General comments:

This manuscript makes a strong contribution to advancing our understanding of passive air sampling by critically evaluating the existing paradigms of sampler deployment and, importantly, identifying a strong source of uncertainty that is being introduced into the data. The overview of the state of passive air sampling is accurate and correctly captures the uncertainties in passive sampling sorbents, configurations, and uptakes that impact PAC quantification by PAS.

The authors take a step-by-step approach in testing a series of hypotheses - this structure is very clear, and very comprehensively integrates the results from the different methods that were combined in the study. The scientific methods and interpretation are very good, and only a few points require further clarification.

Specific points for the authors to clarify:

• Please clarify hypothesis 1 "Can Uptake of Particle-bound Compounds Explain Decreasing Trends in the Amounts Accumulated in PASs?" While I fully agree with the methodology used to test the hypothesis and the conclusions the authors have made with respect to this hypothesis, I question how this phenomenon could lead to decreases in chemicals in the samplers over time. I rather think this would be lead to irregular/non-linear uptake. Does this hypothesis assume that particles can be "blown-off" the sorbents in over time?
• Please consider adding more specificity to the naming of the hypothesis related to degradation (hypothesis 3) to make it clear that this is related to photooxidants, largely ozone. In fact, the outcome of the study seems to be that some other unknown degradation pathway is contributing to the observed patterns, but that aspect is not tested under the 3rd hypothesis.

Review of Title: Uptake Behavior of Polycyclic Aromatic Compounds during Field Calibrations of the XAD-Based Passive Air Sampler Across Seasons and Locations

Author(s): Yuening Li et al.

MS No.: egusphere-2023-2202

General comments

The manuscript discusses the uptake of polycyclic aromatic compounds by a specific passive air sampler for deployments lasting up to 1 year. Contrary to expectations, several compounds did not remain in the linear uptake phase; several PAHs either plateaued, or decreased after about 6 months deployment. The behaviour observed for PAHs was in contrast to results for other semi-volatile organic compounds. Three possible reasons were explored (i) impact of particle-bound PACs; (ii) degradation of PAC by ozone and (iii) equilibration of certain PACs.

None of these were concluded to be responsible for the observed observations.

Overall, the manuscript is well written, and includes good data, figures and tables. There is a lack of additional creativity beyond the 3 reasons investigated by the authors.

Let’s try a few:

1. What amount of XAD is oxidized, and thus losing sorption capacity during the year?
2. The curves depicted in Figure 1 might indicate competition for sorptive sites. And the PACs might simply be replaced by stronger sorbing compounds.

Specific comments

L66 – very detailed list  what is the difference between urban and regions with high traffic density?

L 149 – give better reason why ozone was chosen (NOx or OH)

L193 – reference for chosen air boundary later?

Figure 3 – This is supposed to simulate an equilibration experiment, right? Make that explicit in the caption

Figure 5 would benefit from some descriptive statistics – presumably all compounds but PACs, and PACs on their own

Technical corrections

L145 – sandwich

L335 – Thus it should?