1 Introduction

Air pollution is a serious ecological issue in China’s economically well-developed areas such as Yangtze River Delta, Beijing-Tianjin-Hebei, and Pearl River Delta regions (Wang et al. 2017). Air pollution in these areas not only deteriorates regional and urban air quality but also severely affects health of people and ecosystems (Zhang et al. 2016). Among six air pollutants (e.g., SO2, PM2.5, PM10, NO2, CO, and O3) officially tracked by Chinese air quality monitoring platform (http://www.aqistudy.cn/), ground-level O3 is the most phytotoxic air because it causes severe plant damage and its high regional localized concentration (Feng et al. 2014, 2015a). Although in some areas O3 level was decreased, especially in the USA and Europe (Cooper et al. 2015), ozone monitoring in China showed an increasing trend since the 1990s (Xu et al. 2008, 2016a; Wang et al. 2009), which became especially alarming in the last 5 years (2013–2016) (Lu et al. 2018; Zeng et al. 2019). Ozone pollution observed during summers in China is especially concerned. Particularly high O3 levels were observed in multiple cities during summer 2017: 90th percentile of a daily maximum 8-h average (MDA8) O3 level in 30 out of 74 major cities exceeded 200 μg/m3. Air with such high O3 concentrations corresponds to grade II according to the national air quality standards for residential areas (Lu et al. 2018). Ground-level O3 in China strongly correlated with emissions of NOx, which is a main contributor to O3 formation (Wang et al. 2017). Literature results demonstrate 10% yield decrease for major food crops (such as potato, rice, wheat, soybeans, etc.) at O3 concentrations in air equal to ~40 ppb in comparison with crop yields grown in O3-free air (Feng and Kobayashi 2009). Survey in Beijing and its surroundings revealed 28 plant species or cultivars with typical O3 symptoms (Feng et al. 2014). Thus, food security in China is being threatened or has already been severely affected by current high ground-level O3 concentration. Without special measures and precautions, O3 levels will continue to rise.

This chapter describes mechanism of ground-level O3 formation and summarizes recent development based on the analysis and predictions of spatial-temporal distribution patterns of NOx emission in China. It also discusses how ground-level O3 affects the well-being of plants. Recommendations on adverse impact of ground-level O3 pollution as well as way for its reduction and prevention are also illustrated.

2 Mechanism of O3 Formation

Ozone forms in the atmosphere as a result of visible light-assisted reaction of various nitrogen oxides (formed in a NOx ↔ NO2 + NO reaction) with volatile organic compounds (VOCs), methane (CH4) and carbon monoxide (CO). Specifically, ozone formation is a combination reaction between molecular oxygen (O2) and atomic oxygen (O(3P)) (see Eq. 7.1). Atomic oxygen is obtained by O2 decomposition upon its exposure to short-wavelength ultraviolet (UV) radiation with wavelengths below 240 nm. These reactions are responsible for producing and maintaining protective O3 layer (Chapman 1930).

Troposphere does not have so much UV as other atmospheric layers; thus, NO2 photolytic reaction occurring at wavelengths below 424 nm (as shown in Eq. 7.2) is a primary source of atomic O(3P), which becomes the major O3-producing reaction. In normal and well-balanced environment, freshly formed O3 immediately reacts with NO to regenerate NO2 (see Eq. 7.3). Thus, the whole reaction cycle shown by Eqs. 7.1, 7.2, and 7.3 yields no by- and/or final products when no other competing chemical species are involved:

$$ \mathrm{O}\left({}^3\mathrm{P}\right)+{\mathrm{O}}_2+\mathrm{M}\to {\mathrm{O}}_3+\mathrm{M}, $$
(7.1)
$$ {\mathrm{NO}}_2+\mathrm{hv}\to \mathrm{NO}+\mathrm{O}\Big({}^3\mathrm{P}\Big), $$
(7.2)
$$ {\mathrm{O}}_3+\mathrm{NO}\to {\mathrm{NO}}_2+{\mathrm{O}}_2. $$
(7.3)

Nevertheless, molecules like HO2 and RO2 with very high oxidative activity are also present in the troposphere. These radicals can effectively transform NO to NO2 (see Eqs. 7.4 and 7.5). Without enough NO present in the troposphere, not all freshly formed O3 can react to decompose to O2. Thus, ozone starts to accumulate in the troposphere. Reactions shown in Eqs. 7.2, 7.4, and 7.5 demonstrate “NOx cycle,” which produces O3 without NOx consumption (see Fig. 7.1).

Fig. 7.1
figure 1

Schematic of light-induced mechanism of O3 formation as well as chemical and material balance relationship between ROx- and NOx-based cycles. Reactions and balances marked with red indicate NOx cycle, green corresponds to ROx cycle, blue shows radical initiation reactions, and black demonstrates termination processes. (This figure was adapted from Wang et al. (2017) with permission by Elsevier)

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Another significant chemical cycle affecting O3 formation is “ROx (ROx = OH + HO2 + RO2) radical cycle.” It constantly supplies HO2 and RO2 radicals that easily oxidize NO to NO2. This cycle typically begins with OH-induced degradation of VOCs (see Eq. 7.6), which generates RO2 radicals followed by their conversion to RO (see Eq. 7.5). Just formed RO easily reacts with O2 yielding HO2 (see Eq. 7.7), which then reacts with NO producing OH and NO2 (see Eq. 7.4). Each ROx cycle consumes two NO molecules (and transforms them into NO2), forms two O3 molecules through a typical “NOx cycle” (shown in Eqs. 7.1, 7.2, and 7.3), and recycles NO (from the NOx cycle). Figure 7.1 demonstrates a schematic reflecting these two cycles (one chemical and one photochemical) occurring simultaneously.

$$ {\mathrm{HO}}_2+\mathrm{NO}\to \mathrm{OH}+{\mathrm{NO}}_2, $$
(7.4)
$$ {\mathrm{RO}}_2+\mathrm{NO}\to \mathrm{RO}+{\mathrm{NO}}_2, $$
(7.5)
$$ \mathrm{OH}+\mathrm{RH}+{\mathrm{O}}_2\to {\mathrm{RO}}_2+{\mathrm{H}}_2\mathrm{O}, $$
(7.6)
$$ \mathrm{RO}+{\mathrm{O}}_2\to {\mathrm{HO}}_2+\mathrm{carbonyls}. $$
(7.7)

Both NOx and ROx cycles terminate by their corresponding cross-reaction of ROx and/or NOx. At high NOx concentration, termination by reactions with OH (Eq. 7.8) and RO2 (Eq. 7.9) dominates. The products of these two reactions are organic nitrates (NOz species) and nitric acid. When NOx concentration is low, the main termination reactions are either recombination of hydroperoxyl radicals (see Eq. 7.10) or recombination of RO2 and HO2 radicals (Eq. 7.11), which yield hydrogen and organic peroxides. Thus, one can use NOz concentration as well as H2O2/HNO3 ratio to evaluate atmospheric conditions in terms of NOx concentration (whether it is low or high). These ratios are also used as indicators showing which cycle formation of O3 follows: VOC- or NOx-based.

$$ \mathrm{OH}+{\mathrm{NO}}_2\to {\mathrm{HNO}}_3, $$
(7.8)
$$ {\mathrm{RO}}_2+{\mathrm{NO}}_2\leftrightarrow {\mathrm{RO}}_2{\mathrm{NO}}_2, $$
(7.9)
$$ {\mathrm{H}\mathrm{O}}_2+{\mathrm{H}\mathrm{O}}_2\to {\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{O}}_2, $$
(7.10)
$$ {\mathrm{HO}}_2+{\mathrm{RO}}_2\to {\mathrm{RO}}_2\mathrm{H}+{\mathrm{O}}_2. $$
(7.11)

ROx can also form from the closed-shell molecules. They participate in a standard ROx cycle mentioned above and, thus, also play a substantial role in O3 generation. In contaminated troposphere, ROx radicals form as a result of O3, HONO, and photolysis as well as from the O3-induced cleavage of unsaturated VOCs. Contribution of radicals from different sources varies depending on geographical location (Xue et al. 2016). Relatively recent, new origins of atmospheric radicals as well as their precursors were discovered including previously unknown daytime and nighttime sources of HONO (Kleffmann 2007) and nitryl chloride (ClNO2), respectively. Cl atom released from reaction formed in the dark ClNO2 further reacts with VOCs. Products of this reaction benefit photochemical formation of O3 by a gas-phase mechanism similar to a mechanism involving OH (e.g., Riedel et al. 2014):

$$ \mathrm{RH}+\mathrm{Cl}+{\mathrm{O}}_2\to {\mathrm{RO}}_2+\mathrm{HCl}. $$
(7.12)

All O3 production routes have one common feature: ozone formation demonstrates a nonlinear relationship with concentration of its precursors (VOCs and/or NOx). At low concentrations of NOx/VOCs, NOx cycle generates less O3 produced based on ROx cycle. Thus, NOx/VOC concentration becomes a limiting factor of O3 generation. This situation is known as “NOx-limited O3 formation regime.” In contrast, at high NOx/VOC concentration, the limiting factors of O3 production are intensity, speed, and rate of the ROx-based cycle. This situation is often referred to as “VOC-limited.” However, O3 production through these cycles still remains somewhat complex as it is also influenced by VOC reactivity, NOx/VOC ratios, photochemical aging, meteorological conditions, and biogenic emissions. Dominancy of NOx- or ROx-based cycles is also determined by the sources of reactive organic radicals (e.g., produced by photolysis of O3, HCHO, etc.) as well as by traps of hydrogen radicals (see Eqs. 7.13, 7.14, 7.15, 7.16, and 7.17) (Sillman 1999):

  • Sources:

$$ {\mathrm{O}}_3+\mathrm{hv}\overset{{\mathrm{H}}_2\mathrm{O}\ }{\to }2\mathrm{O}\mathrm{H}, $$
(7.13)
$$ \mathrm{HCHO}+\mathrm{hv}\overset{{\mathrm{O}}_2\ }{\to }{\mathrm{HO}}_2+\mathrm{CO}. $$
(7.14)

  • Sinks:

$$ {\mathrm{H}\mathrm{O}}_2+{\mathrm{H}\mathrm{O}}_2\to {\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{O}}_2, $$
(7.15)
$$ {\mathrm{RO}}_2+{\mathrm{HO}}_2\to \mathrm{ROOH}+{\mathrm{O}}_2, $$
(7.16)
$$ \mathrm{OH}+{\mathrm{NO}}_2\to {\mathrm{HNO}}_3. $$
(7.17)

Many urban areas are either VOC-sensitive or NOx-saturated. Low levels of HOx radicals removed by the reaction products between NO2 and OH are responsible for decreased O3 concentration in urban areas. Additionally, ground-level O3 concentration can be reduced because of so-called “NOx titration”: in areas with substantial NO concentrations in the atmosphere (e.g. from car exhausts), O3 reacts with NO.

Several recent studies (Tang et al. 2012; Tie et al. 2013; Xue et al. 2014) explored O3 formation pathways in China. It is a very important step toward the development of scientific approaches toward regulation and control of ground-level O3 levels. Thus, it was reported that VOC-based formation of O3 dominates in the Pearl River region and in Central Eastern China. At the same time, in the summer, O3 formation in areas with plain and mountain landscapes located in Northern China was sensitive to both VOC and NOx, respectively (Tang et al. 2012). In cities and surrounding areas of Yangtze River Delta and North China Plain as well as PRD regions, O3 formation is VOC-limited (Tie et al. 2013; Xue et al. 2014). In other words, NOx present in Chinese cities induces “titration effect” mentioned above. Unfortunately, this kind of scientific research information is still limited, which slows down full and detailed understanding of processes of ground-level O3 formation. Research of O3-VOC-NOx chemistry in atmosphere over China requires further improvement, which can be achieved by thorough data collection, experimental observations, and theoretical simulations, all of which are crucial for development of effective industrial and governmental policies for ground-level O3 pollution control.

3 Spatial-Temporal Pattern of NOx Emission Over China

Nitrogen dioxide (NO2) and nitrogen oxide (NO), often combined under a general name nitrogen oxides (NOx), are present in the atmosphere as gas in trace amounts and have short life period. Yet, they are very active participants in O3 formation (Seinfeld and Pandis 2006). Anthropogenic activities, especially usage and consumption of fossil fuel, are the major contributors to NOx in the atmosphere especially at high concentrations of NOx near or around densely populated urban and suburban/rural industrial areas as well as around power plants.

China is one of the major contributors to atmospheric NOx pollution in the world as it contributes to ~18% of the global NOx emissions (EDGAR 4.2, EC-JRC/PBL 2011). China’s NOx release increased 1.5 times from 1980 to 1995 as Chinese GDP was increasing by 10% annually during the same time frame (Klimont et al. 2009). Because of Asian economy crisis, NOx emissions started to decline in 1996 and continued to decrease till 2000. Utilization of coal relative to the total energy consumption decreased from 77% to 71%, which was reflected in NOx emission reduction during the 1996–2000 period (Hao et al. 2002; NBS 2010). After the economy started to recover in 2000, NOx emission quickly reached pre-crisis level and even doubled it (Fig. 7.2) as utilization of coal increased twofold in the 2001–2008 time period (NBS 2010). Between 1995 and 2005, NOx emissions increased by 6.3% annually (Zhang et al. 2007). Nevertheless, since strict pollution-control preventive and technical measures were implemented in 2011, fast growth of NOx concentration in the atmosphere slowed down and eventually reversed (see Fig. 2.1 in Chap. 2 and Fig. 5.6 in Chap. 5).

Fig. 7.2
figure 2

NOx emission trend during 1980–2010 period. (This figure was adapted from Gu et al. (2012) with permission by American Chemical Society)

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Chinese government went even further and implemented new regulations to decrease emissions and to improve air quality. The goal was to cut national total NOx emissions by 10% in 2015 relative to emissions in 2011. Several studies showed that OMI NO2 levels increased from 2005 to 2011 followed by a slight decrease (Krotkov et al. 2016; Duncan et al. 2016). Reduction of NOx emissions originating from China after 2011–2012 was confirmed based on satellite data by various researches (e.g., Miyazaki et al. 2017; Souri et al. 2017; Van der A et al. 2017). Latest estimations showed that NOx emission decreased by ~20% during the 2010–2017 period (see Fig. 2.1 in Chap. 2). However, taking a 10-year period covering years from 2005 to 2014, an increasing trend was demonstrated in the amount of NOx emissions in China despite significant year-to-year variations. According to Miyazaki et al. (2017), linear fitting of the NOx emission data over the 10-year period revealed a slope equivalent to 26% increase (see Fig. 7.3).

Fig. 7.3
figure 3

Annual NOx emissions detected in the atmosphere during 2005–2014 monitoring period for India (yellow curve), China (dark blue curve), Europe (cyan curve), Western Europe (cyan dashed line), Southern Africa (red), and the USA (green). Emission values are reported relative to emissions in each country in 2005. (This figure was adapted from Miyazaki et al. (2017) under the Creative Commons Attribution License)

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Especially substantial NOx emissions were detected in large cities such as Wuhan, Nanjing, Tianjin, and Chengdu, in which ground-level concentrations of NOx increased by 42%, 35%, 35%, and 56% per decade, respectively. An overall increase in Eastern China was also detected (see Fig. 7.4). A significant increase in NOx concentration occurred in Western China, especially its Northern regions (~88 to 110°E, 37–48oN): NOx concentration in air in these regions increased by 50–110% per decade. In spite of a general large and positive trend, the three largest Chinese cities (Beijing, Shanghai, and Guangzhou) demonstrated either small reduction or a small 10-year increase in NOx emissions during 2005–2014: −0.6%, −6.2%, and 4.5%, respectively. The same trends were observed specifically for NO2 emissions (Wang et al. 2015).

Fig. 7.4
figure 4

NOx emissions (in 10−11 kg m−2 s−1 per decade) during 2005–2014 period in different countries and regions of Asia (upper left panel). (This figure was adapted from Miyazaki et al. (2017) under the Creative Commons Attribution License)

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NOx emissions also demonstrate spatial variability in China (see Fig. 7.5a). In 2004, regions with highest NOx emissions were mostly located on the East Coast: from Southern Yangtze Delta region around Shanghai to North of Beijing (Zhang et al. 2007). Pearl River Delta (PRD) region of Hong Kong and Guangzhou as well as Sichuan Basin also exhibited high NOx emissions. Regional distribution of NOx emissions shown in Fig. 7.5 corresponds very well to the tropospheric NO2 columns derived from SCIAMACHY measurements in 2004 (Fig. 7.5b): columns with high NO2 concentration were spread across East China and PRD regions in a similar way as spatial distribution of NOx shown in Fig. 7.5a.

Fig. 7.5
figure 5

Spatial distribution of (a) NOx emission and (b) SCIAMACHY tropospheric NO2 vertical columns in 2004. (This figure was adapted from Zhang et al. (2007) with permission by John Wiley and Sons)

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Between 2007 and 2015, provinces with the highest NOx emission in China were Anhui, Shandong, Henan, Hebei, Jiangsu, Guangdong, Zhejiang, Shanxi, Sichuan, and Hubei. Combined, emissions from these provinces accounted for 65% of all NOx emissions in China (van der et al. 2017).

4 Spatial-Temporal Pattern of Ground-Level O3 in China

Fast industrial and urban developments in China led to higher amounts of O3 precursors (mainly NOx and VOCs) participating and assisting O3 formation, which is the reason for high-increasing rate in ground-level O3 concentrations in China (Wang and Mauzerall 2004; Feng et al. 2015a).

Ground-level O3 concentration in China as well as their variations (both spatial and temporal) was thoroughly analyzed. Li et al. (2017a) collected data on ground-level O3 levels at 187 cities from January 2015 to November 2016. Publication and official release of this data revealed significant spatial variation of O3 in China. Averaged O3 concentrations ranged from 50.6 ppb (in Nanchong) to 64.1 ppb (in Yixing, which is an industrial city in the Yangtze River Delta). High O3 levels were also detected in major Chinese metropolitan areas located in Jing-Jin-Ji and in deltas of the Yangtze and Pearl Rivers (see Fig. 7.6). Rapid growth and development of different industries, including transportation and urban branches, in these cities as well as around them is the major driving force responsible for the O3 pollution.

Fig. 7.6
figure 6

Variation of ground-level O3 during 2014–2016 period in China. (This figure was adapted from Li et al. (2017a) with permission by Elsevier)

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Average O3 concentration over China increased from 46.1 ± 8.8 ppb in 2014 to 51.9 ± 7.8 ppb in 2016 (see Fig. 7.6). Significant O3 level increase was observed in the Yangtze River Delta, North China Plain, and Inner Mongolian and even Southeastern Tibetan Plateau. However, levels of NO2 demonstrated a slight decreasing trend (Li et al. 2017a). We already mentioned above that O3 increase in different regions is caused by different factors. Three major factors can be described as follows. (1) Industrialized areas demonstrate increased ground-level O3 concentration because of high VOC emissions (Yuan et al. 2013), which, in turn, leads to more rapid formation of O3. (2) In North China Plain, lowered NOx emission inhibits titration reaction between NO and O3, which also results in high O3 concentrations (Xu et al. 2016b). (3) High O3 levels in Tibet Plateau can be explained by strong stratosphere-troposphere exchange processes because of the narrow troposphere layer in Tibetan Plateau (Skerlak et al. 2014).

Several studies predicted future changes in ground-level O3 concentrations in China. Zhu and Liao (2016) used high-resolution nested grid version of the GEOS-Chem model to simulate changes in ground-level O3 concentrations for the 2000–2050 period in response to the changes in anthropogenic emissions under the RCP2.6, RCP4.5, RCP6.0, and RCP8.5. Annual ground-level O3 concentration predicted from the year 2010 to 2050 for every decade relative to 2000 values under the four RCPs are shown in Fig. 7.7. Under these four RCPs, predicted changes in annual mean ground-level O3 levels showed different trends. RCP8.5 predicted the worst scenario for 2020–2030, and RCP6.0 showed the worst situation over 2040–2050.

Fig. 7.7
figure 7

Predicted (for years 2010–2050) average annual values of ground-level O3 concentrations (ppb) shown relative to the values of 2000 year under the four RCPs. (This figure was adapted from Zhu and Liao (2016) with permission by Elsevier)

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Typically, O3 levels are also significantly higher in the summer than in the winter (Fig. 7.8). Formation rate of O3 depends on solar radiation intensity, and thus shorter light days and insufficient amount of sunlight inhibit O3 formation in winter (Li et al. 2017a). During spring and summer, higher temperatures and stronger solar radiation helps to generate many OH radicals, which react with VOC and further enhance rate of O3 formation. Furthermore, stratosphere-troposphere exchange processes in springs and summers also lead to increased O3 levels (Ou et al. 2015).

Fig. 7.8
figure 8

Seasonal variation of ground-level O3 in China. (This figure was adapted from Li et al. (2017a) with permission by Elsevier)

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5 Impact of Ground-Level O3 on Plants

High O3 concentrations cause severe plant damage through a three-step process: exposure, uptake, and bio-effect. Ozone mainly enters plants through leaf stomata, and the entry of O3 could be controlled by stomata. After O3 molecular enters a substomatal cavity, it quickly binds with molecules of the adjacent cell walls or of the outer cell membrane. The intercellular O3 concertation is almost zero because of very quick chemical reactions occurring in that part of a plant. Nevertheless, the reactive oxygen species (ROS) or biomolecule oxidation products formed as a result of O3 interaction with cellular redox systems originate or support reactions in physiological tissues (Tausz et al. 2007). Schematics of O3 effects on plants are shown in Fig. 7.9 (Renaut et al. 2009). Ground-level O3 caused visible leaf injury, which also reduced photosynthesis and inhibited growth and yield. High O3 level also changes how plants deal with diseases and pests (Krupa et al. 2000).

Fig. 7.9
figure 9

Schematics of main physiological processes for the effects of O3 on plant leaves. Exposure to high O3 levels leads to leaves chlorosis and necrosis. Ozone molecules diffuse through the stomata inside the leaves. Ozone enters mainly through the leaves’ stomata. The major O3 detoxification process incudes ascorbate, glutathione, and SOD. Ozone activates signaling processes based on ethylene, salicylic acid, and jasmonic acids. Different pathways initiate different responses: jasmonic acid pathway inhibits routes based on ethylene and salicylic acid. Photosynthesis rate slows down at both photochemical and biochemical levels during exposure. At the same time, respiration is enhanced including anaplerotic pathway involving PEPc. The following abbreviations were used: AA, SA, and JA stand for ascorbic, salicylic, and jasmonic acids; AAperox indicates ascorbic acid peroxidase; DHAA means dehydroascorbic acid; ET stands for ethylene; GR represents glutathione reductase; GSH and GSSG indicate reduced and oxidized glutathione, respectively; PCD stands for programmed cell death; PEPc stands for phosphoenolpyruvate carboxylase; ROS stands for reactive oxygen species; RuBisCO stands for ribulose-1,5-bisphosphate carboxylase/oxygenase; SOD stands for superoxide dismutase. (This figure was adapted from Renaut et al. (2009) with permission by Elsevier)

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High O3 levels could affect plant leaves, and the visible O3 symptoms of plant leaves could be used to visually quickly assess and predict O3 pollution in the field conditions (Hayes et al. 2007). Feng et al. (2014) examined clearly visible plant injuries in July and August of 2013 in parks, forests, and agricultural regions of Beijing, China. The visible injury was detected in many areas and in 28 different plant species or cultivars. Ozone symptoms were observed more frequently in rural and mountain areas of Northern Beijing, which are located downwind from the city. Less leaves damage was observed in city gardens. Injuries to crop-bearing plants were detected most frequently for genera Phaseolus, Canavalia, and Vigna beans and still observed but less often for watermelon, grapevines, and gourds. Several native trees, such as pines, ailanthus, and ash trees, also showed symptoms. Rose of Sharon, black locust, and Japanese morning glory were among the injured ornamental plants. Examples of O3 symptoms are shown in Fig. 7.10 for some of the plants mentioned above (Feng et al. 2014).

Fig. 7.10
figure 10

Ozone symptoms in plants native to Beijing area of China: Ailanthus altissima (1), Ampelopsis humulifolia (2), Fraxinus rhynchophylla (3), Pinus bungeana (4). Ornamental plants: Robinia pseudoacacia (5) and Hibiscus syriacus (6). Crops: Canavalia gladiate (7), Vigna unguiculata var. heterophylla (8), Benincasa pruriens (9), Luffa cylindrica (10), Citrullus lanatus (11), and Vitis vinifera (12). (This figure was adapted from Feng et al. (2014) with permission by Elsevier)

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Current O3 levels in China are high enough to cause chronic changes in trees such as reduced photosynthesis, decreased productivity, and accelerated leaf senescence. Such severe damage should not be unnoticed because forests and trees are the most important carbon sinks (Wittig et al. 2009).

China is one of the world’s most forest-deficient countries: forests cover only 22% of the total land area, which is significantly less than a 30% global average (Fang et al. 2014). Capacity and strength of carbon sink by boreal forests at midlatitudes of the northern hemisphere are significantly reduced because of ground-level O3 pollution. Li et al. (2017b) quantitatively analyzed data from Chinese studies on temperate and subtropical regions to determine how elevated O3 concentrations affect vegetation growth, biomass, and functional leaf traits of different types of woody plants. Their results indicated that if averaged O3 level is equal to 116 ppb, total biomass of woody plants would decrease by 14% comparing with O3 levels equal to ~21 ppb. Effect of elevated O3 concentration on biomass, growth, leaf characteristics, photosynthetic pigments, gas exchange, chlorophyll fluorescence, and antioxidant parameters for all kinds of trees in China are shown in Fig. 7.11 (Li et al. 2017b).

Fig. 7.11
figure 11

Effect of elevated O3 concentration (eO3) on (a) biomass, growth and leaf characteristics, (b) photosynthetic pigments, gas exchange, chlorophyll fluorescence, and (c) antioxidant parameters of all trees. Data symbols represent percentage of the average value obtained at elevated O3 concentration relative to the average value obtained at control O3 concentration. Error bars reflect 95% bootstrapped confidence intervals. Number of measurements and papers are shown in parentheses, whereas mean elevated O3 concentration and control O3 concentration in square brackets are given along the y-axis. The following abbreviations were used to represent parameters studied: φPSII effective quantum yield of photosystem II, APX ascorbate peroxidase, AsA ascorbic acid, Car carotenoid, CAT catalase, DHAR dehydroascorbate reductase, GR glutathione reductase, LMA leaf mass per area, MDA malondialdehyde, MDAR monoascorbate reductase, SOD superoxide dismutase activity, TAC total antioxidant capacity, WUE water-use efficiency. (This figure was adapted from Li et al. (2017b) with permission by John Wiley and Sons)

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In fact, high O3 level is one of the most damaging pollutants that severely damage economic and biological aspects of crop-bearing plants (Feng et al. 2011). Current ground-level O3 concentration significantly decreased wheat growth as well as its quality and yield according to the meta-analysis, which are shown in Fig. 7.12 (Pleijel et al. 2018). Feng et al. (2015b) reviewed data on how elevated O3 levels affect food crops in China and also demonstrated significant reduction in wheat yield. Based on exposure concentration and stomatal O3 flux-response relationships obtained by O3-FACE experimental data, they predicted that if O3 level continues to increase, wheat crops will produce less yield by 6.4%–14.9% and 14.8%–23.0%, respectively. Thus, if O3 level continues rising, food security in China might be severely compromised. Without certain regulations, precautions, and measures, ground-level O3 levels will continue to rise and to damage forests and crops.

Fig. 7.12
figure 12

Meta-analysis of the effect of non-filtered air (NF) vs. charcoal-filtered air (CF) for agronomically important wheat variables. Values in brackets indicate number of NF-CF comparisons for each variable. Error bars are 95% confidence intervals. (This figure was adapted from Pleijel et al. (2018) with permission by Elsevier)

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6 Mitigation Measures of Ozone Pollution

Ozone causes so much damage to vegetation, and finding a solution on how to control O3 pollution is a worldwide concern. Yet, there is no a single solution to this issue. Below are several policy recommendations and mitigation measures, which should help to decrease O3 impacts and to mitigate risks associated with high O3 levels:

  1. 1.

    Strict control of emissions containing potential agents participating in O3 formation cycle

Strict laws on O3-related precursor (such as VOCs and NOx) emissions are required. Currently, majority of these chemicals are from automotive exhausts. One solution is to impose a Euro standard V for all vehicle exhaust gases throughout China by no later than 2020. Another solution is to decrease personal use of cars and to encourage usage of public transportation in major and overpopulated areas. One way to regulate this is to increase gasoline price and parking fees (Feng et al. 2015a, b).

  1. 2.

    Planting O3-resistant plants

Different plants have different sensitivity under high O3 levels. Thus, O3-resistant genotypes can be selected, reproduced, and/or genetically modified by adding O3-resistant genes. Such genes were already discovered for rice and soybean (Frei et al. 2010; Gillespie et al. 2011). Additionally, there are a large number of studies on the sensitivity of plants to O3. For example, Feng et al. (2018) demonstrated that a lot of O3 sensitivity variation for different woody plants can be explained by interspecific variation in LMA. Thus, ozone-tolerant plants can be chosen based on these rules and planted in certain O3-rich areas to maintain their vegetation and to reduce the damage under high O3 levels.

  1. 3.

    Ozone uptake through plants

Typically, ozone enters a plant through its leaf stomata, after which plants absorb O3 and reduced O3 content in the ambient air. This ability of plants to purify air is an important property that can be used at urban, ecological, and environmental planning and landscaping. However, plant-derived BVOCs can also participate in O3 formation. Thus, high O3 uptake and low-BVOC emission plants should be considered to efficiently control O3 levels.

  1. 4.

    Application of chemical protective agents at specific phenological stages

Ozone-caused damage to plants could be decreased or prevented by application of antioxidants (e.g., glutathione, ascorbic acid, antiozonant ethylenediurea (EDU), etc.) (Feng et al. 2010; Manning et al. 2011). Such chemical agents are indeed widely used in the USA and many European countries to protect crops against high ambient O3. However, the toxicity of EDU in the food chain is yet to be extensively tested, and the phytotoxicity can happen at high doses of EDU (Manning et al. 2011). Thus, most effective approach is to apply such agents at specific phenological stages, such as grain filling in both soybeans and wheat, during rice tillering, etc. It is necessary to apply these chemical protective agents to protect crops from production in high O3 concentration areas. However, further testing of EDU toxicity and its impact on different crops are required through the field experiment.