Reply on RC1

The results are clear and it’s an interesting topic which indeed needs more attention. The main part of the analysis is only applied for the control catchment and not for the treated catchment; based on the assumptions that only the control catchment behaved non-stationary during drought. The primary objectives are based on the concept that a catchment may not show non-stationary rainfall-runoff relationships during changes in climate; this need to be introduced and discussed in depth in the manuscript.


manuscript.
It is critical for the HONO modeling study to clarify why specific parameterization is used. The authors have tried to conduct sensitivity runs and presented results in the SI. However, it is still not convincing why some HONO uptake coefficients were used in the model. Were they based on laboratory experiments, empirical parameters obtained from the field, or simply obtained from other models? These should be clarified. [General Comment]: 1.1 For example,, are these uptake coefficients based on experimental data? Please clarify here how uncertain they are.
[Response]: The selection of uptake coefficients on ground surface and aerosol surface are mainly based on the empirical data derived from either experiments or observations. As the reviewer suggested, we have summarized the variation range of the parameters and several sensitivity results to clarify the associated uncertainties. We referred to some experimental data measured in our laboratory. Experimental data measured on MgO surface fall in the range of 1 -6 ×10 -6 as reported by Ma et al. (2017) and on the hematite surface in the range of 1.9×10 -7-1.6×10 -6 as reported by Liu et al. (2015) . The derived empirical data obtained by VandenBoer et al.(2013) from the field observation fall in the range of 2×10 -6 -1.6×10 -5 . The empirical uptake coefficient used in models varied widely ranging from 10 −7 to 10 -3 (Table S2). The majority γΝΟ2 value employed in literature is about 10 -6 . When the uptake coefficient changes by 10 times, the HONO concentration from the heterogeneous reaction on ground surface changes by a factor of two. The detailed revises refer to: Page 5, Line 170: The selection criteria and possible ranges of the uptake coefficient are discussed in SI. Supplemental Information Page 2, Line 47-55: The selection of uptake coefficients on ground surface and aerosol surface are mainly based on the empirical data derived from either experiments or observations. Experimental data measured on MgO surface fall in the range of 1 -6 ×10 -6 as reported by Ma et al. (2017) and on hematite surface in the range of 1.9×10 -7 -1.6×10 -6 as reported by Liu et al. (2015) . The derived empirical data obtained by VandenBoer et al. (2013) from the field observation fall in the range of 2×10 -6 -1.6×10 -5 . The empirical uptake coefficient used in models varied widely ranging from 10 −7 to 10 -3 (Table S2). The majority γΝΟ2 value employed in literature is about 10 -6 .
[General Comment]: 1.2 Lines 203-205: Please explain why 1.7/H is used in this study and in previous studies, and how uncertain it is.
[Response]: 1.7/H represents the ground surface area density (S/Vg) in the model. Effective surface area of ground can be higher than the geometric surface area due to the presence of trees, buildings, and other surface areas. A factor of 1.4-2.2 for the ratio of effective surface area to geometric surface area was measured by Voogt and Oke (1997 (1997). The result suggests slightly higher concentrations but with similar model performance (details in Figure S4 in Supplemental Information [Response]: The contribution of HONO from acid displacement (5.5% for HNO3 and 0.7% for HCl) is far less than the heterogeneous reaction on the ground surface (86.2%).The dry deposition velocities of HNO3 and HCl in CMAQ is calculated using a big-leaf resistance model (Wesely, 1989;Wesely, 2007). The total resistance to dry deposition (which is the inverse of v) is calculated as the sum of the bulk surface resistance, Rsurf, the aerodynamic resistance, Ra, the quasi-laminar boundary layer resistance, Rbc. Rsurf includes the influence of vegetation, canopy, ground, etc. Considering the average temperature in our study is around 1.6 ℃ which is above the threshold value for low temperatures as suggested in Jaegle's method ( The dry deposition velocities of HNO3 and HCl in CMAQ is calculated using a big-leaf resistance model (Wesely, 1989;Wesely, 2007 It also increases day-time concentrations, however, predicted values are substantially lower than the observed data, which suggests that additional processes (Oswald et al., 2013a;Xing et al., 2017;Romer et al., 2018) are needed to close the gap between observed and predicted day-time HONO concentrations.
[Other Comment]: 1.9 Fig.1: Please explain what the error bars are.
[Response]: We have added the following text in line 300 to explain error bars: Page 9, Line 313: Error bars represent 5%-95% values of all HONO concentrations.
[Other Comment]: 1.10 Line 365: Please provide values for vehicle exhausts.
[Response]: We have added the reported values (0.001-0.008) for vehicle exhausts as follows: Page 10, Line 332: The observed HONO/NO2 ratios ranging between 0.003 and 0.15 are much higher than reported values in the vehicle exhausts (0.001-0.008) which suggests that HONO formation is governed mainly by the secondary production (Kirchstetter et al., 1996;Kurtenbach et al., 2001).
[Other Comment]: 1.11 Line 464: As shown in Fig. 1, daytime HONO was significantly underestimated in the model. Please discuss how this affects OH concentrations.
[Response]: OH concentration is affected not only by the daytime HONO concentration but also by the photolysis rate of HONO. In REV case, we only considered the HONO heterogeneous sources which increase OH concentration as we discussed in section 3.3. Daytime OH concentrations can potentially be higher than the predicted values since daytime HONO concentrations are lower than observed data. However, the aerosol indirect effect may reduce OH concentration as it may slow the HOx formation rate from HONO. A future study incorporating aerosol indirect effect is needed to improve the representation of HONO chemistry in CMAQ and examining its impact on OH concentration. We revised the text as follows: Page 14 Line 462-465: The daytime underestimation of HONO in Fig.1 can potentially lead to the underestimation of OH concentration; however, the aerosol indirect effect may lower the OH concentration by reducing the rates of HOx formation. Therefore, more accurate HONO simulation needs to consider more complex and significant atmospheric chemical processes.
[Other Comment]: 1.12 Fig. 6: It should show the REV case instead of ORI case, as the REV cases are with HONO updates, the main focus of this study.
[Response]: As the reviewer suggested, we have replaced figures in ORI cases to REV cases in the revised manuscript as follows. Page 20 Fig  Please also note the supplement to this comment: https://acp.copernicus.org/preprints/acp-2021-47/acp-2021-47-AC1-supplement.pdf