Why is ozone in South Korea and the Seoul Metropolitan Area so high and increasing?
Nadia Kathryn Colombi1,Daniel J. Jacob1,Laura Hyesung Yang2,Shixian Zhai2,Viral Shah3,4,Stuart K. Grange5,Robert M. Yantosca2,Soontae Kim6,and Hong Liao7Nadia Kathryn Colombi et al.Nadia Kathryn Colombi1,Daniel J. Jacob1,Laura Hyesung Yang2,Shixian Zhai2,Viral Shah3,4,Stuart K. Grange5,Robert M. Yantosca2,Soontae Kim6,and Hong Liao7
1Harvard University, Department of Earth and Planetary Science, Cambridge, MA 02138, USA
2Harvard University, John A. Paulson School of Engineering and Applied Sciences, Cambridge, MA 02138, USA
3NASA Global Modeling and Assimilation Office, Goddard Space Flight Center, Greenbelt, MD 20771, USA
4Science Systems and Applications, Inc., Lanham, MD 20706, USA
5Empa, Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129 8600 Dübendorf, Switzerland
6Ajou University, Department of Environmental and Safety Engineering, Suwon, Gyeonggi 16499, Republic of Korea
7Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, Jiangsu Collaborative Innovation Center of Atmospheric Environment and Equipment Technology, School of Environmental Science and Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China
1Harvard University, Department of Earth and Planetary Science, Cambridge, MA 02138, USA
2Harvard University, John A. Paulson School of Engineering and Applied Sciences, Cambridge, MA 02138, USA
3NASA Global Modeling and Assimilation Office, Goddard Space Flight Center, Greenbelt, MD 20771, USA
4Science Systems and Applications, Inc., Lanham, MD 20706, USA
5Empa, Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129 8600 Dübendorf, Switzerland
6Ajou University, Department of Environmental and Safety Engineering, Suwon, Gyeonggi 16499, Republic of Korea
7Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, Jiangsu Collaborative Innovation Center of Atmospheric Environment and Equipment Technology, School of Environmental Science and Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China
Received: 01 Dec 2022 – Discussion started: 09 Dec 2022
Abstract. Surface ozone pollution in South Korea has increased over the past two decades, despite efforts to decrease emissions, and is pervasively in exceedance of the maximum daily 8-hr average (MDA8) standard of 60 ppb. Here, we investigate the 2015–2019 trends in surface ozone and NO2 concentrations over South Korea and the Seoul Metropolitan Area (SMA), focusing on the 90th percentile MDA8 ozone as an air quality metric. We use a random forest algorithm to remove the effect of meteorological variability on the 2015–2019 trends and find an emission-driven ozone increase of up to 1.5 ppb a-1 in April–May while NO2 decreases by 22 %. GEOS-Chem model simulations including recent chemical updates can successfully simulate surface ozone over South Korea and China as well as the very high free tropospheric ozone observed above 2 km altitude (mean 75 ppb in May–June), and can reproduce the observed 2015–2019 emission-driven ozone trend over the SMA including its seasonality. Further investigation of the model trend for May, when meteorology-corrected ozone and its increase are the highest, reveals that a decrease in South Korea NOx emissions is the main driver for the SMA ozone increase. Although this result implies that decreasing volatile organic compound (VOC) emissions is necessary to decrease ozone, we find that SMA ozone would still remain above 80 ppb even if all anthropogenic emissions in South Korea were shut off. China contributes only 8 ppb to this elevated South Korea background and ship emissions contribute only a few ppb. Zeroing out all anthropogenic emissions in East Asia in the model indicates a remarkably high external background of 56 ppb, consistent with the high concentrations observed in the free troposphere, implying that the air quality standard in South Korea is not practically achievable unless this background external to East Asia can be decreased.
The manuscript by Colombi et al. (2022) explores the 2015-2019 trends in surface ozone and NO2 concentrations over South Korea and the Seoul Metropolitan Area (SMA) using observations and the GEOS-Chem model. The authors 1) quantified emissions-driven trend in ozone and NO2 concentrations by removing the effect of meteorological variability during the period, 2) successfully simulated surface ozone over South Korea and China by including recent model chemical updates, and 3) identified factors deriving high surface ozone and increasing trends over SMA.
The manuscript is well structured and the findings are useful not only to academia but also to policymakers. I support the publication of this manuscript with minor revisions mostly asking for clarification.
Specific comments
Use of 90th percentile MDA8 as a metric
As highlighted in the introduction (L45 - 46), there is no monitoring site that complies with the MDA8 national air quality standard in South Korea. It is more urgent to grasp the overall status of MDA8 (mean or median MDA8 as a proxy) or the low concentrations (10th percentile MDA8 as a proxy) rather than high concentrations (90th percentile MDA8 as a proxy) for practical applications. Therefore, I recommend using mean, median, or 10th percentile MDA8 rather than 90th percentile MDA8 as a metric.
Figure 1 and 2 lower panels
Adding the same metric for South Korea with different color will be helpful in showing relative ozone pollution in SMA compared to the national average.
Section 3 and 4 titles (L105)
These sections are for SMA only. Please clarify in the title, too.
Technical corrections
L25 – 26: ~find an emission-driven … while NO2 decreases by 22%.
Please rephrase. It is unclear whether the NO2 decrease is from emissions or surface observations.
L33: ~, we find that SMA ozone would still remain above 80 ppb …
What about other parts of South Korea? Because the analysis of South Korea and SMA are mixed, the focus often gets vague.
L43 – 46: In 2015 … continued to increase (NIER, 2020).
MDA8 of 60 ppb is a national air quality standard not limited to SMA.
L123: ~ for NO2
A period is missing next to NO2.
L153: ~thus the seasonality … VOC-limited conditions.
Please rephrase. I don’t understand what you mean here.
Surface ozone, detrimental to human and ecosystem health, is very high and increasing in South Korea. Using a global model of the atmosphere, we found that emissions from South Korea and China contribute equally to the high ozone observed. We found that in the absence of all anthropogenic emissions over East Asia, ozone is still very high, implying that the air quality standard in South Korea is not practically achievable unless this background external to East Asia can be decreased.
Surface ozone, detrimental to human and ecosystem health, is very high and increasing in South...
