1. Introduction
Since the implementation of the Clean Air Action Plan in China, the quality of ambient air has steadily improved. However, in recent years, the issue of ozone (O
3) pollution has become increasingly prominent, with O
3 concentrations showing a generally upward fluctuating trend [
1-
4]. In 2021, 2022, and 2023, the 90th percentiles of the daily maximum 8 h average O
3 concentrations in the 2 + 26 cities of the Beijing-Tianjin-Hebei region and surrounding areas were 171, 179, and 181 µg·m
−3, respectively. The recently issued Action Plan for Continuous Improvement of Air Quality in 2023 has expanded the 2 + 26 cities to 2 + 36. In 2024, the average O
3 concentration in the 2 + 36 cities was 179 µg·m
−3, which is a 0.6% increase from the previous year. These O
3 concentrations exceed the secondary grade (160 µg·m
−3) according to the National Ambient Air Quality Standard (NAAQS) [
5] and are significantly higher than the latest World Health Organization (WHO) standard (60 µg·m
−3) to prevent health risks from long-term O
3 exposure [
6]. Moreover, the number of days with O
3 as the primary pollutant has surpassed that of days with fine particulate matter (PM
2.5) as the primary pollutant, indicating that O
3 has become a key factor affecting air quality. As a typical secondary pollutant, O
3 mainly arises from atmospheric photochemical reaction processes, and its precise control remains a significant challenge [
7].
2. Major causes of O3 pollution
2.1. NOx and VOCs: Key precursors to O3 formation
Nitrogen oxides (NO
x) and volatile organic compounds (VOCs) are essential precursors for the formation of O
3. The reaction pathways for O
3 formation are generally understood (
Fig. 1 [
8]). O
3 is primarily formed through the photochemical oxidation of VOCs in the presence of NO
x, which generates HO
x (HO
2-OH) and RO
x (RO
2-RO) (R: alkyl) radicals. Next, the RO
x/HO
x cycles are coupled with the NO
x cycle (NO
2-NO), promoting O
3 accumulation and concentration increase. Studies have found that the formation of O
3 involves a complex nonlinear relationship between NO
x and VOCs [
9,
10]. The empirical kinetic modeling approach (EKMA) can be used to assess the sensitivity of O
3 concentration to those of its precursors. Based on the ratio of VOCs to NO
x, the sensitivity range of O
3 formation can be divided into the NO
x-limited regime, VOC-limited regime, and transitional regime, providing scientific support for O
3 pollution control [
11]. The optimal reduction ratio of NO
x and VOC emissions for controlling O
3 can be obtained from the EKMA curve based on the specific conditions of different regions. However, if the control ratio of NO
x to VOCs is not appropriate, it may lead to ineffective O
3 control or even a rebound in O
3 concentration, making it challenging to control O
3 pollution. Previous studies have indicated that anthropogenic NO
x emissions in China decreased by 21% during 2013-2017, whereas reductions in VOCs lagged significantly [
12,
13]. The resulting imbalance in precursor emission reduction would exacerbate O
3 levels due to the reduced NO titration [
14], especially in most urban and industrial regions of China, where O
3 formation is currently in the VOC-limited regime [
15,
16]. Nevertheless, it should be noted that a greater proportion of NO
x emission reductions would be beneficial for mitigating O
3 pollution in most rural areas of China, given that they are under NO
x-limited conditions [
8,
17,
18]. More importantly, thanks to substantial reductions in NO
x emissions, most urban regions in China are shifting to a transitional regime [
19-
22]. This finding also suggests that substantially reducing NO
x (as discussed in Section 3.1) is a feasible solution for controlling O
3 pollution in China.
2.2. Solar radiation and temperature: Key meteorological factors influencing O3 formation
O
3 is a secondary product of photochemical reactions, and strong solar radiation is an essential condition for these reactions. Studies have shown that PM
2.5 can affect solar radiation flux by directly absorbing and scattering sunlight [
23], thereby influencing O
3 formation. In recent years, the significant reduction in PM
2.5 concentrations in China has tended to increase the near-surface solar radiation, which is favorable for O
3 formation. Moreover, as PM
2.5 concentrations decrease, the process by which particulate matter quenches free radicals is weakened, stimulating O
3 formation [
12,
24]. The result is a “seesaw relationship” between PM
2.5 and O
3 under certain conditions. On the other hand, PM
2.5 and O
3 share a certain homology, as the secondary reactions of common precursors such as NO
x and VOCs can contribute to both PM
2.5 and O
3 simultaneously. Additionally, as O
3 concentrations increase, they can drive the oxidation processes that lead to the formation of PM
2.5. Based on statistical results from the China National Environmental Monitoring Center, it has been observed that, as PM
2.5 concentrations decrease (<50 µg·m
−3), the correlation coefficient between PM
2.5 and O
3 shifts from negative to positive [
25]. This finding indicates that the “seesaw relationship” between PM
2.5 and O
3 is being broken, thanks to continuous improvements in air quality in China. In southern China, this “seesaw relationship” has already been disrupted, with a positive correlation between PM
2.5 and O
3, showing that their coordinated control can be achieved [
25,
26].
Temperature is another key meteorological factor that affects O
3 formation, with solar radiation being an important driver of surface temperature. An increase in temperature will accelerate the rate of photochemical reactions. Moreover, recent studies indicate that, under global warming, rising temperatures will further promote emissions of biogenic VOCs, as well as non-combustion anthropogenic VOCs (e.g., volatile chemical products, VCPs) [
27-
29]. Both effects will promote the formation of O
3, exacerbating O
3 pollution. Compared with the global scale, the impact of temperature changes on O
3 formation in China exhibits unique characteristics. High anthropogenic emissions, resulting from rapid industrialization and urbanization, significantly amplify the temperature-O
3 relationship in China due to the abundance of O
3 precursors such as NO
x and VOCs [
30].
In summary, the increase in surface O
3 pollution in China over the past decade is driven by a complex interplay of uncoordinated reductions in the emissions of NO
x and VOCs, increased irradiance, and a reduced heterogeneous sink of radicals induced by decreases in PM
2.5 concentrations and by meteorological variability [
12-
14,
31,
32]. Nevertheless, the dominant factors resulting in O
3 increase in different regions of China are still under debate and require further investigation.
3. Feasible strategies for O3 pollution control
3.1. Precursor control strategy: Substantial NOx reduction as a feasible approach
Considering that O
3 formation in most urban and industrial regions of China is in a VOC-limited regime, a much larger reduction in VOC emissions versus NO
x emissions is required in order to achieve a reduction in O
3 concentrations. However, VOC emissions from anthropogenic sources are widely dispersed and complex, and a significant portion comes from biogenic sources. Both source control and end-of-pipe control lack mature and effective technological solutions, making substantial reductions in VOCs difficult to achieve in the short term, even though VOCs have considerable emission reduction potential [
33]. In contrast, NO
x sources are well defined and involve combustion processes, which primarily include stationary combustion plants and the internal combustion engines of transportation vehicles. Furthermore, the corresponding control techniques are very mature. The selective catalytic reduction (SCR) of NO
x by NH
3 (NH
3-SCR) technique for NO
x control has already been widely implemented in coal-fired power plants, and this technique is also rapidly being adopted in the non-electric sector [
34,
35]. For vehicles, the NO
x in gasoline vehicle exhaust can be efficiently removed using the three-way catalytic technique, while NO
x emissions from diesel vehicles can be reduced using the SCR of NO
x by urea (urea-SCR) technique. In addition, the China VI emission standards for heavy-duty vehicles have been fully implemented, and the regulatory agency is strengthening the supervision of in-use vehicles. These measures are expected to significantly reduce NO
x emissions from transportation vehicles [
36,
37]. In the short term, since VOC concentrations cannot be reduced as quickly as NO
x concentrations, a substantial reduction in NO
x to shift to a NO
x-limited regime is an effective and more realistic approach to control O
3 pollution.
Smog-chamber experimental simulation results have shown that moderately reducing NO
x keeps O
3 formation in the VOC-limited regime, leading to a rebound in O
3 concentrations. This mirrors the current situation in urban areas of China. However, with the ongoing control of industrial and motor vehicle emissions, along with the substitution of non-fossil renewable energy within the framework of China’s “dual-carbon” goal, NO
x levels are expected to decrease rapidly [
38]. It should be acknowledged that the potential for NO
x emissions reduction is not yet particularly clear, although studies have explored this topic. Zhang et al. [
39] showed that NO
x emissions in China can be decreased by 56.1% compared with baseline 2014 emissions through the application of end-of-pipe measures. Further renewable energy adoption can reduce NO
x emissions further by up to 89.9%. Guo et al. [
40] found that, by maximizing the strict implementation of emissions reduction technologies in the industrial, transportation, and power sectors, global anthropogenic NO
x emissions could be reduced by 52% by 2050 compared with their 2015 levels. Based on the pollutant emissions inventory for 2010 in China, simulation results of the chemical transport model showed that the NO
x emissions reduction rate had to be greater than 20%-60% to achieve a transition from VOC-limited to NO
x-limited regimes in the Beijing-Tianjin-Hebei region in 2014 [
41]. Our results from both smog-chamber experimental simulations and box-model simulations have indicated that, for most urban regions in China, where O
3 formation is in the VOCs-limited regime, O
3 concentrations will start to decrease when NO
x is reduced by about 85%, causing a shift to the NO
x-limited regime (
Fig. 2(a)), thus controlling O
3 pollution effectively [
42].
Results from field observations have also indicated that a slight reduction in NO
2 concentrations indeed led to a significant increase in O
3 concentrations in the early stages of the coronavirus disease 2019 (COVID-19) epidemic (
Fig. 2(b)). During the strictest lockdown periods, NO
2 concentrations decreased by around 70% and the increasing trend in O
3 concentrations reversed, dropping to the same levels as in 2019 (
Fig. 2(b)) [
43]. Therefore, further strengthening NO
x emissions reduction is an effective and practical strategy for regional O
3 pollution control.
3.2. Direct ozone-decomposition technology
In the short term, when it is difficult to effectively control O
3 by reducing its precursors, the use of direct decomposition technology to remove O
3 from the atmosphere is a feasible supplementary solution for controlling O
3 pollution. Since O
3 is a gaseous pollutant and its decomposition into oxygen is an exothermic reaction, this process is thermodynamically feasible. Thus, developing efficient catalytic materials can enable the direct decomposition of low-concentration O
3 in the ambient air. Given that catalysts can efficiently decompose O
3 under ambient temperature, high relative humidity, and high space velocity conditions [
44-
46], they can be coated on artificial surfaces (e.g., building surfaces) to increase O
3 decomposition activity while retaining the original functions of the coatings. For example, catalytic exterior wall coatings can be made by adding 3%-7% of O
3-decomposition catalyst to ordinary coatings [
47]. The transition-metal catalyst used for such applications has a low cost. Overall, the cost of exterior wall coatings that can decompose O
3 is 0.2-1.5 times higher (an increase of 5-15 CNY·kg
−1 coating) than the cost of ordinary coatings, depending on the amount of catalyst added. It is particularly notable that these functional coatings can be used during the construction or renovation of buildings. Laboratory and field-test results have shown that functional coatings can effectively decompose O
3 in the atmosphere, with the average O
3-decomposition efficiency ranging from 5.5% to 33.2% (
Fig. 3). Moreover, the closer the distance to the functional coatings, the greater the observed O
3-decomposition efficiency [
48]. Promoting the use of O
3 direct decomposition technology and its material products on artificial surfaces such as building surfaces, hardened ground, and vehicle radiators can eliminate O
3 in the atmospheric environment without additional energy consumption. Based on field-test results, the cost of using this technology to ensure that the O
3 concentrations in the 2 + 26 cities meet the secondary grade (160 µg·m
−3) of NAAQS in 2035 is estimated to be about 13 billion CNY, which is one-tenth the cost of synergistic control of VOCs and NO
x as reported by Ding et al. [
1]. The field application of this catalytic material, which utilizes natural photothermal conditions to achieve the spontaneous catalytic decomposition of low-concentration gaseous pollutants (e.g., O
3) in the atmosphere, provides a practical foundation for building so-called “environmental catalytic city” and has significant environmental implications for the design and construction of “self-purifying city” [
49,
50].
This direct O3-decomposition technology offers significant technical advantages in China. The extensive urbanization and high-density construction in Chinese cities provide a large surface area for widespread application, enabling localized O3 reduction in pollution hotspots. This approach offers an economical and durable solution aligned with China’s rapid construction activities. Moreover, the integration of this technology supports China’s green building initiatives and sustainable development goals, presenting a scalable and innovative strategy to address O3 pollution in China.
PDF
(812KB)
