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

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.

Cost-Effectiveness of NOX and VOC Co-operative Controls for PM2.5 and O3 Mitigation in the Context of China’s Carbon Neutrality

“…In 2030, when compared to the NO X -AVOC-A scenario, the NO X -only scenario leads to an increase of 87.8% in annual O 3 -related premature deaths in mainland China, 57.0% in the 2 + 26 cities, 147.0% in the YRD, and a staggering 1239.0% in PRD. The leading cities of these city clusters, such as Shanghai, Beijing and Guangzhou, are highly developed with intense NO X emissions, where the O 3 formation regimes have been identified as VOCs-limited, indicating that short-term O 3 declines in these cities heavily rely on AVOC reductions. However, regarding city cluster, the area of PRD (201 grids) is much smaller than that of 2 + 26 cities (819 grids) and PRD (1029 grids), and thus, the urban concentration of PRD is denser, resulting in its overall higher NO X emission intensity, and in turn leading to the heightened sensitivity of O 3 to AVOCs .…”

Mitigating China’s Ozone Pollution with More Balanced Health Benefits

“…At present, the concentration ratio of HCHO to NO 2 based on satellite monitoring is a major approach to characterize the O 3 formation regime on a large scale. The leading cities of these city clusters such as Shanghai, Beijing, and Guangzhou were highly developed with intense NO X emissions and it has been demonstrated that the O 3 formation regimes in these megacities were all VOC-limited, whereby the ratios of HCHO/NO 2 were less than 1. , However, the total area of PRD (201 grids) is much smaller than that of YRD (1029 grids) and “2 + 26” cities (819 grids), and thus the overall urbanization level of PRD was higher. In contrast, the proportions of rural areas in YRD and “2 + 26” cities were larger, which led to lower urban coverage rates.…”

Elucidating the impacts of COVID-19 lockdown on air quality and ozone chemical characteristics in India

“…In urban grid cells, our results suggest that the FNR transition range is 0.6-1.4. Lee, et al 85 studied FNR values in nine megacities in East Asia and suggested the ozone chemical regime to be NOx-saturated when the FNR is below 1.5. In rural grid cells, the union of the FNR transition range, for 2019BAU and 2020BAU scenarios, is 0.6-1.7.…”