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.

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 ...”