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

Preprints

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

Preprints

23 Mar 2026

| 23 Mar 2026

Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

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

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.

Download & links

Preprint (PDF, 17882 KB)

Supplement (19865 KB)

Download & links

Preprint (17882 KB)
Metadata XML
Supplement (19865 KB)
BibTeX
EndNote

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

Status: open (until 15 May 2026)

Post a comment Subscribe to comment alert

RC1:
'Comment on egusphere-2026-816', Anonymous Referee #1, 17 Apr 2026 reply
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.

Reply
Citation: https://doi.org/10.5194/egusphere-2026-816-RC1
RC2: 'Comment on egusphere-2026-816', Anonymous Referee #2, 17 Apr 2026 reply

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 ...”

Reply

Citation: https://doi.org/10.5194/egusphere-2026-816-RC2

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

Viewed

Total article views: 190 (including HTML, PDF, and XML)

HTML	PDF	XML	Total	Supplement	BibTeX	EndNote
87	95	8	190	81	9	8

HTML: 87
PDF: 95
XML: 8
Total: 190
Supplement: 81
BibTeX: 9
EndNote: 8

Views and downloads (calculated since 23 Mar 2026)

Month	HTML	PDF	XML	Total
Mar 2026	66	31	3	100
Apr 2026	21	64	5	90

Cumulative views and downloads (calculated since 23 Mar 2026)

Month	HTML	PDF	XML	Total
Mar 2026	66	31	3	100
Apr 2026	21	64	5	90

Viewed (geographical distribution)

Total article views: 173 (including HTML, PDF, and XML) Thereof 173 with geography defined and 0 with unknown origin.

Country	#	Views	%

Latest update: 20 Apr 2026

Short summary

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.


Total:	0
HTML:	0
PDF:	0
XML:	0