the manuscript developed an updated NOx emission inventory in East Asia (constraint with GEMS) and analyze source-receptor relationship between China and S. Korea during 2022 winter-spring transition. The manuscript is well-written. the  logic is clear, method being sound, and results are convincing. I only find few minor limitations need to address as follows:

1) line 28, it is better to provide the lifetime of NOx in days, cause the transport across yellow sea can also be short.

2) line 32. it is better to add some reference for long-range transport studies in Korea domain. there are many published during KORUS-AQ campaign, and you can compare your results here. 

3) line 72, need to cite paper for Korea-China transport studies for example: 

eck et al, 2020 "Influence of cloud, fog, and high relative humidity during pollution transport events in South Korea: Aerosol properties and PM_2.5 variability"

Jordan et al, 2020 "Investigation of factors controlling PM_2.5 variability across the South Korean Peninsula during KORUS-AQ"

Nault et al, 2018, "Secondary organic aerosol production from local emissions dominates the organic aerosol budget over Seoul, South Korea, during KORUS-AQ"

Lee et al, 2020 "Sensitivity of simulated PM_2.5concentrations over northeast Asia to different secondary organic aerosol modules during the KORUS-AQ campaign"

Choi et al, 2019. "Impacts of local vs. trans-boundary emissions from different sectors on PM_2.5 exposure in South Korea during the KORUS-AQ campaign"

Tang et al, 2023 "WRF-Chem quantification of transport events and emissions sensitivity in Korea during KORUS-AQ"

Bae et al, 2020 "Long-range transport influence on key chemical components of PM_2.5 in the Seoul Metropolitan Area, South Korea, during the years 2012-2016"

4) line 90 to 120. need to mention Dr. Woo's KORUS-AQ v5 emission and also recent emission used for ASIA-AQ. those are state-of-art emissions widely used in research community, and require an acknowledgement.

5) line 120, please provide evidence than long-range transport only occur within PBL, or rephrase. 

6) line 130, need a brief description of what schemes, initial and boundary conditions you used here. I saw you sum them up in supplemental materials, but it is useful to introduce them in the manuscript as well.

7) Figure 1. do you have plan to account N.Korea emission impact, it is much close to S.Korea and should have more close impact on S.Korea air quality? if not, please rephrase and explain reason. 

8) line 162, why do not use Dr. Woo's emission prepared for ASIA-AQ? it should be more accurate and widely used. if not, please address the reason in the manuscript.

9) line 208, "assume a linear relationship between NO2 column and emission". linear relationship is not real. please provide your reason why assuming linear? or any reference to prove the method.

10) line 212, nighttime NO2 chemistry is very import for NOx distribution, giving that GEMS has no nighttime observation to constrain. please provide expected limitation of your study that ignore nighttime NOx chemistry.

11) figure 2. when comparing GEMS no2 column with WRF-CMAQ model results, it is required that GEMS no2 is covered to use vertical profile from the same model. the default one for GEMS is NOT WRF-CMAQ. please provide how you do the conversion?

Review of “Local and transboundary contributions to nitrogen loadings across East Asia using CMAQ-ISAM and GEMS-informed emissions inventory during the winter-spring transition” by J. Park et al.

In this manuscript, the authors report on a study investigating the contribution of transboundary transport to NOy levels in the Republic of Korea and China for January to May 2021. Using the CMAQ-ISAM, they first update the NOx emission inventory used by inverting GEMS NO2 observations. They then separate the region of interest into 4 areas and use tagged NOy in the model to evaluate the contribution of different source areas to NOy levels in East Asia during the winter spring transition.

The manuscript discusses an interesting topic. It is well structured and clearly written, and the methods and conclusions appear valid. However, there are several critical aspects which need to be addressed and clarified before the manuscript can be accepted for publication.

Major comments:

In the title and in other parts of the manuscript, the authors talk about “nitrogen loading”. When I first read this, I expected that this would include ammonia and particulate nitrates, but this is not the case. Please use more precise wording to make clear, what exactly is covered by this study.

If I understood the manuscript right, the definition of NOy used does not include NOx. This makes sense if one would like to focus on transport, but it can be misleading if the results are interpreted in the way of “how much of the reactive nitrogen pollution is due to transnational transport”. I think it would be better to include NOx in NOy. If that’s not possible, the authors should at least include a column in the tables giving the fraction that transported NOy contributes to NOy + NOx assuming the latter is local.

I’m completely confused by Figures 5 and 6. If I did not misunderstand the figures, they are supposed to show the absolute mixing ratio of NOy that is present throughout East Asia and originates in one of the selected source regions. However, the maps show clear hotspots in polluted regions such as SMA even when the contribution is from another source region. Shouldn’t these maps show smooth distributions outside the source regions? And shouldn’t we see a reduction with distance from the source region? Please explain or correct the figures.

I do not follow the discussion on the impact of wind speed on the fraction of transported NOy. In the manuscript, it is claimed that low wind speeds lead to accumulation of pollutants (so far, I agree) and that this leads to a larger contribution of transported NOy. This I do not understand, as accumulation (and dilution) will work in a similar way for locally produced and transported NOy, and therefore, the ratio is not changed. In my opinion, the only relevant quantities are a) how much NOy is transported over the borders into the selected region and b) how much NOy is produced within this region. Please explain.

Minor comments:

L149: While the PBL is an interesting region for air pollution studies, I’m surprised that the authors assume that it is also the altitude region where the relevant transport processes take place.

L198: The sentence on the use of averaging kernels is rather vague and should be formulated more precisely.

L219: Note that GEMS does not take instantaneous snapshots of NO2 but scans for 30 minutes from East to West.

L261: Introduce SMA

Section 2.3: It is well known from validation studies that version 2 of the GEMS tropospheric NO2 product as well as GEMS cloud fractions still have some issues. This should be briefly discussed and the impact on emissions and the quantification of NOy transport be mentioned.

Figure 2: Are the model fields sampled at the times of valid GEMS measurements?

Figure 2: I do not understand why the model with a posteriori emissions overestimates NO2 over large regions of Northern and Central China and Korea in February and March. With the assumption of local linearity, I would have expected the model to always do a good job on local hotspots such as SMA.

Figure 2: What does the term “hourly” in the figure caption refer to?

Line 323: A decline in anthropogenic emissions will increase the relative contribution of transported NOy only if the other regions do not see a similar decline in emissions.

Figure 4: Introduce ICO, BCO, OTH

Lines 368 and 373: See my major comment on the impact of wind speed. I do not agree with the arguments made here about “rapid passing through” and “longer lingering” as wind speed affects all NOy, not just the transported NOy.

Figure 5: The ppb scale does not seem correct to me – 5x10E4 ppb of NOy?

Figure 5: What does the term total (sum) NOy imply?