Synergistic regulation of air quality and thermal environments by urban forests: insights from a fixed observational study

Abstract

Urban forest construction is widely regarded as a key strategy for improving air quality and mitigating urban heat island effects. However, the interactions between the urban thermal environment and air quality are influenced by a range of biophysical and meteorological factors, including canopy structure, air pollutant dispersion, and temperature-dependent chemical reactions, making them highly dynamic and complex. Thus, this study conducted field measurements at 10 m (within urban forests) and 20 m (outside the forests) to record concentrations of PM_2.5, PM₁₀, CO, NO₂, SO₂ and O₃, as well as wind speed, wind direction, air temperature, relative humidity, and atmospheric pressure on polluted, heat-stressed, and control days. The 10 m level captures conditions within the canopy, where temperature, humidity, and pollutant levels are influenced by shading, transpiration, and leaf deposition. In contrast, the 20 m level reflects ambient conditions above the canopy, where atmospheric mixing and external sources dominate. Comparing these two heights helps reveal how urban forests regulate air quality and thermal environments, clarifying the mechanisms underlying their interactions. The study further assessed the influence of urban forests on air pollutant concentrations, air temperature, physiological equivalent temperature (PET), linear regression relationships, and trade-offs between air quality and thermal environments. The results showed that urban forests reduced PM_2.5, PM₁₀ and O₃ concentrations while increasing CO, NO and NO₂ concentrations. The concentration of PM_2.5 within the urban forest was more than 20% lower than that observed outside the forest. Air pollutant concentrations correlated more strongly with air temperature than with PET. The relationship between air temperature and NO₂ and O₃ was stronger on heat-stressed and control days than on polluted days. Urban forests reduced the trade-off between air temperature and PM_2.5, O₃ concentrations but increased the trade-off between air temperature and CO, NO and NO₂ concentrations. These findings offer a clearer understanding of how they influence the coupled dynamics of air quality and thermal environments, thereby support more effective nature-based strategies for urban environmental management.

Keywords

Urban forests / Synergistic regulation / Air pollution / Heat stress / Trade-off

References

[1] Abhijith KV, Kumar P, Gallagher J, McNabola A, Baldauf R, Pilla F, Broderick B, Di Sabatino S, Pulvirenti B. Air pollution abatement performances of green infrastructure in open road and built-up street canyon environments–a review. Atmos Environ, 2017, 162: 71-86.

[2] Aghdam FZ, Hasanlou M, Dehghanijabbarlou M. Quantifying urban air quality through multispectral satellite imagery and Google Earth Engine. J Atmos Sol Terr Phys, 2024, 261. ArticleID: 106301

[3] Araminienė V, Sicard P, Černiauskas V, Coulibaly F, Varnagirytė-Kabašinskienė I. Estimation of air pollution removal capacity by urban vegetation from very high-resolution satellite images in Lithuania. Urban Clim, 2023, 51. ArticleID: 101594

[4] Arunab KS, Mathew A. Quantifying urban heat island and pollutant nexus: a novel geospatial approach. Sustain Cities Soc, 2024, 101. ArticleID: 105117

[5] Beele E, Aerts R, Reyniers M, Somers B. Spatial configuration of green space matters: associations between urban land cover and air temperature. Landsc Urban Plan, 2024, 249. ArticleID: 105121

[6] Cao C, Lee X, Liu SD, Schultz N, Xiao W, Zhang M, Zhao L. Urban heat islands in China enhanced by haze pollution. Nat Commun, 2016, 7. ArticleID: 12509

[7] Cao W, Zhou W, Yu WD, Wu T. Combined effects of urban forests on land surface temperature and PM_2.5 pollution in the winter and summer. Sustain Cities Soc, 2024, 104. ArticleID: 105309

[8] Chen DS, Sun WW, Shi JC, Johnson BA, Tan ML, Pan QQ, Li WQ, Yang XD, Zhang F. Utilizing GaoFen-2 derived urban green space information to predict local surface temperature. Urban for Urban Green, 2024, 99. ArticleID: 128463

[9] Chen DL, Yan JL, Sun NX, Sun W, Zhang WK, Long YC, Yin S. Selective capture of PM_2.5 by urban trees: the role of leaf wax composition and physiological traits in air quality enhancement. J Hazard Mater, 2024, 478. ArticleID: 135428

[10] Eisenman TS, Churkina G, Jariwala SP, Kumar P, Lovasi GS, Pataki DE, Weinberger KR, Whitlow TH. Urban trees, air quality, and asthma: an interdisciplinary review. Landsc Urban Plan, 2019, 187: 47-59.

[11] Endreny TA. Strategically growing the urban forest will improve our world. Nat Commun, 2018, 9. ArticleID: 1160

[12] Fallmann J, Forkel R, Emeis S. Secondary effects of urban heat island mitigation measures on air quality. Atmos Environ, 2016, 125: 199-211.

[13] Fu RN, Pađen I, García-Sánchez C. Should we care about the level of detail in trees when running urban microscale simulations?. Sustain Cities Soc, 2024, 101. ArticleID: 105143

[14] Han L, Zhang RJ, Wang JQ, Cao SJ. Spatial synergistic effect of urban green space ecosystem on air pollution and heat island effect. Urban Clim, 2024, 55. ArticleID: 101940

[15] Jeanjean APR, Buccolieri R, Eddy J, Monks PS, Leigh RJ. Air quality affected by trees in real street canyons: the case of Marylebone neighbourhood in central London. Urban for Urban Green, 2017, 22: 41-53.

[16] Lai KY, Kumari S, Gallacher J, Webster C, Sarkar C. Nexus between residential air pollution and physiological stress is moderated by greenness. Nat Cities, 2024, 1(3): 225-237.

[17] Li XB, Wang DS, Lu QC, Peng ZR, Lu SJ, Li B, Li C. Three-dimensional investigation of ozone pollution in the lower troposphere using an unmanned aerial vehicle platform. Environ Pollut, 2017, 224: 107-116.

[18] Li HD, Meier F, Lee X, Chakraborty T, Liu JF, Schaap M, Sodoudi S. Interaction between urban heat island and urban pollution island during summer in Berlin. Sci Total Environ, 2018, 636: 818-828.

[19] Li ZT, Zhang H, Juan YH, Lee YT, Wen CY, Yang AS. Effects of urban tree planting on thermal comfort and air quality in the street canyon in a subtropical climate. Sustain Cities Soc, 2023, 91. ArticleID: 104334

[20] Li HW, Zhao YL, Wang CH, Ürge-Vorsatz D, Carmeliet J, Bardhan R. Cooling efficacy of trees across cities is determined by background climate, urban morphology, and tree trait. Commun Earth Environ, 2024, 5. ArticleID: 754

[21] Lin XL, Chamecki M, Yu XP. Aerodynamic and deposition effects of street trees on PM_2.5 concentration: from street to neighborhood scale. Build Environ, 2020, 185. ArticleID: 107291

[22] Liu X, Shi XQ, He HD, Peng ZR. Distribution characteristics of submicron particle influenced by vegetation in residential areas using instrumented unmanned aerial vehicle measurements. Sustain Cities Soc, 2022, 78. ArticleID: 103616

[23] Long YC, Zhao Y, Sun NX, Xu Y, Xue WK, Yin S. Summer urban synergistic effects of anthropogenic pollutants and low-molecular-weight biogenic volatile organic compounds on secondary organic aerosol presented byPM₁. Sci Total Environ, 2025, 964. ArticleID: 178572

[24] Lu KF, He HD, Wang HW, Li XB, Peng ZR. Characterizing temporal and vertical distribution patterns of traffic-emitted pollutants near an elevated expressway in urban residential areas. Build Environ, 2020, 172. ArticleID: 106678

[25] Miao CP, He XY, Gao Z, Chen W, He BJ. Assessing the vertical synergies between outdoor thermal comfort and air quality in an urban street canyon based on field measurements. Build Environ, 2023, 227. ArticleID: 109810

[26] Miao CP, Li PP, Huang YQ, Sun YX, Chen W, Yu S. Coupling outdoor air quality with thermal comfort in the presence of street trees: a pilot investigation in Shenyang, Northeast China. J for Res, 2023, 34(3): 831-839.

[27] Miao C, Wang J, Wang D. Research progress on urban forest ecosystem services and multifunctionality. Int J Environ Sci Technol, 2025, 22(12): 11557-11566.

[28] Ngarambe J, Joen SJ, Han CH, Yun GY. Exploring the relationship between particulate matter, CO, SO₂, NO₂, O₃ and urban heat island in Seoul, Korea. J Hazard Mater, 2021, 403. ArticleID: 123615

[29] Oke TR. The micrometeorology of the urban forest. Philos Trans R Soc Lond B Biol Sci, 1989, 324(1223): 335-349.

[30] Rendon P, Love N, Pawlak C, Yost J, Ritter M, Doremus J. Street tree diversity and urban heat. Urban for Urban Green, 2024, 91. ArticleID: 128180

[31] Santunione G, Barbieri A, Sgarbi E. Analysis of particulate matter (PM) trapped by four different plant species in an urban forest: quantification and characterization. Trees People, 2024, 16. ArticleID: 100585

[32] Sarrat C, Lemonsu A, Masson V, Guedalia D. Impact of urban heat island on regional atmospheric pollution. Atmos Environ, 2006, 40(10): 1743-1758.

[33] Sharma S, Hussain S, Kumar P, Singh AN. Urban trees’ potential for regulatory services in the urban environment: an exploration of carbon sequestration. Environ Monit Assess, 2024, 196(6. ArticleID: 504

[34] Shenyang Municipal Bureau of Ecology and Environment (2024) 2023 Bulletin of Environmental quality in Shenyang

[35] Steinparzer M, Schaubmayr J, Godbold DL, Rewald B. Particulate matter accumulation by tree foliage is driven by leaf habit types, urbanization- and pollution levels. Environ Pollut, 2023, 335. ArticleID: 122289

[36] Tamaskani Esfehankalateh A, Ngarambe J, Yun GY. Influence of tree canopy coverage and leaf area density on urban heat island mitigation. Sustainability, 2021, 13(13. ArticleID: 7496

[37] Ulpiani G. On the linkage between urban heat island and urban pollution island: three-decade literature review towards a conceptual framework. Sci Total Environ, 2021, 751. ArticleID: 141727

[38] United Nations. World urbanization prospects: the 2018 revision (ST/ESA/SER.A/420), 2019. New York, United Nations, Departement of Economic and Social Affairs Population Division

[39] Vigevani I, Corsini D, Comin S, Fini A, Ferrini F. Methods to quantify particle air pollution removal by urban vegetation: a review. Atmos Environ X, 2024, 21. ArticleID: 100233

[40] Wang AJ, Ren C, Wang JQ, Feng ZB, Kumar P, Haghighat F, Cao SJ. Health assessment and mitigating solutions to heat-pollution induced by urban traffic. J Clean Prod, 2024, 434. ArticleID: 140097

[41] Wang C, Jin JL, Davies C, Chen WY. Urban forests as nature-based solutions: a comprehensive overview of the national forest city action in China. Curr for Rep, 2024, 10(2): 119-132.

[42] Wang CC, Ren ZB, Zhang P, Guo YJ, Hong SY, Hong WH, Wang XY, Geng RX, Meng FY. Impact of vegetation coverage and configuration on urban temperatures: a comparative study of 31 provincial capital cities in China. J for Res, 2024, 35(1): 142.

[43] Wang MK, Kong DL, Mo XH, Wang YH, Yang QP, Kardol P, Valverde-Barrantes OJ, Simpson MJ, Zeng H, Reich PB, Bergmann J, Tharayil N, Wang JJ. Molecular-level carbon traits underlie the multidimensional fine root economics space. Nat Plants, 2024, 10(6): 901-909.

[44] Wang XY, Ren ZB, Wang CC, Zhang P, Hong SY, Geng RX, Zhang BY. Positive or negative relationship between forest fragmentation and carbon sequestration capacity across different urban agglomerations in China. J for Res, 2026, 37: 14.

[45] Wróblewska K, Jeong BR. Effectiveness of plants and green infrastructure utilization in ambient particulate matter removal. Environ Sci Eur, 2021, 33(1): 110.

[46] Wu J, Wang Q, Xu C, Lun X, Wang L, Gao Y, Huang L, Zhang Q, Li L, Liu B, Liu H, Xu L (2024) Biogenic volatile organic compounds in forest therapy base: A source of air pollutants or a healthcare function. Sci Total Environ 931: 172944.

[47] Xu D, Bai TT, Yang L, Zhou YY, Chen B, Xu HF, Song YZ, Yuan Y, Cui YZ, Meng L, Xia ZQ, Chen M, Xu ZC, Zhao P, Dong GH, Zhang L, Zhao JC, Wu WB, Wang W, Zhao L, Cheng J, Ciais P. Quantifying the cooling effect of urban greening driven by ecological restoration projects in China. Environ Sci Technol, 2024, 58(47): 20990-21001.

[48] Yao L, Li T, Xu MX, Xu Y. How the landscape features of urban green space impact seasonal land surface temperatures at a city-block-scale: an urban heat island study in Beijing, China. Urban for Urban Green, 2020, 52. ArticleID: 126704

[49] Yli-Pelkonen V, Setälä H, Viippola V. Urban forests near roads do not reduce gaseous air pollutant concentrations but have an impact on particles levels. Landsc Urban Plan, 2017, 158: 39-47.

[50] Zhu ST, Tang JY, Zhou XL, Li P, Liu ZL, Zhang CC, Zou ZY, Li T, Peng CH. Spatiotemporal analysis of the impact of urban landscape forms on PM_2.5 in China from 2001 to 2020. Int J Digit Earth, 2023, 16(1): 3417-3434.

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