194/egusphere-2026-816-RC1
- AC1: 'Reply to Review 1 of EGUSPHERE-2026-816', Jochen Stutz, 10 Jul 2026
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-816/egusphere-2026-816-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2026-816-AC1
RC2:
'Comment on egusphere-2026-816', Anonymous Referee #2, 17 Apr 2026

The article “Vertical distribution of pollution trapped by wintertime surface inversions in Fairbanks, Alaska, during the ALPACA experiment” by Stutz et al., provides a quantitative characterization of pollutant profiles and their vertical distribution using transport models and experimental chemistry. It proposes an interpretation of various coupled transport–chemistry mechanisms to investigate wintertime air pollution. Understanding pollution processes in polluted Fairbanks city requires integrating data from emissions, transport models, and chemical processes.
The article shows excellent agreement between experimental observations and model results, providing a strong understanding of atmospheric parameters during the ALPACA campaign.
Some minor specific comments and suggestions should be addressed before publication.
Please explore the discussion on the possible interference, well documented in the literature, on the data obtained from monitors, especially NOx monitor. How these extreme conditions would affect the measurements? How the interfering species will act in the extreme RH and Temperature conditions?
Even if the following paper, Kuhn et al., 2026, will add details on the implication for multiphase chemistry, please add in this article a brief input about those implications.
Line 152: C(z) = Ca + Cb × (1/1 + exp(−(z − H)/M)) probably should be C(z) = Ca + Cb × 1/(1 + exp(−(z − H)/M))
Figure 2: Please open a bracket for ppb on the y-axis for HCHO
Figure 4: y-axis „surface ...”

Citation: https://doi.org/10.5194/egusphere-2026-816-RC2
- AC2: 'Reply to Review 2 of EGUSPHERE-2026-816', Jochen Stutz, 10 Jul 2026
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-816/egusphere-2026-816-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2026-816-AC2

Peer review completion

AR – Author's response | RR – Referee report | ED – Editor decision | EF – Editorial file upload

AR by Jochen Stutz on behalf of the Authors (10 Jul 2026) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (13 Jul 2026) by Chiara Giorio

RR by Anonymous Referee #2 (25 Jul 2026)

RR by Anonymous Referee #3 (04 Aug 2026)

Suggestions for revision or reasons for rejection

Hide

RR by Anonymous Referee #4 (04 Aug 2026)

Suggestions for revision or reasons for rejection

Hide

ED: Reconsider after major revisions (06 Aug 2026) by Chiara Giorio

AR by Jochen Stutz on behalf of the Authors (31 Aug 2026) Author's response Author's tracked changes Manuscript

ED: Publish as is (05 Sep 2026) by Chiara Giorio

AR by Jochen Stutz on behalf of the Authors (14 Sep 2026) Author's response Manuscript

Journal article(s) based on this preprint

12 Oct 2026

Vertical distribution of pollution trapped by wintertime surface inversions in Fairbanks, Alaska, during the ALPACA experiment

Jochen Stutz, Jonas Kuhn, Sol Cooperdock, Sarah Johnson, Fangzhou Guo, James H. Flynn, Meeta Cesler-Maloney, and William R. Simpson

Atmos. Chem. Phys., 26, 14345–14365, https://doi.org/10.5194/acp-26-14345-2026,https://doi.org/10.5194/acp-26-14345-2026, 2026

Short summary

Jochen Stutz, Jonas Kuhn, Sol Cooperdock, Sarah Johnson, Fangzhou Guo, James H. Flynn, Meeta Cesler-Maloney, and William R. Simpson

Supplement

https://doi.org/10.5194/egusphere-2026-816-supplement

Data sets

Longpath Differential Optical Absorption Spectroscopy (LP-DOAS) observations of vertical trace gas profiles in Fairbanks, Alaska, during the Alaskan Layered Pollution and Chemical Analysis (ALPACA) experiment (2022) Jochen Stutz et al. https://doi.org/10.18739/A21G0HW9H

Gas and meteorological measurements at the CTC site and Birch Hill in Fairbanks, Alaska, during the ALPACA-2022 field study William Simpson et al. https://doi.org/10.18739/A27D2Q87W

Jochen Stutz, Jonas Kuhn, Sol Cooperdock, Sarah Johnson, Fangzhou Guo, James H. Flynn, Meeta Cesler-Maloney, and William R. Simpson

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Northern cities often experience periods of wintertime air pollution, which are associated with surface temperature inversions and weak winds. In Fairbanks, Alaska, pollutants accumulate in a 20–40 m high surface layer with SO₂ and NO₂ mixing ratios of up to 35 ppb and 65 ppb, respectively. This surface pollution is caused by the balance of emissions, chemistry, and inefficient mixing with clean background air. This balance has to be considered for air pollution mitigation strategies.


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Vertical distribution of pollution trapped by wintertime surface inversions in Fairbanks, Alaska, during the ALPACA experiment

Journal article(s) based on this preprint

Interactive discussion

Peer review completion

Suggestions for revision or reasons for rejection

Suggestions for revision or reasons for rejection

Interactive discussion

Peer review completion

Suggestions for revision or reasons for rejection

Suggestions for revision or reasons for rejection

Journal article(s) based on this preprint

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Data sets

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