|The authors have made an effort to address the questions raised by the reviewers. The model sensitivity tests as performed by the authors should be helpful for further understanding the sources and formation mechanisms of nitrate aerosols in the UTLS over the TP/SASM region. The authors’ responses are generally fine, but additional issues arising need to be addressed further. Below are my specific comments and suggestions. |
It is interesting to see that compared to surface NO3-, nitrate aerosols in the UTLS over the TP/SASM region are not so sensitive to a 50% reduction of anthropogenic NOx emissions in Asia. What could one learn from this result with regards to the sources and formation mechanism of nitrate aerosols (and their gas precursors) in the UTLS over the TP/SASM region? Specifically, does it mean that anthropogenic NOx emissions in Asia (including South and Southeast Asia) do not have large impacts on nitrate aerosols in the UTLS over the TP /SASM region? The authors may want to give a short discussion on the implications of this sensitivity result, instead of merely arguing that nitrate is more important than sulfate for the UTLS aerosol layer even under a situation of 50% NOx emission reduction. The authors are suggested to use a different title for Sect. 7, .e.g. “Sensitivities of simulated nitrate in the UTLS to anthropogenic NOx, NH3 and SO2 emissions in Asia”.
PM2.5 is an important parameter usually used for air quality study, and it might not have to be used to describe the aerosols in the UTLS. Was PM2.5 simulated as a tracer in GEOS-Chem or it was calculated just by summing simulated SO4=, NO3-, NH4+, BC and OC? Mineral dust and sea salt aerosols, which were simulated and used for the calculation of extinction coefficient in this study, should also be taken into account for PM2.5. Otherwise, more clear definition/descriptions should be given in the text. In the abstract (Line37-38), for example, it might be written as “defined as the sum of sulfate, nitrate, ammonium, black carbon, and organic carbon aerosols in this study”. In Fig. 7, the sub-panels for PM2.5 can be omitted, and instead the distributions of mineral dust and sea salt aerosols might be displayed.
The physical, chemical and dynamical processes related to the formation of the UTLS aerosol layer are complex and need further investigations in future studies. Therefore, I would suggest that a more detailed description of the aerosol module used in this study be given for the convenience of comparisons of future work with this study. How many size bins or modes of the aerosols were adopted in the GEOS-Chem used in this study? What ionic aerosols were considered in the ISORROPIA II used in this study, NH4+/SO4=/NO3-, or NH4+/Na+/K+/Ca2+/Ma2+/SO4=/NO3-/Cl-/CO3=, or others? Were the aerosols treated as internal, or external mixed, or both (the same question as in my first round review)? One can see from Table 3 that OC and BC concentrations are constant for different simulations, indicating that they are external mixed.
With respect to the formation mechanisms for SO4= and NO3- in the UTLS (Line 547-553), simple comparison of the changes in the gas-phase oxidation of SO2 and the gas-particle equilibria of NO3- with temperature (altitude) might not be sufficient to explain the difference in their vertical distributions. Note that the reaction of NO2 (not NO as written in Line 546) with OH also changes with temperature, the same as for the reaction of SO2 with OH. Was H2SO4 simulated in the GEOS-Chem? If so, comparisons of both the formation and nucleation rates of H2SO4 and HNO3 might be helpful. It seems that the gas-phase oxidation of SO2 was not addressed in the work of X.Y. Zhang et al. (2012), not mentioned to the change of its reaction rate with temperature. Therefore, this work should not be cited here (Ling 550).