in the process of urbanization,population agglomeration and industrial concentration lead to high-intensity emissions of air pollutants in the process of production and living.When the total emission of air pollutants exceeds the atmospheric environmental capacity,that is,the self-purification capacity of the atmosphere is not enough to remove the emitted air pollutants,the pollutants will accumulate in the atmosphere and undergo a complex secondary transformation process,which will lead to combined air pollution and pose a major threat to human health and sustainable development。

Environmental micro-interface process is an important reason for the formation of combined air pollution.in many developing countries In the world today,due to the interfacial catalysis of high concentrations of atmospheric particulate matter,the oxidation capacity of the atmosphere is enhanced,and the conversion of gaseous pollutants to particulate pollutants is accelerated,thus forming a new kind of atmospheric"haze chemistry"pollution^[1,2]。 For example,the（NO_x）of mine dust particles and nitrogen oxides can greatly accelerate the interfacial oxidation of sulfur dioxide（SO₂）,resulting in the rapid growth of sulfate during severe haze in China^[3]。 Black carbon particles can catalyze and activate H₂O,O₂and NO_xunder natural conditions to produce active oxygen species,enhance atmospheric oxidation,and significantly contribute to atmospheric secondary pollution^[4⇓⇓~7]。 On the other hand,the modification of the underlying surface by human activities is also considered to have an important impact on atmospheric chemical processes.For example,the use of titanium dioxide（TiO₂）materials on building surfaces may have an important contribution to atmospheric nitrogen pentoxide（N₂O₅）,gaseous sulfuric acid（H₂SO₄）,and the formation of new particles^[8,9]。

The interfacial catalytic process in the atmosphere not only plays an important role in the formation of secondary pollutants(such as fine particulate matter,（PM_2.5）,ozone,（O₃）,etc.),but also purifies the primary gaseous pollutants in the atmosphere.Rapid urbanization has transformed the underlying surface of the atmosphere,providing conditions for human intervention to purify air pollutants,and the regulation of urban underlying surface of the atmosphere is expected to play an important role in the fine management of air pollution,and achieve significant health effects in densely populated areas 。

environmental catalysis includes artificial environmental catalysis and Spontaneous environmental catalysis in nature.spontaneous environmental catalysis refers to the study of interfacial catalytic processes in heterogeneous atmospheric chemistry related to the earth's surface and atmospheric particles by treating the entire earth's atmosphere as a reactor of light and heat^[10]。 Anthropogenic environmental catalysis is mainly the study and use of catalytic science and technology to eliminate pollutants that have been produced.When the photocatalytic material is irradiated by photons,electrons in the valence band are excited to jump to the conduction band,holes are formed in the valence band,and the electrons and the holes respectively react with O₂and H₂O（or surface hydroxyl groups)to form superoxide radicals and hydroxyl radicals,thereby oxidizing and degrading pollutants.The room temperature catalytic material does not need an additional heat source,and can play a catalytic role in purifying environmental pollutants under actual atmospheric conditions.Photocatalytic technology and room temperature catalytic technology provide a technical basis for the application of catalytic materials to purify air in urban environment."Environmental catalytic city"refers to a city in which catalytic materials are coated on the inside and outside of buildings and hardened ground in the city,so that the spontaneous catalytic purification of low-concentration gaseous pollutants in the environment can be achieved under the conditions of light and heat in nature 。

Nowadays,many countries are facing severe combined pollution problems of PM_2.5and O₃,and there is still a lack of mature coordinated control strategies.In the process of PM_2.5treatment,there is a dilemma that the concentration of PM_2.5decreases and the pollution of O₃deteriorates.The O₃in the troposphere near the ground is a typical secondary pollutant,which is mainly produced by the atmospheric photochemical reaction between volatile organic compounds(VOCs)and NO_xemitted by human beings.NO_xand VOCs jointly affect the concentration of O₃in the tropospheric atmosphere.At present,O₃pollution in most urban areas in China is mainly controlled by VOCs.In theory,to achieve a rapid reduction of O₃concentration in urban areas,VOCs emission reduction should be much larger than NO_x.However,due to the dispersion and complexity of anthropogenic VOCs emissions,there is a lack of mature and effective control technology for unorganized emissions such as solvent use and residential life,and a considerable part of VOCs come from natural sources,which is difficult to control artificially.Therefore,it is difficult to achieve significant emission reduction of VOCs in the short term.At the same time,further reduction of PM_2.5concentration requires substantial emission reduction of NO_x^[11,12]。 In the process of slow emission reduction of VOCs and rapid emission reduction of NO_x,it will inevitably lead to the rebound of O₃.Because O₃is a gaseous pollutant,and the decomposition of O₃into oxygen is an exothermic reaction,which is thermodynamically feasible,the development of efficient catalytic materials can achieve the direct decomposition of low concentrations of O₃in the atmosphere 。

At present,through long-term research,many domestic scientific research teams have developed a series of catalysts with independent intellectual property rights,which can efficiently catalyze and decompose O₃at room/low temperature,high humidity and high space velocity^{[13⇓⇓⇓⇓⇓⇓⇓⇓~22]}。 Researchers have systematically studied the active sites and reaction mechanism of the catalytic decomposition reaction of O₃,and proposed three limiting factors for the efficient decomposition of O₃under room/low temperature conditions:insufficient active sites,competitive adsorption of water molecules to the active sites,and non-desorption of intermediate oxygen species^{[13,23⇓~25]}。 The research group has successfully developed a new type of efficient and stable O₃decomposition catalyst by regulating the type/nature of active sites and the reaction path,and revealed a new hydrogen transfer reaction mechanism.It breaks through the three factors that limit the efficient decomposition of O₃under room temperature/low temperature conditions,and realizes the improvement of room temperature/low temperature performance and stability of O₃catalytic decomposition materials under high humidity and high space velocity conditions^{[26⇓⇓⇓⇓~31]}。 Based on the catalyst's characteristics of efficient decomposition of O₃at room temperature,high humidity and high space velocity(million volume space velocity),the atmospheric haze chemistry research team and enterprises cooperated to add the catalyst to building exterior wall coatings.The activity of decomposing O₃is increased while the original function of the coating is retained,and the results of laboratory and field tests show that the functional coating can effectively decompose O₃^[32]。 In 2021,the field application and field evaluation of the 5200 m²were carried out in Daxing District of Beijing.Compared with the site within 1 km,it was proved that the removal efficiency of O₃was between 5%and 20%within the construction range of Daxing 5200 m².Promote the direct decomposition technology of O₃and its material products,apply them to the exterior walls of new buildings and old urban renovation buildings,hardened ground,motor vehicle radiator surface,etc.,to build an"environmental catalytic city",which can eliminate O₃in the atmospheric environment without external energy consumption,and effectively slow down the rebound of O₃pollution in the process of deep emission reduction of NO_x 。

In addition to ozone,the application objects of"environmental catalytic city"also include other atmospheric pollutants,such as a series of research and application exploration have been carried out in the environmental catalytic purification of NO_xat home and abroad,and some purification technologies of VOCs also have application potential^[33⇓~35]。 In addition,indoor air quality is closely related to human health,and indoor air pollution removal is also an important application scenario of"environmental catalytic city"。

According to the 2022 China Eco-environmental Status Bulletin,in 2022,the 90th percentile concentration of the daily maximum 8-hour average concentration of O₃in 339 cities at prefecture level and above in China is 90~194μg/m³,with an average of 145μg/m³,which is 5.8%higher than that in 2021;92 of 339 cities have exceeded the standard of O₃,accounting for 27.1%.Compared with other air pollutants such as PM_2.5,O₃has become the primary pollutant in most regions of China in recent years.In 2022,the days with O₃as the primary pollutant exceeding the standard accounted for 47.9%of the total days exceeding the standard.In summer,almost all the primary pollutants in the polluted days nationwide are O₃.In the Pearl River Delta region,the proportion of O₃as the primary pollutant has exceeded 90%in the whole year.What is more serious is that the maximum concentration of 8 hours on the 1st day of UNK in key cities in China in 2022 increased by about 18%compared with 2013.According to the monthly variation,the months with high O₃concentration spread from spring and autumn to early spring and late autumn,and the duration increased.From the regional distribution,the O₃pollution is gradually developing from point to patch,and the impact area is increasing.On the whole,O₃pollution is deteriorating^[36⇓~38]。 On the one hand,with the smooth implementation of the Air Pollution Prevention and Control Action Plan(2013-2017)and the Blue Sky Defense Action Plan(2018-2020),the prevention and control of air pollution has entered the deep water area,making it more difficult to further reduce the emission of primary pollutants,and the demand for precise control has become increasingly prominent.On the other hand,according to the requirements of the Action Plan for Continuous Air Quality Improvement,by 2025,the concentration of PM_2.5in cities at prefecture level and above will be reduced by 10%compared with 2020,and the total emissions of NO_xand VOCs will be reduced by more than 10%compared with 2020.Due to the complex nonlinear relationship between O₃pollution and precursor emissions,there is a possibility of further deterioration of O₃pollution in China,and the coordinated control of PM_2.5and O₃has become an urgent task of air pollution prevention and control in China^[39⇓~41]。 Under the background of"double carbon"goal,air pollution control needs more low-carbon and effective control measures in addition to source emission reduction,which puts forward new and higher requirements for further improving the theory of"environmental catalytic city"and promoting its practical application。

It should be pointed out that the materials that can be used in"environmental catalytic city"must have the characteristics of high performance,long life and low cost.The main tasks to improve the theory of"environmental catalytic city"include:(1)to systematically study the heterogeneous chemical processes of air pollutants at the interface of typical cities by means of laboratory simulation,field test and numerical simulation evaluation.Scientifically assess the possible atmospheric environmental impacts of the interface process,such as whether it will lead to the generation of other pollutants,promote the development and use of building materials with self-purification function for urban air pollution,and build an environmentally friendly urban interface;(2)Construct the method and technical system of environmental catalytic city effect evaluation,and scientifically and quantitatively evaluate the implementation cost,air pollutant purification effect and health benefits of the environmental catalytic city plan;(3)Expanding the technical system suitable for the construction of environmental catalytic cities,developing new efficient,stable and low-cost purification technologies for different pollutants(（O₃,NO_x,SO₂,NH₃,VOCs),and expanding the scope of environmental catalytic cities to purify pollutants;(4)Explore the applicable technical system of environmental catalysis city under different climate and pollution conditions in China,and develop the technical scheme of environmental catalysis city to meet different needs;(5)Explore the application of functional materials in environmental catalysis of urban indoor and outdoor air purification,and build a"self-purification city"in an all-round way 。