This manuscript investigates atmospheric particulate matter (PM) and associated trace elements in the Upper Indo-Gangetic Plain (U-IGP) and Central Indo-Gangetic Plain (I-IGP), aiming to understand their concentrations, seasonal variations, and sources.

While the study covers an important topic, there are several critical issues that need to be addressed. The rationale for selecting the U-IGP region over other areas is not well explained, leading to logical inconsistencies in the study's focus. Furthermore, the manuscript conflates the definitions of heavy metals and trace elements and does not clearly delineate their significance or relationship. The analysis of seasonal variations, especially concerning crustal elements and dust, lacks clarity, and the use of the term "dust" is ambiguous—it's unclear whether it refers to natural or anthropogenic sources. The discussion on meteorological influences, health impacts, and the PMF method is either insufficient or overly generalized. The presentation quality also needs improvement, with figures that do not effectively convey the key findings, and a confusing structure that hampers the overall readability.

In its current form, the manuscript does not meet the standards required for publication. And in particular the authors need to think about why it should be published as a MEASUREMENT REPORT type of article, or where the scientific highlights need to be highlighted for this type of article. Major revisions are necessary to address the scientific and presentation-related issues. Here are my specific recommendations:

Lines 39-40:

Why is it UPPER IGP and not examining IGPs of other orientations? There is a logical problem with the reason for the point selection here.

Lines 42-44:

Has there been a change in major contributors? If not, then the word while should not be used after the comma.

Lines 64-73:

The review is confusing and here confuses the definitions of heavy metals and trace elements, and does not systematically address the relationship between the two and their respective importance.

Lines 74-75:

Just because PM is elevated doesn't mean it's not important. The order of magnitude that PM can reach needs to be given to help the reader determine how important it is.

Line 77:

I don't think it's climate change (on a long-term scale), more that it's a change in meteorological conditions.

Lines 80-82:

I still don't understand why the U-IGP was chosen - was it because the atmospheric issues were not prominent, or was it for some other reason?

Line 94:

What's the meaning of SA?

Lines 95-98:

Are these statements intrinsically related? What do the differences between warm and cold season crustal elements and Cl indicate? Is dust here dust or anthropogenic source dust and does the 50%+ contribution of dust conflict with the results of this paper?

Also, are some of the important sources covered in this paper all reported in previous work with source profiles? That is, are the sources in this paper reliably and consistently referenced?

Lines 99-100:

The reference here is strange. Dust itself is not a secondary source of oxidation.

Lines 125-126:

Please label in Figure S1, which regions are represented by U, C, and L respectively. If the authors consider the U-IGP to be important, then it is perfectly acceptable to put a diagram of the study area in the text.

Lines 140-150:

Because this paper is a MEASUREMENT REPORT, it is still very different from a typical research paper. So please elaborate on the representativeness of the sites selected for this study to confirm the scientific increment and value of this report.

Line 158:

Please check that the number of valid digits is the same.

Section 2.2:

There is also confusion here about the formulation of the instrument. For example, lines 168-178 are also all about instruments, so why are they not integrated into subsection 2.2?

Line 218:

Why don't you just write here that you use the XX-based PMF method, which seems to be a more mainstream term? I know there has been a lot of recent research based on a priori information, so please refer to a more specialized way of presenting it.

Lines 232-234:

So please describe what a priori information was included and what uncertainties, or key issues, were resolved.

Lines 329-334:

Is it reasonable to need to consider COLD and WARM? Is it necessary to give the change in temperature (not Table 1) to testify to the reasonableness of the division, except for Figure 2c because there is a break in time, and both a and b are continuous? Or consider bringing up the junction date discussion.

Line 351:

Why is Si, a typical crustal element, relevant to combustion?

Line 356:

Is there strict evidence of favorable meteorological conditions? Supporting evidence on meteorological conditions in the text also needs attention.

Lines 371-373:

What does EF have to do with the source?

Lines 377-378:

I think if EF is relevant to the source, or how to reveal enhancement, it is necessary to put it in the text. Besides, considering meteorological effects, how can the authors define it only as an effect of anthropogenic activities?

Lines 378-388:

How do authors consider health effects? Especially other elements not mentioned here. I.e., how do the authors synthesize elements with high concentrations but weak health effects, and elements with low concentrations but strong effects? Or looking back is the representation in 3.2 appropriate?

Lines 411-412:

Cl-rich is not a source, just a phenomenon. Here's hoping the authors can parse to the source of the emissions.

What is the difference between SFC1 and SFC2? What sources are characterized respectively?

Figures 4 and 5:

I don't see much point in doing diurnal differences. Because the source contributions don't look like much of a percentage change, except for a change in concentration.

The authors need to reconsider integrating the two graphs.

Lines 462-465:

It is not very meaningful to discuss concentration variations because the boundary layer itself has daily variations, as in Table 1. What needs to be considered is the contribution, or percentage, to the total PM.

Figure 6:

The vertical coordinate of this figure is strange, if it is μg m-3, the contribution of individual sources to the mass concentration does not correspond to Figures 4-5. In addition, uncertainty ranges for daily changes need to be given.

Section 3.4:

I think there is a need to highlight the core innovative points or findings about METEOROLOGY. The authors didn't find out what the real meteorological drivers of PM changes in the cold/warm seasons are, and too much reporting on the rise and fall of PM will only increase the time spent reading the literature, making the real innovation of this section obscured.

Review of “Measurement report: Sources and meteorology influencing highly-time resolved PM2.5 trace elements at 3 urban sites in extremely polluted Indo Gangetic Plain in India” by Shukla et al. 

This manuscript looks at the elemental composition of PM_2.5 at three sites in the Indo-Gangetic Plain.  One of the sampling sites is in a region that is less well represented in the literature.  The authors used source apportionment to understand the different emission sources of elemental PM_2.5 and explored the spatial and temporal variation to be used for mitigation strategies. 

The topic is interesting and of importance to the community, but revisions are necessary before publication.

Major comments

Section 3.3: It would help the reader to have a description of the different source profiles at the beginning of this section.  This information is interspersed in the text or buried in the supplemental.  In general, as this paper is focused on pollution sources, it is necessary to link these profiles to possible specific sources and/or atmospheric processes - Specific questions that I had in regards to this were:

• -What is the difference (in composition and likely sources) between SFC1 and SFC2
• -Are Cl-rich and S-rich also related to combustion, as it was discussed the main sources of Cl and S were combustion in the previous section, or are there sources?
• -What are the sources of Cu-rich

Section 3.2 focuses on the importance of Pb, Ni, As and (to a lesser extent) Cr in regards to health effects, but there is no discussion of the sources of these elements.  This would be valuable to include, either based on distribution across the source profiles, or correlations with the time series of the different profiles. 

The authors extensively discuss the observed differences between the warm and cold periods and frequently refer to the role of meteorology.  However, they do not clearly propose what meteorological processes are driving the differences.  For example, is it differences in temperature/RH, different wind speed and direction patterns, a difference in boundary layer height, etc.  Furthermore – Can the authors exclude that it is not due to differences in seasonal trends in emissions rather than meteorology (for example the seasonal variation in crop-residue burning, house-hold combustion for heating).  These differences would still be of importance, but they are not, strictly speaking, due to meteorology.      

Minor Comments

Line 356: What is meant by “conventional sources”?

Line 356: Due to the focus of meteorology in this paper, can you expand on what you mean by “favorable meteorological conditions” in this specific case?

Line 434:  Is there further evidence or previous research that supports that the Cl-rich aerosol is related to aqueous phase reactions, rather than increased emissions during this period?

Typographical:

-There are numerous grammatical errors throughout the manuscript.  Although the meaning is generally clear some sentences are hard to follow, and I recommend the authors thoroughly proofread the manuscript.  Some specific recommendations are included below. 

Sometimes PM_2.5 is subscripted (i.e. line 103) while at other points (line 79) it is not.  It should be consistent. 

Line 336: Parentheses do not match.

Line 403: “These findings indicate…” I am having trouble following this sentence

Line 423: What is RC?

I would recommend using the terminology “warm and cold periods” rather than just “warm and cold” for example at line 422, 434, etc