Reply to 'Comment on acp-2022-123', Anonymous Referee #1, 24 Mar 2022

Wenjie Wang¹, David D. Parrish², Siwen Wang¹, Fengxia Bao¹, Ruijing Ni³, Xin Li⁴, Suding Yang⁴, Hongli Wang⁵, Yafang Cheng³, Hang Su¹*, 

¹ Multiphase Chemistry Department, Max Planck Institute for Chemistry, Mainz, 55128, Germany.

² David.D.Parrish, LLC, Boulder, CO, 80303, USA;

³ Minerva Research Group, Max Planck Institute for Chemistry, Mainz 55128, Germany.

⁴ State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing, 100871, China

⁵ State Environmental Protection Key Laboratory of Formation and Prevention of Urban Air Pollution Complex, Shanghai Academy of Environmental Sciences, Shanghai 200233, China.

We are grateful for the insightful and constructive comments provided by Anonymous Referee #1 of our paper (Wang et al., 2022). We plan to fully respond to all referee comments when we revise our paper following closure of the open discussion period. However, we wish to respond to the following comment by Referee #1 while the discussion is still open, so that posting of additional comments regarding this issue will be possible:

1. Line 95-97: In general, the authors can do better in catching up the more recent studies of ozone trends and ozone sensitivity in China. An example here, there are also studies pointing out the increases in tropospheric ozone in northern mid-latitudes extend to more recent years (e.g. 2017) than 2000 (Cooper et al., 2020; Gaudel et al., 2020).

We believe that this comment misinterprets the results of the cited studies. Cooper et al. (2020), Gaudel et al. (2020) and a more recent paper (Chang et al., 2022), all including the same three lead authors, present careful linear trend analyses of a wide selection of tropospheric ozone data sets, all including measurements at northern midlatitudes, but covering somewhat different time periods. These three studies all utilize linear trend analyses to quantify the mean temporal trend over the past two to three decades included in the respective ozone measurements; thus they reveal the total net ozone change over that period, but do not provide information regarding how the ozone trend varied during that time. In contrast, Parrish et al. (2020) present a non-linear analysis of the northern midlatitude ozone changes over the past four decades; this non-linear analysis not only quantifies the net ozone change over that time period, but also provides additional information regarding variation of the ozone trend during that time. Table 1 compares the net trends quantified over four different time periods by the four analyses.

Overall, the results given in Table 1 agree that only small net changes have occurred in northern midlatitude ozone over the specified time periods; all derived trends are in the range of  -1.14 to +1.41 ppb/decade. These trends are of significantly smaller magnitude than the net trends reported for earlier time periods; for example Logan et al. (2012) found that ozone over Europe increased by 6.5 to 10 ppb during the 1978–1989 period, and Cooper et al. (2010) report a strong increase of 6.3 ± 3.4 ppb/decade in springtime ozone across western North America during 1995–2008. The net trends in Table 1 are small because they reflect the overall ozone change due to the increases during the 1990’s and early 2000s, and the decreases that followed the maximum concentrations reached in the mid-2000s (Parrish et al., 2020); the analyses presented in all four papers referenced in Table 1 are consistent with this picture. This non-linear character is also reflected in the analysis of Cooper et al. (2020), which found a negative trend of significantly greater magnitude for the 2000-2017 period, which included less of the early increase, compared to the 1995-2017 period.

We conclude that our original discussion on lines 95-97 in Wang et al. (2022) is correct:  “The baseline ozone concentrations at northern midlatitudes increased at an average rate of ~ 0.60 ppb year⁻¹ from 1980 to 2000 (Parrish et al., 2020).” However, as also reported by Parrish et al. (2020) that increase “… has ended, with a maximum reached in the mid‐2000s, followed by slow decrease (average = −0.09 ± 0.08 ppb year⁻¹ from 2000 to the present).” The assertion of Referee #1 that increases in tropospheric ozone in northern mid-latitudes extend to more recent years (e.g. 2017) is not correct.

Table 1. Comparison of mean temporal trend derived from linear and non-linear trend analyses

^a Calculated from the quadratic fit with the parameter values given in their Table 2 for all baseline data sets.

^b Weighted mean of 12 northern midlatitude trends given in their Table 2. Each trend is weighted by the inverse square of the confidence limit reported for the trend.

^c Weighted mean of 5 northern midlatitude trends given for the 950-250 hPa tropospheric ozone column in their Table S1a.

^d Weighted mean of 15 northern midlatitude trends given in their Table 2.

References

Chang, K.-L., Cooper, O. R., Gaudel, A., Allaart, M., Ancellet, G., Clark, H., et al.: Impact of the COVID-19 economic downturn on tropospheric ozone trends: An uncertainty weighted data synthesis for quantifying regional anomalies above western North America and Europe. AGU Advances, 3, e2021AV000542. https://doi.org/10.1029/2021AV000542. 2022.

Cooper, O. R., Parrish, D. D., Stohl, A., Trainer, M., Nédélec, P., Thouret, V., Cammas, J. P., Oltmans, S. J., Johnson, B. J. Tarasick, D., et al.: Increasing springtime ozone mixing ratios in the free troposphere over western North America, Nature 463 (7279):344–48, doi:10.1038/nature08708, 2010.

Cooper, O. R., Schultz, M. G., Schroeder, S., Chang, K.-L., Gaudel, A., et al.: Multi-decadal surface ozone trends at globally distributed remote locations, Elem Sci Anth, 8, 23, http://doi.org/10.1525/elementa.420, 2020.

Gaudel, A., Cooper, O. R., Chang, K.-L., Bourgeois, I., Ziemke, J. R., Strode, S. A., Oman, L. D., Sellitto, P., Nédélec, P., Blot, R., Thouret, V., and Granier, C.: Aircraft observations since the 1990s reveal increases of tropospheric ozone at multiple locations across the Northern Hemisphere, Science Advances, 6, eaba8272, http://doi.org/10.1126/sciadv.aba8272, 2020.

Logan, J. A., Staehelin, J., Megretskaia, I. A., Cammas, J.‐P., Thouret, V., Claude, H., et al.: Changes in ozone over Europe: Analysis of ozone measurements from sondes, regular aircraft (MOZAIC) and alpine surface sites. Journal of Geophysical Research, 117, D09301, https://doi.org/10.1029/2011JD016952, 2012.

Parrish, D. D., Derwent, R. G., Steinbrecht, W., Stübi, R., Van Malderen, R., Steinbacher, M., Trickl, T., Ries, L., and Xu, X.: Zonal similarity of long‐term changes and seasonal cycles of baseline ozone at northern midlatitudes, J. Geophys. Res.-Atmos., 125, e2019JD031908, 2020.

Wang, W., Parrish, D. D., Wang, S., Bao, F., Ni, R., Li, X., Yang, S., Wang, H., Cheng, Y., Su, H.: Long-term trend of ozone pollution in China during 2014-2020: distinct seasonal and spatial characteristics, https://doi.org/10.5194/acp-2022-123, 2022.