The issue of harmonization of GHG and air pollutant inventories has been discussed and attempted at regional and global level since a while...with mixed the sucess. The recognized importance of SLCFs led to another attempt of harmonizatiion with GHG inventory process; in fact IPCC tried that some 10-15 years ago but at the time the conclusion was that the uncertainties and data availability is not satisfactory; at the time the focus was on black carbon. The new attempt appears more promising and the efforst is supported by a larger group of scientists. This paper provides a good account of key advantages and challenges associated with the development of the harmonized (or coordinated) inventories that could serve several purposes, even though the process is driven by (and the needs) of IPCC. 

My impression is that the authors occasionally deter from the main purpose of the IPCC TF and discuss this development as a way to develop an all-purpose inventory, which in my view is neither feasible nor desirable. Such discussion appears in few places refering to the multipurpose of inventories and is inlcuded also in COnclusions (see from line 541). The key reasons and end uses for this specific work and process are clear and are not meant to replace some of the existing work and established systems like the European EEA/EMEP Guidebook and the EU or UNECE reporting system. These serve differnet purposes. I agree that learning, exchange of information, benefit from resourves available within well established national and international systems shall be utilized. 

The other more general comment is about lack of reference and discussion of the EEA/EMEP Emission Inventory Guidebook and the UNECE (Air Convention) supported process that lead to it. Interestingly, here the air pollutant inventory system (developped in the 90's) was extended and an attempt of harmonization with the IPCC formats was made over time, simplifying some of the air pollutatn reporting but at the cost of detaching the reporting categories from the sources of emissions, loosing important detail for NMVOCs, and making further enhancements to improve inventories of particualte matter more difficult. Neverthless, EEA/EMEP guidebook and methods continue to be developed, used by all UNECE countries and many other countries of the world, even if the methods are often not very representative for developing countries. Several developers and users of EEA/EMEP guidebook and methodology are members of the IPCC TF. I believe the paper shall include brief discussiona nd refernce to this, for example in section 2.3, but also in Figure 1, where one coudl mention the only internationaly (not globally) accepted reporting system and methods (beyond scientific tools and attepts, like EDGAR or CEDS and some specific models).

I guess, the authors have spent some time disussing the order in the paper and decided for air pollutants, GHG, SLCF and i was surprised but the start of the discussion with the air pollution impacts on health as entry point to this paper - is this because of the attempt to make the link to SDGs and Paris Agreement? I was wondering myself what woudl be best. Considerig the fact that this the IPCC driven process that attempts to bring and update methods to include SLCFs along the GHGs, I woudl probably start with GHG section followed by the SLCF discsussion that woudl link to air pollutant inventories and experience as this includes pretty much all SLCFs, apart from CH4 and HFCs that are already included under GHG inventories. I was also wondering if the 'impact' sections in air pollutant and also GHG sections shall not be part of the introduction to the paper giving background to inventoies in general.

Finally, I am not entirely sure what purpose Figure 2 serves - in the context of this paper. Maybe additional reason for this question is my personal believe that the concept of euivalent particulate matter emissions is not very useful and not only bacuse of the spatial scales at which air pollution is truly relevant, rather than large scale (national or regional averages) but also because of changing atmosphere composition the assumed relationships on the roles of each species contributing to ambient PM are far from constant.

Few more specific comments:

line 24: one coudl say 'grennhouse gases and air pollutants' instead of 'gases and particultes' 

line 26: for completness, fugitive sources shall be mentioned too, not only incomplete combution, since NMVOC, NH3, and even fugitive PM play a role in formation of pollution

line 41: I'd say 'recent decades' rather than 'recent years'

line 55: I am not sure if 'verify' is the correct word here; maybe 'monitor progress in implementation' woudl be more representative

line 63: here and above few references woudl be useful. for example in line 63 a refernce to EPA (when  referring to the US) and the EEA/EMEP guidebook and LRTAP Convention (when refering to Europe) could be mentioned ; see also general comments above

line 66: 'this pattern'? what pattern? Are the authors referring to strong increase in pollution and then decline driven by introduction of controls? Agree, but the previous sentences mostly speak about role of inventories and various multiraterla efforst...nothing like this happened in CHina and there is no harmonized and officail emission inventory for China even now. 

line 110-113: It has been the case in the past, but for several countries already now the role of non-combusion emissions (NH3) has been increasing strongly and even in Europe key primary source of PM is biomass for heating rather than fossil fuels. And so I disagree with this statement as a univeral truth.

line 120: How relevant is this statement in perspecctive of this paper? How does this relate to harmonization of emisison inventory methods and challenges?

line 131: Suggest adding 'often' beofre 'major' as it is not always the case, especially if we consider that the text is referring to various temporal and spatial scales where locally (or even regionally - say heavy industry, or poorly controlled industrial nd power coal boilers) carbonaceous aerosols might not be the major part of PM2.5.

line 137-141: This is the section where i think reference to the internationally accepted and used (while not global) EEA/EMEP guidebook and LRTAP process shall be added.  

line 146-147: Not only in these countries; Europe has undertaken action way beore any systematic officailyl agreed and harmonized inventory systems were introduced. The observation of healtha and ecosystem damage and link with the air pollution was enoiugh to triger action. Of course progress and triggeering more efficient and coordinated action required agreed methods to report emissions. 

line 163: Maybe i missunderstnad the statments here but i believe thqt in CHina or Vietnam (just to give two examples) the observed PM2.5 is also playing that role. But maybe i am missing something here. Further, this whole paragraph coudl benefit from refernces to some of the regulations, e.g, NEC Directive in Europe, and also for other regions there are good referneces to established regulatory and enforcement frameworks. For some of the less informed reads it might be useful inforamtion.

from line 173: as above, few more refernces woudl be useful, especially to the the Air Convention, since Gothenburg Protocol is referred to in the text.

line 281: Here at the end it woudl be useful to add refernce to Kigali Amendment to the Monteral Protocol since there are specific regioanl/coutnry requirements given there.

line 283: This paragrpah begins with a statement that I do not agree with. Interntional reporting requirements within the EU and also within the UNECE LRTAP Convention have triggered and actualyl insitutionalized regular update of methods and national emission reporting. 

Line 294: As above, this last sentence in this paragrpah is not correct. The air pollution inventories reported within the UNECE COnvention are often updated also for historical years and it is regualrly documented in annual submissions and informative reports submitted with it.

line 372: 'improve air quality coudl unmask' - in this particular statement it owudl approrpaite to make it clear it refers to PM2.5 as for ozone reduction (as the authros discuss further in the paper) the picture is might be different.

line 454: I think a refernce to the methods used and established in Europe and beyond could be referred to here as they are harmoinized and broadly available and in fact applicable with some exceptions; the global GHG inventory and default emission factors proposed in the IPCC methods are also strongly limted for some GHGs and particular sectors and regions.

line 519: But is the large naumber of metrics the issue? I think the problem is rather the problem that one needs a value judgment with respect to why a particualr metric is used as their choice will strongly affedct the relevance of species, sources and therefore policy message.

line 535: reference to MOVES was made also earlier, one coudl also mention other tools like COPERT for Europe, also mobile sources method used widely in Europe and elsewhere. In fact also MOVES is used in some other countries.

line 541: I mentioned this before in the general section; i think this discussion divers a little from the main purpose of the paper. These different needs will always remain and there is no need (and it is not feasible) to develop a 'one for all' inventory. 

line 845: With changing SO2 emissions role of NH3 in formation of ammonium particulkes will be (is) changing

line 863: In fact the paper that is quoted in this paper (Klimont et al 2017) does include mineral PM2.5 at a global level and even national estimates can be derived from the gridded global files. Additionally, maybe relevant, Philip et al (https://doi.org/10.1088/1748-9326/aa65a4) discusses also the role of fugitive and miner dust. 

line 875: Maybe adding a refer to Gomez et al (2022; https://www.nature.com/articles/s41467-021-27624-7 ) might be useful

In this perspective article, the authors present an overview of how GHG and air pollutant emission inventories have developed in the past motivated by different impacts, uses and mitigation goals, highlight challenges and benefits of coordinated GHG and air pollutant emissions inventory development and provide recommendations for such a coordinated reporting within the context of the UNFCCC and IPCC frameworks. Although aerosols, ozone and their precursors gases were recognized by IPCC as climate forcing agents as early as the first assessment report, no effort has been made thus far to define a globally consistent framework for reporting national inventories of short-lived climate forcers. This is partly also due to the fact that SLCFs (except methane which is also a well-mixed greenhouse gas) are not currently part of the mitigation goals. The historical perspectives and recommendations made in this article will be useful for informing the development of the new SLCF emissions reporting guidance initiated by the IPCC.

Some specific comments that authors may consider:

L28: Mannuci et al only highlight health effects of PM. Please cite studies that also consider ozone and ecosystem effects of air pollution

L29: CH₄ is considered short-lived (IPCC AR6 chapter 6) and is much shorter lived than well-mixed greenhouse gases such as CO₂ and N₂O.

L36: Naik et al (2021) should be cited as Szopa et al (2021)

L39: Replace air pollutant emissions with precursor emissions

L124-128: should be revised to: Inventories of air pollutants include emissions of compounds that are directly hazardous to human health (e.g., nitrogen oxides (NOx = NO2 + NO)) or produce secondary pollutants with detrimental health and ecosystem effects (e.g., tropospheric ozone). These inventories generally include ozone precursor gases such as carbon monoxide (CO), non-methane volatile organic compounds (NMVOCs), and NOx, and primary aerosols (including BC, OC) as well as aerosol precursor gases such as ammonia (NH3), and SO₂

L132-135: There seems to be a point of confusion about the level of direct PM_2.5 emissions and their effect on climate. Can some references be given here?

L135: replace “as as well” with “as well as”

151-153: Can reference be provided here?

174-176: A reference would be useful here.

L182-184: Reference please?

L211-214: Tracking compliance will likely be a major reason for detailed inventories even if low SDI countries adopt high controls. Unless the fleet moves over to clean cars (e.g., EV), I don’t think one could ignore the source.

L214: Insert a “.” between ‘access Therefore’

Section2.1 : Why only air pollution impacts are discussed here? Why not the climate change impacts of air pollutants and the uncedrtainty associated with lack of consistent information on their emissions trends, which is a signifcant motivating factor for IPCC to develop reporting methodology for SLCFs.

L249-250: what about methane? Methane’s effect on climate is in the short-term

L251-254: This statement needs to be caveated since methane, a GHG, is short-lived and its concentrations have been helpful in developing top-down emissions inventories

L271: I don’t understand this construct – is this ±?

L274: define EDGAR

L288: reference?

L376-381: References would be useful here.

L384-387: Also Szopa et al (2021)

L409: reference?

L504-506: reference?

L508: equivalant should be equivalent throughout the manuscript

L515-519: Allen et al (2022) https://www.nature.com/articles/s41612-021-00226-2 could also be useful here.