Zhang et al. analysed the 3-hydroxy fatty acids in inhalable particles from urban Tianjin at 2m and 200m in different seasons to investigate the vertical, seasonal and diurnal variations in airborne endotoxins. They then identified the potential influencing factors of 3-hydroxy fatty acids and endotoxin levels. They found higher total endotoxin concentrations at ground level compared to 200 m, peaking in winter due to the combined contributions of near-ground emissions from both natural and anthropogenic sources, as well as different control factors affecting the levels of short-chain and long-chain 3-hydroxy fatty acids and endotoxins. The study aligns with the journal's scope. However, there are a few shortcomings (outlined below) in the current version of the manuscript that require attention.

Major concerns:

Lines 216-217, The authors concluded that “the endotoxins at 220 m in Tianjin might be originated either from the vertical transport of ground emissions or long-range transport from marine areas”. However, the conclusion drawn here differs from or contradicts the concentration-weighted trajectory (CWT) analysis in Section 3.1. The evidence should support one another, and the authors should present a consistent statement throughout the manuscript.

From Table 2, we can see that the concentrations of endotoxin in PM₁₀, PM_2.5-10, and PM_2.5 from the same region are significantly different. Therefore, it is not reasonable to make a direct comparison of the endotoxin concentrations of samples from different regions with different sample types.

Although both NO₂ and O₃ are oxidants, their effects on the concentration of endotoxins appear to be contrasting (Figures 7b and c). In this case, I would not suggest combining them when considering the seasonal variation of endotoxin with oxidant.

Specific comments:

Lines 74-76, According to Binding et al., 2004, it is easier to produce unsaturated 3-fatty acids with acid hydrolysis rather than alkaline hydrolysis.

Lines 115-119, Abbreviations of EtOAc, CAN, CMPI, TEA and DMED should be introduced for their first occurrence.

Line 208, The author states that there was no obvious diurnal difference in total endotoxin levels; however, the variation is actually very large, especially in Winter.

Line 322-323, This sentence is not clear to me, please rephrase it.

Figure 5, Please explain the TSP and DECOS in the figure caption.

The manuscript presents a valuable investigation into the vertical and seasonal distribution of airborne endotoxins in Tianjin, China, using 3-hydroxy fatty acids (3-OH-FAs) as biomarkers. The study addresses an important gap in understanding endotoxin dynamics in urban boundary layers, particularly in coastal megacities. The methodology is robust, combining advanced analytical techniques (UHPLC-MS) with comprehensive environmental monitoring. However, several aspects require clarification or improvement to enhance the manuscript's impact and readability.

Major comments:

1. Justify the use of the factor "4" (lipid A carries 4 mol 3-OH-FAs) and "8000" (average endotoxin molecular weight). Cite specific literature for these assumptions, especially the factor “4”.

2. The higher endotoxin levels at 2 m vs. 220 m are attributed to "near-surface emissions," but specific sources (e.g., soil, traffic, marine) should be quantified or discussed.

3. The near-ground winter peak in PM10 from urban Tianjin is linked to "enhanced emissions," but is this due to local heating (biomass/coal combustion) or meteorological inversions? 

4. Figure 1: The diurnal/nocturnal variations in 3-OH-FAs are intriguing but lack statistical significance testing (e.g., p-values for day/night differences).

5. Figure 5: Include error bars for literature data where available to facilitate comparison.

Minor comments:

1. Update citations to include recent studies on urban endotoxins.

2. Delete (b-c) in second line of Figure 4.