|1. Abstract, around L20: Before discussing the model attribution, please add a few sentences stating how well the model captures the observed ozone interannual variability (r-squared = ?) and that the model significantly underestimates long-term increases in surface ozone over East Asia. Such statements should also be included in the Conclusion before discussing any attribution results. |
2. Introduction, Page 3, around Line 19: Move the discussions of future ozone changes to the next paragraph and add a few sentences at the end of the previous paragraph to clearly state (1) Asian NOx emissions have more than tripled over the past 20-30 years (Granier et al., 2011; Zhang Q. et al., HillBoll et al., 2013); (2) Consequently, outflow of Asian pollution has increased significantly and has contributed to raising springtime ozone observed over western North America (Refs, e.g., Lin et al., act-2016-1093).
In the response to reviewers, the authors stated that there is no significant trend in Asian ozone outflow because the offsetting effects of North American and European emission reductions. However, you are looking at outflow of ozone immediately downwind of Asia, which should be dominated by the trends of Asian NOx emissions. The early modeling results of Lamarque et al. (2010) and Parrish et al. (2014) are based on free-running chemistry-climate models that generate their own meteorology and thus are not expected to reproduce the influence of meteorological variability on observed ozone trends. As the authors nicely demonstrated in this manuscript, the meteorologically-driven ozone interannual variability is quite large. Therefore, the trend analysis presented in this article (e.g., Fig.5) should be compared to more recent work using long-term model simulations driven by observed meteorology.
3. Specific comments on the figures and associated discussions:
Figure 1: please discuss how the trends in NOx emissions used in this study compare to the trends derived from satellite NO2 columns for the overlapping period 1996-2005? Can the NOx emission biases explain the biases in simulated ozone trends shown in Figure 5?
Figure 3 and Table 4: The correlations reported in Table 4 mainly reflect the prominent variability on monthly/seasonal time scales, as opposed to interannual time scales. It is important to evaluate how well the model captures the observed interannual variability – the focus of the paper. Thus, the referee suggests conducting the comparison and interannual correlations for each season (DJF, MAM, JJA, SON) over the study period.
Figure 4: The ozonesondes sampling frequency is less than 4 to 5 profiles per month at these sites. These weekly ozonesondes are too infrequent to capture the actual interannual variability and long-term trends of seasonal mean ozone in surface air and aloft, given the large meteorologically-driven ozone variability. The referee suggests only using ozonesonde data for evaluating the simulated monthly mean ozone climatology.
Figure 5: Again, the comparison of observed and simulated ozone trends should be made on a seasonal basis!
Figures 6 and 7: It would be very useful to the readers of the paper if the authors could show a pressure-latitude cross-section similar to Figure 6, but for the linear trends of seasonal mean ozone in ppb yr-1 over 1986-2006 (highlight where the trends are statistically significant at the 95% confidence level using boxes or stippling). You can then discuss the vertical and latitudinal distribution of changes in Asian pollution outflow during the historical period. The proposed analysis will also provide additional insights into whether the lack of significant ozone trends currently shown in Figure 7 is due to spatial averaging.
4. Discussion on the influence of climate change: In fact, many studies suggest that a warming climate would most likely worsen regional air stagnation events and thus decrease outflow of pollution from a source region. Your conclusion that future climate change will lead to increases in Asian ozone outflow in spring and summer seems to contradict with the findings in the published literature. Furthermore, increasing water vapor in a warming climate will lead to lower ozone at remote locations.