|The authors present the results of a forecasting system that assimilates both initial hourly aerosol concentration and emission fluxes in order to improve the forecasting of particulate matter concentrations over China. To evaluate the performance of this system the forecasted concentrations are contrasted on one hand with independent observations not assimilated by the system and on the other hand against a control run without any assimilation and a forecast experiment only assimilating initial conditions but no emissions. The forecast is conduct for all China but a more in depth analysis is conducted in three regions experiencing stronger pollution levels. These three regions are the Beijing-Tianjin-Hebei region, the Yangtze River delta and the Pearl River Delta. The authors present results illustrating that the forecast assimilating initial conditions and emissions performs much better than the control simulation. Performance analysis in the three above-mentioned regions suggests that the system achieves improvements for almost all 48-h forecast in two of them while in the third one the improvement is more limited. Similarly the performance of the joint assimilation compared to the one only assimilating initial conditions shows improvement in two of the regions. |
The results presented in the manuscript are interesting, however the authors conduct only a shallow analysis of the results and do not discuss how some of the assumptions made in the system affect the result. Although I recommend this paper for publication I would suggest the authors extend the discussion of the results addressing some of the topics highlighted below. When presenting a new inversion system, in addition of presenting the main results (if it works or not), the limitations of the system and their impact should also be presented.
The authors assume prior emissions constant in time but it is well known that emissions are not constant throughout the day. Why were emissions considered constant throughout the day and also throughout the week? How much of the improvement in performance of the system comes from this assumption? How much better does the control perform when variable emissions within the day are allowed? Furthermore, the implications of not perturbing emissions of elemental carbon and organic carbon should be included in the manuscript. How does this affect the forecast? How realistic is the result provided by the system with this constrain?
The authors examine first the performance of the system by comparing the analysis of both assimilation experiments (expC and expJ) to the observations and then the forecast. It is interesting to note that when the analysis of both experiments are examined a better performance is obtained in PRD and JJJ when only initial conditions (IC) are assimilated (i.e. expC). However, when comparing the forecasts between both experiments, expJ performs better than the forecast of expC. What are the implications of this result? Furthermore, the authors provide a too simplistic analysis of the performance of the forecast in the three regions. Yes it is true that expJ improves with respect to the control and expC in YRD and PRD, but this is mostly for daytime, during night-time the improvement is very similar in three regions. In YRD, the performance is actually deteriorated during nighttime and in JJJ there is either deterioration or no improvement after 24 hr forecast for both assimilation experiments. Although the authors suggest that this is mainly to a good performance of the model during nighttime, this is not enough I believe. Why is the performance of the control run better during night? Why does the assimilation have so little impact during night? Why should the model have better performance for nocturnal conditions? Was it tuned under such conditions? Do the a priori emissions provided, the ones considered constant, correspond to night emissions? I would suggest the authors spend a bit more trying to address this issue as they have done so far.
If the difference between the control run and expC can be seen as the contribution of assimilating concentrations, can the difference between expC and expJ as the impact of assimilating emissions? If so, is it really worth if to assimilate both? Why wasn’t there and expE conducted where only emissions were assimilated? Figure 8 suggests that in most of the days in the three cities, the fact of assimilating only IC has little impact on the forecast. Figure 9 also illustrates that most of the improvement comes when emissions are assimilated. What if only emissions were to be assimilated, could that be enough? I suggest the authors include a discussion section where this is addressed.
The assimilations system needs further description. The authors describe how the observation error covariance matrix (R) is defined but do not do the same for the background error covariance matrix (Pb). How is Pb defined? The authors should explain this in the manuscript. Furthermore, observations from 77 stations were assimilated and observations from another set of 77 stations were used for verification purposes. However, in the three regions of interest in the manuscript; namely JJJ, YRD, and PRD, it is not clear how many stations were assimilated and how many were used in the verification. This number is provided in the caption of Figure 1 but should also be included in the text. Please also clarify if all these verification stations are used to compute the statistics presented in Figure 9.
Lines 30-31: Acronyms should be defined.
Lines 79-81: Structure of the paper described is not consistent with actual structure of the paper. There are 6 sections in the manuscript and only 5 according to text in last sentence of section 1.
Line 131: Sub index i should be defined. It is clear from the text what it stands for but should be introduced anyway.
Line 132: Why is it t-2 for the emissions and t-1 for the concentrations (line 131)? Is it a mistake and it should be t-1 for both? If not, please explain.
Line 147: Please explain which criteria was applied to define the limits (0.1 and 1.25) to the spread of (Ki,t)inf. How were they defined?
Line 150: Why are the negative values set to 0.001 and not simply 0? Please explain.
Line 322: Remove “which is a limitation of this manuscript”. It is already stated in lines 300 and 301.
Lines 352-356: Explain the criteria used to select the stations that would be used for verification and those used in the assimilation? How many of each are in the different regions. The total number of stations in each region is provided but it is not said how many of them are for validation or verification purposes.
Line 371: Why are hourly concentrations above 800 μg m-3 considered unrealistic? Hasn’t had China intense pollution events where this limit was exceeded in terms of hourly concentration? In any case, this should be argued much better if observations are removed. Also, why are observations where the departure of the ensemble mean of the first guess exceeds 100 μg m-3 removed? What
Line 408: What is the impact of considering that no correlations exist between emission variables. What is the impact on the assimilation and the forecast?.
Lines 460-461: What is it, are the emissions perturbed or not in expC? According to this line not, but according to the statement in lines 450-452, the emissions are perturbed by adding random noise.
Lines 566-570: Where are the numbers in this paragraph coming from? Please explain and present them.
Line 609: Replace “analysing” with “analysis”.
Line 649: What exactly is “dramatic”? How large is that? Please replace.
Lines 1097-1101: Authors should specify if the analysis presented in the figures include all verification stations in each region or only some of them. In addition, authors should also clarify to which dates the analysis presented in the figures corresponds.