Satellite-Based Diagnosis and Numerical Verification of Ozone Formation Regimes over Nine Megacities in East Asia

Hyo-Jung Lee,Lim-Seok Chang,Geum-Hee Yang,Xiong Liu,Juseon Bak,Gonzalo González Abad,Jong-Min Kim,Jae-Bum Lee,Cheol-Hee Kim,Daniel A Jaffe,Hyun-Young Jo,Yu-Jin Jo

doi:10.3390/rs14051285

Abstract

Urban photochemical ozone (O3) formation regimes (NOx- and VOC-limited regimes) at nine megacities in East Asia were diagnosed based on near-surface O3 columns from 900 to 700 hPa, nitrogen dioxide (NO2), and formaldehyde (HCHO), which were inferred from measurements by ozone-monitoring instruments (OMI) for 2014–2018. The nine megacities included Beijing, Tianjin, Hebei, Shandong, Shanghai, Seoul, Busan, Tokyo, and Osaka. The space-borne HCHO–to–NO2 ratio (FNR) inferred from the OMI was applied to nine megacities and verified by a series of sensitivity tests of Weather Research and Forecasting model with Chemistry (WRF-Chem) simulations by halving the NOx and VOC emissions. The results showed that the satellite-based FNRs ranged from 1.20 to 2.62 and the regimes over the nine megacities were identified as almost NOx-saturated conditions, while the domain-averaged FNR in East Asia was >2. The results of WRF–Chem sensitivity modeling show that O3 increased when the NOx emissions reduced, whereas VOC emission reduction showed a significant decrease in O3, confirming the characteristics of VOC-limited conditions in all of the nine megacities. When both NOx and VOC emissions were reduced, O3 decreased in most cities, but increased in the three lowest-FNRs megacities, such as Shanghai, Seoul, and Tokyo, where weakened O3 titration caused by NOx reduction had a larger enough effect to offset O3 suppression induced by the decrease in VOCs. Our model results, therefore, indicated that the immediate VOC emission reduction is a key controlling factor to decrease megacity O3 in East Asia, and also suggested that both VOC and NOx reductions may not be of broad utility in O3 abatement in megacities and should be considered judiciously in highly NOx-saturated cities in East Asia.

Highlights

The WRF–Chem model evaluations against surface observations were performed by employing statistical metrics: Pearson correlation coefficient (R), Index of Agreement (IOA), normalized mean bias (NMB), and root–mean–square–error (RMSE)
Photochemical O3 formation is influenced by volatile organic compounds (VOCs)/NOx ratios, and its formation regimes are generally classified as either NOx -limited regimes based on non-linear O3 –NOx –VOC reactions
The identification of the O3 formation regime is an important tool for making O3 -reduction policies, such as NOx vs. VOC control priority

Summary

Introduction

Ozone (O3 ) is increasing in East Asia at a higher rate because of the rapid industrialization and urbanization over the years, and has been regulated since the 2000s in East Asian countries [1–4]. In China, total nitrogen oxides (NOx = NO (nitric oxide) + NO2 (nitrogen dioxide)) have been on a downward trend since 2013 due to China’s implementation of. China’s Five-Year Plan (FYP) air pollution control policies during 2013–2017 [1,2]. Korea has enforced stringent regulation policies for NOx since the early 2000s [3,4], and in Japan, O3 reduction measures have been imposed to reduce traffic emissions since the. 1990s, resulting in the reduction of volatile organic compounds (VOCs) and NOx emissions, Remote Sens. Studies on the impacts of O3 regulation and emission reduction policies have been conducted targeting

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Conclusion