This study examined the impacts of SO2 emissions from the United States, Europe, and China on tropospheric oxidants and ozone concentrations using GFDL-AM3 nudged towards NCEP wind velocities. They found that OH and HO2 decrease throughout the northern hemisphere mid-troposphere by up to 10%, as well as up to 4% decrease in O3 in the free troposphere and at the surface due to the presence of SO2. They also indicated that aerosols impact oxidation primarily through heterogeneous reactive uptake pathways over photolysis pathways. They topic is interesting and within the scope of the journal. The manuscript is well organized. Before it can be considered to be accepted, a few comments shoud be addressed.

General comments: This study investigates the responses of tropospheric oxidants to SO2 emissions from the United States, Europe, and China. In this case, I would like to see what are the response differences between these three regional sources. The authors only mentioned the oxidative response is greatest for the China perturbation simulation due to this largest emission. More detailed analysis/information should be given.

Specific comments:

Title: This study focused on the SO2 emissions from the United States, Europe, and China instead of the whole northern hemisphere mid-latitude anthropogenic sulfur dioxide emissions. I would suggest to make it more specific.

Lines 19-21: This study zeroed out the regional emissions individually. But this sentence is more like all emissions were zeroed out.

Line 40: The OH response to historical anthropogenic emissions renged from a 12.7% decrease to a 14.6%. Which anthropogenic emissions are inferred here, aerosol, GHGs, ozone?

Line 65: The authors mentioned absolute humidity, temperature, and ultraviolet radiation are thought to predominantly control OH abundance. Aerosol change can also influence humidity and temperature. Why they were not disscussed? 

Line 90: Which aspects of the model were evaluated against observations? Since this study focused on the impacts of SO2 on HOx, NOx, and O3, at least, SO2, sulfate and ozone concentrations over China, US and Europe shoud be evaluated.

Line 121: How the authors test the significance of the difference with only one year simulation? The test requires two arrays. With one year simulation for both ctrl and sensitivity simulations, I am not sure how these two arrays were created.

Line 134: The figures only show the relative change in percentage. I do not see the largest perturbation in China, although it shoud be true in absolute concentration change.

Lines 139-144: Many recent studies also analyzed the long-range transport of aerosols between continents and to the Arctic (e.g., Yang et al., 2017, 2018; Ren et al., 2020). The authors may consider to cite more relavent studies.

References:

Yang, Y., Wang, H., Smith, S. J., Easter, R., Ma, P.-L., Qian, Y., Yu, H., Li, C., and Rasch, P. J.: Global source attribution of sul- fate concentration and direct and indirect radiative forcing, At- mos. Chem. Phys., 17, 8903–8922, https://doi.org/10.5194/acp- 17-8903-2017, 2017.

Yang, Y., Wang, H., Smith, S. J., Easter, R. C., and Rasch, P. J.: Sulfate aerosol in the Arctic: Source attribution and radiative forcing, J. Geophys. Res., 123, 1899–1918, https://doi.org/10.1002/2017JD027298, 2018.

Ren, L., Yang, Y., Wang, H., Zhang, R., Wang, P., and Liao, H.: Source attribution of Arctic black carbon and sulfate aerosols and associated Arctic surface warming during 1980–2018, Atmos. Chem. Phys., 20, 9067–9085, https://doi.org/10.5194/acp-20-9067-2020, 2020.

This study documents the impact of anthropogenic SO2 emissions from China, Europe and the US , on tropospheric oxidants in the GFDL AM3 model. The authors test this by conducting individual simulations where the SO2 emissions in each region are systematically switched off. They find SO2 emissions and subsequent formation of sulphate aerosols lead to a decrease in OH and HO2, increase in NOx and a decrease ozone. Changes in concentrations are focussed predominantly in the northern hemisphere. The chemical effect, via heterogenous uptake of these species by aerosol particles dominates over the photolysis effect , whereby aerosol alter the photolysis rates of O3 and NO2. This is a pertinent and timely study given current strategies to improve air quality, particularly in Asia and ensuring all atmospheric chemistry aspects and pollutants impacting this are considered. It is a well written piece of work and clearly laid out.

I therefore find it is suitable for publication in ACP subject to some minor comments which should be addresses prior to publication.

Minor comments:

- Does the model include nitrate, and if so how does this impact sulfate formation and subsequent impact on oxidants ?

- The changes in surface ozone (of the order of 3ppb/year) , this would equate to less than 10% or so of the surface ozone in the source regions studied here and is likely well within the model standard deviation. Can the authors comment on the role of the model uncertainty /variability on the significance of these results and potential implications for AQ. Indeed, in L199/200 the authors refers to clean air technologies driving up O3 by “a few ppb” in summertime. A comment on the relative significance of these changes for local air quality should be provided.

- L147: how much of the reduction in OH is attributed to the heterogenous uptake of sulphate by HO2 versus increase sink via oxidation of SO2+OH? Can the authors separate out these contributions to OH loss. The analysis presented focus on the radiative effect (via changes in photolysis) and the chemical effect ( heterogeneous uptake) but what about the additional aerosol oxidation chemistry that is also taking place and how that impacts the oxidant concentrations?

Technical comments:

L21: The sentence “Regional SO2 emissions ….. increasing the NOx lifetime as the heterogeneous sink on sulfate aerosol declines” is confusing as the sink on sulfate of OH, NO3 and N2O5 are increased which leads to the reduction in NOx sinks. Please rephrase.

L132: for the presence of --> due to the presence of

L134: The zeroing of 2015 emissions --> the zeroing of 2015 SO2 emissions

L138/9: from a zeroing of China SO2 --> the increase in sulphate clearly doesn’t come for the zeroing of the emissions, suggest the statement is rephrased.