Severe Surface Ozone Pollution in China: A Global Perspective

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Severe surface ozone pollution in China: a global perspective Xiao Lu, Jiayun Hong, Lin Zhang, Owen R. Cooper, Martin Schultz, Xiaobin Xu, Tao Wang, Meng Gao, Yuanhong Zhao, and Yuanhang Zhang Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.8b00366 • Publication Date (Web): 26 Jul 2018 Downloaded from http://pubs.acs.org on July 26, 2018

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Environmental Science & Technology Letters

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Severe surface ozone pollution in China: a global perspective

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Xiao Lu1, Jiayun Hong1, Lin Zhang1*, Owen R. Cooper2,3, Martin G. Schultz4,

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Xiaobin Xu5, Tao Wang6, Meng Gao7, Yuanhong Zhao1, Yuanhang Zhang8*

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(1) Laboratory for Climate and Ocean-Atmosphere Studies, Department of

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Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing

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100871, China

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(2) Cooperative Institute for Research in Environmental Sciences (CIRES), University

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of Colorado, Boulder, CO, 80309, US

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(3) NOAA Earth System Research Laboratory, Boulder, CO, 80305, US

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(4) Jülich Supercomputing Centre, Forschungszentrum Jülich, 52425, Germany

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(5) State Key Laboratory of Severe Weather & Key Laboratory for Atmospheric

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Chemistry of China

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Meteorological Sciences, Beijing, 100081, China

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(6) Department of Civil and Environmental Engineering, The Hong Kong Polytechnic

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University, Hong Kong, 99907, China

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(7) School of Engineering and Applied Sciences, Harvard University, Cambridge, MA,

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02138, USA

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(8) State Key Joint Laboratory of Environmental Simulation and Pollution Control,

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College of Environmental Sciences and Engineering, Peking University, Beijing

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100871, China

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Correspondence to: Lin Zhang ([email protected]) and Yuanhang Zhang

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([email protected])

Meteorological Administration, Chinese Academy of

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ACS Paragon Plus Environment


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Abstract

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The nationwide extent of surface ozone pollution in China and its comparison to the

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global ozone distribution have not been recognized due to the sparsity of Chinese

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monitoring sites before 2012. Here we address this issue by using the latest 5-year

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(2013-2017) surface ozone measurements from the Chinese monitoring network,

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combined with the recent Tropospheric Ozone Assessment Report (TOAR) database

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for other industrialized regions such as Japan, South Korea, Europe, and the US

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(JKEU). We use various human health and vegetation exposure metrics. We find that

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although the median ozone values are comparable between Chinese and JKEU cities,

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the magnitude and frequency of high ozone events are much larger in China. The

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national warm-season (April-September) 4th highest daily maximum 8-h average

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(4MDA8) ozone (86.0 ppb) and number of days with MDA8 greater than 70 ppb

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(NDGT70, 29.7 days) in China are, respectively, 6.3-30% (range of regional mean

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differences) and 93-575% higher than the JKEU regional averages. Health exposure

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metrics such as warm-season mean MDA8 and annual SOMO35 (Sum of Ozone

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Means Over 35 ppb) are 6.3-16% and 25-95% higher in China. We also find an

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increase of surface ozone over China in 2016-2017 relative to 2013-2014. Our results

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show that on the regional scale human and vegetation exposure to ozone is greater in

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China than other developed regions of the world with comprehensive ozone

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monitoring.

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TOC Graphic

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1. Introduction

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Surface ozone is an air pollutant detrimental to human health and vegetation growth1.

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There is evidence of risks of respiratory and cardiovascular mortality due to

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short-term (acute) exposure to high ambient ozone, and recent studies also reveal

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long-term (chronic) exposure effects even at low ozone levels2-4. Ozone near the

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surface is mainly generated by photochemical oxidation of carbon monoxide (CO)

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and volatile organic compounds (VOCs) in the presence of nitrogen oxides

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(NOx=NO+NO2) and sunlight. Severe ozone pollution in many European and United

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States (US) urban areas has now been extensively alleviated owing to stringent

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emission control measures since the 1990s5-8. In the meantime, South and East Asia

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have experienced rapid urbanization and industrialization, causing significant

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increases in anthropogenic ozone precursor emissions9,10 and potentially shifting the

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worldwide air pollution hotspots to these populous regions11.

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Eastern China experiences severe fine particulate matter (PM2.5) pollution in winter12,

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and this issue has been the main focus of the government’s air pollution control

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strategy over the past 5 years13. More recently, summertime ozone pollution has

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become an emerging concern in China. The past summer of 2017 recorded

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particularly high surface ozone in a large number of Chinese cities, with the 90th

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percentile of the daily maximum 8-h average (MDA8) ozone in 30 of the 74 major

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cities exceeding 200 ug m-3 (the Grade II national air quality standard for protection

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of residential areas)14. Previous studies from field observations and satellite retrievals

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have indicated severe ozone pollution in China, e.g. observed hourly maximum ozone

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concentrations in China frequently exceeded 150 ppb15,16. These values are

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comparable to or higher than the present-day annual 4th highest MDA8 (4MDA8,

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approximately 98th percentile) ozone values in the Los Angeles Basin (about 110

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ppb)8, an area with the most severe ozone pollution in the US. In contrast to the

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general decreasing ozone in the US and Europe5, available surface ozone observations

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have shown significant positive trends in China since 199017-21. Despite the increasing 3



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attention, the severity of ozone pollution in China compared with other industrialized

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regions of the world has not been quantified based on extensive ozone monitoring.

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This study aims to understand the current status of observed surface ozone pollution

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in China from a global perspective by comparison to ozone observations in other

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industrialized regions of the Northern Hemisphere, as revealed by ozone metrics

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relevant to human health and crop/ecosystem productivity. The data come from

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China’s recently available nationwide surface ozone measurements and from the

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Tropospheric

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http://www.igacproject.org/activities/TOAR) organized by the International Global

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Atmospheric Chemistry Project (IGAC)22. We also analyze changes of ozone

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pollution in China over the past 5 years based on the different ozone exposure

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metrics.

Ozone

Assessment

Report

(TOAR,

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2. Materials and Methods

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The China National Environmental Monitoring Center (CNEMC) network.

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Surface air pollutants in mainland China are monitored by the CNEMC of the

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Ministry of Environmental Protection in China (MEPC) and data are reported hourly

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(http://106.37.208.233:20035/). This nationwide observational network, designed for

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monitoring urban and suburban air pollution in mainland China, became operational

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in 2013 at 74 major cities, and by 2017 it includes 1597 non-rural sites covering 454

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cities (Figure S1). These measurements now document the air quality in Chinese

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cities and have been applied in recent studies23-26. In this study we use the hourly

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measurements of ozone, carbon monoxide (CO), nitrogen dioxide (NO2), sulfur

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dioxide (SO2), and fine particulate matter (PM2.5) from 2013 to 2017. Quality controls

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for all species have been applied following previous studies23 to remove data outliers

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(Supporting Information).

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TOAR surface ozone database. We use the TOAR surface ozone dataset (accessed

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from https://doi.org/10.1594/PANGAEA.876108)27 for a global comparison of 4


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ground-level ozone. The TOAR surface ozone database includes ozone statistics and

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metrics relevant to studies on climate, human health and vegetation exposure,

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calculated from hourly ozone measurements at more than 9,000 monitoring sites

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around the world since the 1970s22. Procedures for data harmonization, quality control,

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and metric calculation are described by Schultz et al. (2017)22. Here we analyze the

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latest 5-year (2010-2014) aggregated ozone metrics as well as the long-term

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(1980-2014) yearly ozone metrics from the TOAR database. Recent reports suggest

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that levels of ozone or its precursors in the US and Europe have remained relatively

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stable or slightly decreased after 201428,29. We use the warm-season (April-September)

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metrics, except for SOMO35 (Sum of Ozone Means Over 35 ppb) that is aggregated

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annually. While the TOAR database includes thousands of observational sites in

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Europe, US, Japan, and Korea, it only has data from 32 sites located in China (half are

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in Hong Kong). Our analysis of the CNEMC observations (not included in the TOAR

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surface ozone database) thus fills this gap.

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Ozone metrics for human health and crop/ecosystem exposure. Table 1 lists the 11

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ozone metrics analyzed in this study. These metrics include standard statistics such as

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median, 98th-percentile (Perc98), MDA8, and daytime average (DTAvg), as well as

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metrics for assessing human health and ecosystem exposure impacts30. MDA8 is a

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widely used daily ozone metric for air quality regulation and human health impact

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studies in many regions such as the US, Europe, and China. The response of human

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health to ozone exposure is controlled by ozone level, exposure duration frequency,

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and physical activity4,30. Following TOAR, ozone exposure is evaluated in our study

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by short-term human exposure metrics such as daily maximum 1-hour (MDA1) and

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MDA8, and total number of days with MDA8 >70 ppb (NDGT70), or by metrics that

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consider the cumulative exposure (e.g. annual SOMO35). Risk estimates from

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epidemiological studies based on these exposure metrics are summarized in ref (4)

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and have been recently applied in China31. Ozone damage to vegetation is cumulative

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and can be disproportionally larger for high ozone concentrations30. Here we use the

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AOT40 and W126 metrics which correlate with crop yields32,33 as indicators of 5



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vegetation exposure. We calculated all metrics based on hourly CNEMC

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measurements following the TOAR definitions, procedures, and data completeness

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requirements (Table S1). We also calculated the annual total numbers of days with the

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ozone level exceeding the Chinese Grade II (Exceedance) ozone air quality standard

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(Table 1).

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3. Results and discussion

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Spatiotemporal distribution of ozone air quality across mainland China. Figure 1

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shows the spatial distribution of annual mean MDA8 (annual AVGMDA8) in Chinese

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cities averaged for 2013-2017. MDA8 and MDA1 are also used to determine

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exceedances of the national ozone air quality standard (Table 1) in China. The annual

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AVGMDA8 and AVGMDA1 ozone values averaged for the Chinese sites are 41.2±6.3

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ppb (87.9±13.5 ug m-3) and 48.2±7.2 ppb (103.6±15.4 ug m-3), respectively. During

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2013-2017 there were over 27 days per year that exceeded the Grade II air quality

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standard, averaged over the monitoring sites. Spatially, ozone hotspots (annual

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AVGMDA8 > 50 ppb and Exceedance > 40 days) extend across eastern China

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especially in the North China Plain (NCP) and the Yangtze River Delta (YRD),

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mainly induced by high anthropogenic sources of ozone precursors. Severe ozone

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pollution also occurs in some western cities, e.g., in the central Gansu Province where

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Exceedance II is greater than 80 days, comparable to the eastern regions. This is likely

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due to unique topography (valley basins in mountainous regions) coupled with high

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local ozone precursor emissions from the petrochemical industry and vehicles34.

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The mean MDA8 ozone over China peaks in summer due to strong photochemistry,

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similar to other regions at northern mid-latitudes, but the patterns vary among

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different regions (Figure S2 and S3). MDA8 ozone levels in the YRD and NCP peak

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in May (60 ppb for the regional average) and June (68 ppb), respectively, while the

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averaged MDA8 value in the Pearl River Delta (PRD) is highest in October (57 ppb).

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This difference is due to the timing of the Asian summer monsoon, which brings

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cloudy and rainy weather, marine air, and strong deep convection, all unfavorable for 6


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ozone chemical production and accumulation35, 36. As a consequence, ozone pollution

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over YRD and PRD generally decreases during the summer monsoon, and peaks

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before its arrival or after its retreat.

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Comparison with other northern mid-latitude regions. Figures 2 and S4 compare

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different human health and vegetation exposure metrics for warm-season surface

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ozone in China, to those in other industrialized regions including Japan and South

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Korea (JK), Europe (EU), and the US (hereafter referred as JKEU). We use the

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non-rural sites (Supporting Information) from the TOAR database. We show that

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warm-season Median and mean DTAvg ozone metric values averaged for China

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(30.2±8.1 ppb and 41.6±8.6 ppb, respectively) are comparable to other regions, e.g.,

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Median values of 29.0 ppb over JK, 31.0 ppb over EU, and 33.5 ppb over the US

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(Figure S4). Active local anthropogenic emissions and photochemistry lead to high

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DTAvg ozone in East Asia and California37, 38. High Median and DTAvg values also

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occur at high elevations such as western China, the European Alps, and the western

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US, reflecting the typical increase of ozone with altitude39-42.

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The severity of Chinese surface ozone pollution becomes distinct when comparing

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metrics such as 4MDA8 and Perc98, which focus on the high end of the ozone

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distribution and are likely caused by local pollution episodes. As shown in Figure 2

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and Figure S4, 4MDA8 and Perc98 ozone values averaged over the Chinese sites are

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86.0±14.4 ppb and 80.7±14.1 ppb, respectively, approximately 20-25% higher than

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the averages of European and US sites. High 4MDA8 ozone values above 100 ppb are

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widely observed in the NCP, YRD and PRD (Figure 2). High ozone days also occur

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much more frequently in China, as indicated by the NDGT70 metric (Figure 2). The

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NDGT70 value averages 29.7±22.0 days over the Chinese sites, approximately twice

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the value in JK, a factor of 6 higher than EU, and a factor of 3 higher than the US.

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NDGT70 values above 70 days are common in eastern China, while in other regions

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only a few sites in California reach such a high frequency of exceedance.

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In addition to NDGT70, we compare two other health exposure metrics (AVGMDA8

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in Figure 2 and SOMO35 in Figure S4). The national warm-season mean AVGMDA8

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value in China (50 ppb) is 6.3%-16% higher than those found in JKEU (43.1-47.0

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ppb), while the mean annual SOMO35 value in China (4.3±1.7 ×103 ppb day) is

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25-95% higher than the regional mean SOMO35 values of 2.2-3.4 ×103 ppb day in

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JKEU cities. The AVGMDA8 and SOMO35 values are particularly high in populous

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eastern China (Table S2), e.g. AVGMDA8 > 55 ppb in NCP and SOMO35 > 5.5 ×103

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ppb day in YRD. To our knowledge, no other region of the world, with extensive

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ozone monitoring, has such a large population exposed to such high and frequent

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ozone pollution episodes.

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Comparisons of vegetation exposure metrics also indicate the potential for greater

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ozone-induced plant damage in China. The AOT40 (Figure 2) and W126 (Figure S4)

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metrics in China are 2.2±1.1×104 and 2.9±1.7×104 ppb hour, which are 35-100% and

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50-170% higher than the JKEU regional means, respectively. W126 shows a larger

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difference due to its weighting towards higher ozone levels. High AOT40 (>4×104

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ppb hour) and W126 (>6×104 ppb hour) values are widespread in eastern and central

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China. Previous field surveys at limited locations, and coarse-resolution modeling

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studies (e.g., 2 degrees) estimated that ozone pollution in China could decrease the

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wheat yield by 6.4-14.9% and the annual net primary production by about 14%33, 43.

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These studies focused on ozone pollution before 2010, and thus we may expect even

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larger effects for the more recent conditions recorded by the nationwide network.

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Changes in surface ozone pollution over China in 2013-2017. The 5-year CNEMC

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measurements allow us to examine the interannual variability of surface ozone

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pollution over China. Here we focus on the sites in 74 major Chinese cities (Figure S1)

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where continuous observations from 2013 to 2017 are available.

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Figure 3 shows the evolution of different ozone metrics for the Chinese sites over

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2013-2017 compared with those in Japan, Europe, and the US over 1980-2014. Figure 8


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S5 shows interannual variations of additional ozone metrics averaged over these

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Chinese sites during 2013-2017. We find that all ozone metrics show continuous

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increases over the 5 years in China. Averaged over the 74 cities, the DTAvg, 4MDA8,

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and Perc98 values increased at rates of 3.7-6.2% year-1, while the ozone exposure

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metrics SOMO35, AOT40, and W126 increased at higher rates (11.9-15.3% year-1).

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The increases are statistically significant in 2016-2017 relative to 2013-2014 based on

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analysis of variance tests (Table S4), and are consistent with previous studies of

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long-term ozone increases at a limited number of sites across China17-21. Even though

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the 5-year period is too short to derive statistically robust trend estimates, these results

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indicate an increasing severity of human and crops/ecosystems ozone exposure across

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China. We find the largest increases of ozone exposure in eastern and central China,

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especially in the NCP and YRD, overlapping with the most populous areas (Figure

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S6). From an historical perspective, present-day ozone in the major Chinese cities is

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comparable to or even higher than the 1980 levels in the US when emission controls

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were just beginning to have an impact on reducing ozone (Figure 3).

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The Chinese State Council implemented the Action Plan on Air Pollution Prevention

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and Control in September 2013, resulting in nationwide reductions of ambient SO2,

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CO, NO2, and PM2.5 levels, as shown in Figure S7. The emerging severity of ozone

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pollution in China raises a new challenge to current emission control actions. Previous

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observational and modeling analyses showed that ozone chemical production in the

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NCP, YRD, and PRD regions are likely VOC-sensitive or mixed-sensitive16,44-49.

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Recent bottom-up emission estimates and satellite formaldehyde observations

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indicated increasing VOCs levels in eastern China that can be attributed to

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anthropogenic sources 20,50. For those VOC-sensitive regions, either decreasing NOx

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levels or increasing VOCs levels could potentially enhance ozone pollution. Therefore

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VOC controls could be explored as a possible control strategy. Reduced PM2.5 levels

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over 2013-2017 may also cause an increase of ozone due to impacts on ozone

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photochemistry and heterogeneous chemistry on aerosol surfaces51. In addition, the

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interannual variability of surface ozone can be influenced by meteorological 9



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conditions. The summer of 2017 had hotter and drier weather conditions compared to

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previous years (Figure S8), which favored ozone production and led to higher ozone

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levels. Further studies are needed to better quantify the contributions of emission

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changes and weather variability on recent trends in surface ozone over China.

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We conclude that China has become a global hotspot of present-day surface ozone

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pollution, and human and vegetation exposure in China is greater than in JKEU.

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Although statistics such as Median and mean DTAvg, which focus on the mid-range

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of the ozone distribution, are comparable to JKEU, the magnitude and frequency of

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high ozone events are much greater in China. While high ozone pollution also occurs

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in some other developing regions, such as India in the pre-summer monsoon season

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(April-May)36 and Mexico City52, the lack of accessible ozone observations prevents

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us from comparing those regions to China and JKEU. China’s surface ozone air

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quality deteriorated in 2016-2017 compared to 2013-2014, indicating that current

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emission control measures have not been effective for reducing ozone air pollution.

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Supporting Information

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The CNEMC data quality control (SI 1); station type in the TOAR database (SI 2);

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calculation of the ozone metrics (Table S1), ozone metric values over NCP, YRD, and

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PRD (Table S2); ozone metric values over China, Japan/South Korea, Europe, and the

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US (Table S3)；ANOVA test of year on year ozone increase (Table S4); site locations

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(Figure S1); Annual cycle of MDA8 over China (Figure S2); seasonal and spatial

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distribution of MDA8 over China (Figure S3); additional ozone metrics over China,

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Europe, and the US (Figure S4); interannual variability of ozone metrics (Figure S5

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and S6); interannual variations of national mean NO2, CO, SO2, and PM2.5 (Figure S7);

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surface temperature and relative humidity anomalies in summer 2017 (Figure S8).

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Notes

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The authors declare no competing financial interest.

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Acknowledgements

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This work was funded by the National Key Research and Development Program of

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China

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(2014CB441303), the National Natural Science Foundation of China (41475112), and

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the Clean Air Research Project in China (Grant No. 201509001). We acknowledge the

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Tropospheric Ozone Assessment Report (TOAR) initiative for providing the surface

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ozone data used in this study.

(2017YFC0210102),

China’s

National

Basic

296 297

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Research

Program


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Table 1. Description of ozone metrics used in this study. More details of the

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calculation methods are provided in Table S1. Metrics [unit]

Definition

Aggregation period

50th-percentile of hourly concentrations. 98th-percentile of hourly concentrations. daytime average ozone is the average of hourly ozone concentrations for the 12-h period from 08:00 to 19:59 local time. MDA1 [ppb] daily maximum 1-hour average. AVGMDA1 represents mean MDA1 in the aggregation period. MDA8 [ppb] daily maximum 8-hour average. AVGMDA8 represents mean MDA8 in the aggregation period. 4MDA8 [ppb] 4th highest MDA8. SOMO35 [ppb the sum of positive differences between MDA8 day] and a cut-off concentration of 35 ppb. NDGT70 total number of days with MDA8 > 70 ppb. [day] AOT40 accumulative hourly ozone concentrations above [ppb hour] 40 ppb. Daily 12-h AOT40 is calculated using hourly values for the 12-h period from 08:00 to 19:59 local time, and warm-season total AOT40 is presented. W126 Daily W126 is calculated using the formula: [ppb hour] W126 = ∑ w × C , where C denotes hourly ozone concentration in ppb for the 12-h period from 08:00 to 19:59 local time., w is the weighting index defined as Median [ppb] Perc98 [ppb] DTAvg [ppb]

w = ∙(∙

April-September April-September April-September

annual and April-September annual and April-September April-September annual; April-September April-September

April-September

, M=4403, A = 126.

/ )

Exceedance [day]

Number of days with ozone concentration annual exceeding the Chinese Grade II national air quality standard, defined as MDA8 >160 µg m-3 or MDA1 > 200 µg m-3 over residential, industrial, and rural areas.

300 301

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302 303 304 305 306

Figure 1. Distribution of annual mean MDA8 (left) and ozone air quality exceedance (right) averaged over 2013-2017. Only sites with at least 3-year measurements are included. Mean values and standard deviations over the sites are shown inset.

13



307 308 309 310 311 312 313 314 315 316

Figure 2. Comparison of April-September ozone metrics (4MDA8, NDGT70, AVGMDA8, AOT40) between China (including CNEMC sites and TOAR Chinese sites) and other industrialized regions (Japan and Korea (JK), Europe (EU), and the US. Only sites with at least 3-year measurements are included. Values inset are regional mean and standard deviation averaged over the N sites. The differences between China and other industrialized regions are statistically significant (p

Severe Surface Ozone Pollution in China: A Global Perspective

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