**1. Introduction**

Air pollution has been considered as the most serious environmental problem for human health, associated with some million deaths worldwide per year. Among them, SOx was eliminated by desulfurization equipment in factories, however, nitrogen oxides (NOx = NO+NO2) have not well regulated. As NO2 is one of the precursors of O3, emission of NO2 in Asia is estimated to be about four times higher than that of Europa and the U.S.A. [1–5]. Carbon dioxide (CO2) increases global warming and increasing tropospheric O3 are important global environmental issues today [6]. Cities have to cope with the challenges derived from poor air quality impacting the wellbeing of human health and citizen.

China has implemented various policies and measures to control air pollution and promote urban greening as well as forest rehabilitation since 1999 [7]. In 2004, China officially launched the construction of national forest cities, and gradually promoted and organized the construction of forest cities on a large scale across the country as proposed by the Chinese leader, Mr. JP Xi (2016). In 2019, forest city construction was included in Chinese Law as a legal guarantee for land greening and its protection. Urban greens, such as avenue trees and park trees, provide comfortable conditions for city residents, and a moderate environment in megacities in China [8] and in Japan [9, 10], and in small cities in Europe [11]. To efficiently reduce O3 in cities, it is important to define suitable urban forest management, including proper species selection, with a focus on the reduction ability of O3 via flux data [12, 13], and other air pollutants, such as PM [14–17], biogenic emission rates, allergenic effects [18] and maintenance requirements.

An epidemiological investigation is the most objective and practical method to determine the degree of O3 damage by investigating the typical characteristics of O3 damage to plants in the natural environment [19, 20]. The highest O3 concentrations primarily occurred in July and August in northern China and the central part of Japan [21–23], and in September or April and May in southern China [23–26], in April–June in northern Japan [21]. Ozone causes cellular damage in plants, reducing stomatal control, lower CO2 assimilation rates, and the occurrence of visible leaf injury [9, 27, 28]. These effects often accelerate senescence, diminish green leaf area and biomass, allocation of photosynthates to roots [29] and reduce photosynthetic capacity [28, 30, 31]. The investigation of visible injury of O3 to the urban forest can not only judge whether the current concentration of O3 in the air has reached the level harmful to the urban plants (e.g., [32]), but also determine the situation of urban exposure to O3 pollution, and provide objective evidence for plant sensitivity evaluation [29].

Plants of green-roof are exposed to severe environments; heat, drought, PM, airpollutions, etc. even with intensive management of the people [11, 33, 34]. There are many factors reducing the vigor and health of trees in avenue, parks, and suburban forests, etc. With the aggravation of O3 pollution in urban air, the number of injured plant species in urban vegetation increased significantly, and the vitality of urban green resources was affected. Study shows that most plant functional types suffer a substantial decline in LAI as O3 level increases [35]. Studies on the effects of O3 on plants commonly used the following methods: manipulative exposure experiment: O3- FACE) and epidemiological investigation (e.g., [9]). The effects of single or mixed pollutants on plant functional traits were studied through controlled experiments, such as OTC or artificial fumigation, to judge the resistance/sensitivity of plants to single pollutants [36] and multiple stresses.

We should know more about the negative effects of elevated O3 on environmental conditions for plants as well as residents in big cities and surrounding areas through supporting service of ecological services; that is, plant primary productivities and nutrient cycling [37]. Moreover, we should consider biodiversity conservation, including plant-insect interactions for our future resources (e.g., pollination) as affected by elevated O3 (e.g., [38–43]).

For accessing the methods of improving environmental conditions in cities and the vicinities, we state the role of urban green, declining symptoms, ecosystem vigor and health of big cities, especially under increasing ground-level O3 and its dynamics. We showed the effects of environmental changes derived from elevated O3 and other environmental factors on the heath and vigor of green infrastructure. Based on this evidence, we discussed a plausible understanding of the construction and

maintenance of urban greening. Moreover, to attain the international Sustainable Development Goals (SDGs), the interactions in urban and suburban greens should be better understood to maintain the green infrastructure.
