Junshi Xu is an Assistant Professor in the Department of Geography and Deputy Programme Director of the Master of Transport Policy & Planning at The University of Hong Kong. Before joining HKU, he was a Research Associate in Civil & Mineral Engineering at the University of Toronto, where he earned his Ph.D. in Transportation Engineering. He also served as Director of Positive Zero Transport Futures, a living lab initiative focused on transport decarbonization and societal impact.
At HKU, Professor Xu leads the PEACE (People-oriented, Equitable, Accessible, Community-focused, and Environmental) Lab. His research asks how cities can move people and goods while reducing environmental burdens, protecting health, and advancing environmental justice. The lab develops data-driven, simulation-based, and AI-enabled approaches to examine how mobility systems shape environmental exposures and population health, particularly concerning emerging and unregulated air pollutants.
Current projects in the PEACE Lab investigate how mobility, urban activity, and environmental stressors co-evolve across space and time. The group combines retrospective analysis of observed mobility and exposure patterns with scenario-based modelling of future interventions. This work aims to reveal unequal exposure burdens and inform interventions for healthier, lower-carbon, and more equitable cities.
Research Interests
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Urban mobility and transport decarbonization
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Air quality, environmental monitoring, and exposure assessment
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AI, geospatial data science, and urban sensing
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Environmental justice, public health, and equitable urban systems
Join Us
The PEACE Lab welcomes undergraduate researchers, graduate students, summer interns, visiting students, and postdoctoral researchers interested in urban mobility, environmental exposure, and healthy urban systems. We especially welcome applicants with strengths or interests in transportation engineering, data science, AI modelling, remote sensing, urban sensing, or environmental modelling. Current methodological interests include foundation models, transformers, LLMs, multimodal and generative AI, causal AI, and spatiotemporal machine learning.
Selected Ongoing Research Projects
The projects listed below are selected ongoing studies currently recruiting participants. Additional projects are underway in the PEACE Lab across urban mobility, air quality, environmental exposure, public health, and environmental justice.
A 60-participant cohort study involving three waves of one-week monitoring among working-age adults living in Sham Shui Po, Central & Western, Kwun Tong, and Tuen Mun.
A city-wide study aiming to recruit 500 participants to examine environmental stressors and mental health across Hong Kong.
Selected Publications
2026
Zhu, M., Ye, X., Wang, Y., Niu, C., Qiu, W., & Xu, J. (2026). Generative AI and Causal-Spatial Modeling for Understanding Nighttime Pedestrian Risk in Urban Systems. Computers, Environment and Urban Systems. https://doi.org/10.1016/j.compenvurbsys.2026.10243
Niu, C., Loo, B., & Xu, J. (2026). How Does Autonomous Ride-Hailing Pricing Shape Modal Shifts and Accessibility Equity? Journal of Transport Geography. https://doi.org/10.1016/j.jtrangeo.2026.104676
Niu, C., Chen, Q., Wang, A., & Xu, J. (2026). Disentangling Near-Road Emission Inequities through Data-Driven Spatiotemporal Traffic Dynamics. Environmental Science & Technology. https://doi.org/10.1021/acs.est.5c14619
Saeedi, M., Zalzal, J., Ganji, A., Xu, J., Goodfellow, S. D., & Hatzopoulou, M. (2026). Bridging the Gap Between Data Reproduction and Prediction: The Impact of Feature Selection and Cross-Validation Strategies on Prediction of Ambient Ultrafine Particles Collected with Mobile Monitoring. Environmental Science & Technology. https://doi.org/10.1021/acs.est.5c12601
2025
Zhang, Q., Zakaria, C., Hatzopoulou, M., Xu, J., HubkaRao, T., Tchana, S., Rothman, L., Sadeghi, A., & Batomen, B. (2025). Eye on the Street: Computer Vision for Spatial-Temporal Mapping of Street Safety Elements. UbiComp Companion 2025. https://doi.org/10.1145/3714394.3756280
Xu, J., Ganji, A., Saeedi, M., Jeong, C., Su, Y., Munoz, T., Lloyd, M., Weichenthal, S., Evans, G., & Hatzopoulou, M. (2025). Unveiling the Impact of Wildfires on Nanoparticle Characteristics and Health Disparities through Mobile and Fixed-Site Monitoring in Toronto, Canada. Environmental Science & Technology. https://doi.org/10.1021/acs.est.4c08675
Batisse, E., Lloyd, M., Cavanaugh, A., Ganji, A., Xu, J., Hatzopoulou, M.,…Weichenthal, S. (2025). Examining the social distributions in neighbourhood black carbon and ultrafine particles in Montreal and Toronto, Canada. Environment international, 198, 109395. https://doi.org/10.1016/j.envint.2025.109395
Ganji, A., Lloyd, M., Xu, J., Weichenthal, S., & Hatzopoulou, M. (2025). Traffic-related air pollution backcasting using convolutional neural network and long short-term memory approach. The Science of the total environment, 976, 179286. https://doi.org/10.1016/j.scitotenv.2025.179286
Jaafar, W., Zalzal, J., Xu, J., Ganji, A., & Hatzopoulou, M. (2025). Refining Air Pollution Exposure Estimates: A Comparison of Citywide and Neighborhood Land Use Regression Models in Toronto. Environmental science & technology, 59(35), 18783-18794. https://doi.org/10.1021/acs.est.5c08371
Saeedi, M., Xu, J., Ahmed, U., Roorda, M., & Hatzopoulou, M. (2025). Urban Air Pollution Data Collection, Mapping, and Prediction Using Mobile Sensors Installed on Courier Trucks. Transportation Research Record, 2679(7), 236-250. https://doi.org/10.1177/03611981251324208
Sun, X., Zou, R., & Xu, J. (2025). Impact of jobs-housing balance on neighborhood traffic and air quality. Urban climate, 60, 102361. https://doi.org/10.1016/j.uclim.2025.102361
Wang, K., Li, M., Xu, J., Hatzopoulou, M., & Nurul Habib, K. (2025). Behavioural nudging for greener travel: A discrete choice experiment in the Greater Toronto Area. Energy (Oxford), 322, 135745. https://doi.org/10.1016/j.energy.2025.135745
Weichenthal, S., Lavigne, E., Lloyd, M., Venuta, A., Ganji, A., Xu, J.,…Hatzopoulou, M. (2025). Within-city spatiotemporal variations in outdoor ultrafine particles and emergency room visits for cardiovascular outcomes. Environmental epidemiology, 9(5), e419. https://doi.org/10.1097/EE9.0000000000000419
Zhang, L., Song, G., Zhang, Z., Zhai, Z., Xu, J., Fan, P., & Ding, Y. (2025). Can Eco-Driving Evaluation Cross Cities? Data Localization and Behavioral Heterogeneity from Beijing to Toronto. Sustainability, 17(9), 3957. https://doi.org/10.3390/su17093957
2024
Ganji, A., Saeedi, M., Lloyd, M., Xu, J., Weichenthal, S. and Hatzopoulou, M. (2024). "Air pollution prediction and backcasting through a combination of mobile monitoring and historical on-road traffic emission inventories." Science of The Total Environment 915: 170075. DOI:https://doi.org/10.1016/j.scitotenv.2024.170075
Lloyd, M., Olaniyan, T., Ganji, A., Xu, J., Venuta, A., Simon, L., . . . Weichenthal, S. (2024). "Airborne nanoparticle concentrations are associated with brain cancer incidence in Canada’s two largest cities." ISEE Conference Abstracts 2024(1). DOI:https://doi.org/10.1289/isee.2024.0090
Lloyd, M., Olaniyan, T., Ganji, A., Xu, J., Venuta, A., Simon, L., . . . Weichenthal, S. (2024). "Airborne Nanoparticle Concentrations Are Associated with Increased Mortality Risk in Canada’s Two Largest Cities." American Journal of Respiratory and Critical Care Medicine 210(11): 1338-1347. DOI:https://doi.org/10.1164/rccm.202311-2013oc
Lloyd, M., Olaniyan, T., Ganji, A., Xu, J., Simon, L., Zhang, M., . . . Weichenthal, S. (2024). "Airborne ultrafine particle concentrations and brain cancer incidence in Canada’s two largest cities." Environment International 193: 109088. DOI:https://doi.org/10.1016/j.envint.2024.109088
Xu, J., Farrar, E., Jeong, C.-H., Jaafar, W., Anwar, D., Nielsen, I., . . . Hatzopoulou, M. (2024). "Aircraft Activities and Ultrafine Particle Concentrations near a City Airport: Insights from a Measurement Campaign in Toronto, Canada." ACS ES&T Air 1(8): 801-814. DOI:https://doi.org/10.1021/acsestair.3c00114
Farrar, E., Kobayaa, N., Jaafar, W., Torbatian, S., Gamage, S. M., Brook, J., . . . Hatzopoulou, M. (2024). "Campus–Community Partnership to Characterize Air Pollution in a Neighborhood Impacted by Major Transportation Infrastructure." ACS ES&T Air 1(12): 1601-1616. DOI:https://doi.org/10.1021/acsestair.4c00155
Jaafar, W., Xu, J., Farrar, E., Jeong, C.-H., Ganji, A., Evans, G. and Hatzopoulou, M. (2024). "Challenges and opportunities of low-cost sensors in capturing the impacts of construction activities on neighborhood air quality." Building and Environment 254: 111363. DOI:https://doi.org/10.1016/j.buildenv.2024.111363
Xu, J., Saeedi, M., Zalzal, J., Zhang, M., Ganji, A., Mallinen, K., . . . Hatzopoulou, M. (2024). "Exploring the triple burden of social disadvantage, mobility poverty, and exposure to traffic-related air pollution." Science of The Total Environment 920: 170947. DOI:https://doi.org/10.1016/j.scitotenv.2024.170947
Weichenthal, S., Lloyd, M., Ganji, A., Simon, L., Xu, J., Venuta, A., . . . Hatzopoulou, M. (2024). "Long-Term Exposure to Outdoor Ultrafine Particles and Black Carbon and Effects on Mortality in Montreal and Toronto, Canada." Res Rep Health Eff Inst 2024(217): 1-63.
Venuta, A., Lloyd, M., Ganji, A., Xu, J., Simon, L., Zhang, M., . . . Weichenthal, S. (2024). "Predicting within-city spatiotemporal variations in daily median outdoor ultrafine particle number concentrations and size in Montreal and Toronto, Canada." Environmental Epidemiology 8(4): e323. DOI:https://doi.org/10.1097/ee9.0000000000000323
Torbatian, S., Saleh, M., Xu, J., Minet, L., Gamage, S. M., Yazgi, D., . . . Hatzopoulou, M. (2024). "Societal Co-benefits of Zero-Emission Vehicles in the Freight Industry." Environmental Science & Technology 58(18): 7814-7825. DOI:https://doi.org/10.1021/acs.est.3c08867
2023
Lloyd, M., A. Ganji, J. Xu, A. Venuta, L. Simon, M. Zhang, M. Saeedi, S. Yamanouchi, J. Apte, K. Hong, M. Hatzopoulou, S. Weichenthal. Predicting spatial variations in annual average outdoor ultrafine particle concentrations in Montreal and Toronto, Canada: Integrating land use regression and deep learning models. 2023. Environment International, 178, 108106. https://doi.org/10.1016/j.envint.2023.108106
Ganji, A., O. Youssefi, J. Xu, K. Mallinen, M. Lloyd, A. Wang, A. Bakhtari, S. Weichenthal, M. Hatzopoulou. 2023. Design, calibration, and testing of a mobile sensor system for air pollution and built environment data collection: The Urban Scanner platform. Environmental Pollution, 120720. https://doi.org/10.1016/j.envpol.2022.120720
Z. Zhai, J. Xu, M. Zhang, A. Wang, M. Hatzopoulou. Quantifying start emissions and impact of reducing cold and warm starts for gasoline and hybrid vehicles. 2023. Atmospheric Pollution Research, 101646. https://doi.org/10.1016/j.apr.2022.101646
2022
Xu, J., M. Zhang, A. Ganji, K. Mallinen, A. Wang, M. Lloyd, A. Venuta, L. Simon, J. Kang, J. Gong, Y. Zamel, S. Weichenthal, M. Hatzopoulou. 2022. Prediction of short-term ultrafine particle exposures using real-time street-level images paired with air quality measurements. Environmental Science & Technology, 56, 12886 - 12897. https://doi.org/10.1021/acs.est.2c03193
Ahmed, U., R. Tu, J. Xu, G. Amirjamshidi, M. Hatzopoulou, M. J. Roorda. 2023. GPS-based traffic conditions classification using machine learning approaches. Transportation Research Record, 036119812211113. https://doi.org/10.1177/03611981221111370
Zhai Z., J. Xu, G. Song, M. Hatzopoulou. Comparative analysis of drive-cycles, speed limit violations, and emissions in two cities: Toronto and Beijing. 2022. Science of the Total Environment, 811 (x), 152323. https://doi.org/10.1016/j.scitotenv.2021.152323
Tu, R., J. Xu, T. Li, H. Chen. 2022. Effective and acceptable eco-driving guidance for human-driving vehicles: a review. International Journal of Environmental Research and Public Health, 19 (12). https://doi.org/10.3390/ijerph19127310
Tu, R., J. Xu, A. Wang, M. Zhang, Z. Zhai, M. Hatzopoulou. 2022. Real-world emissions and fuel consumption of gasoline and hybrid light duty vehicles under local and regulatory drive cycles. Science of the Total Environment, 805, 150407. https://doi.org/10.1016/j.scitotenv.2021.150407
Wang, A., J. Xu, M. Zhang, Z. Zhai, G. Song, M. Hatzopoulou. 2022. Emissions and fuel consumption of a hybrid electric vehicle in real-world metropolitan traffic conditions. Applied Energy, 306 (PB), 118077. https://doi.org/10.1016/j.apenergy.2021.118077
2021
Xu, J., R. Tu, U. Ahmed, G. Amirjamshidi, M. Hatzopoulou, M. J. Roorda. 2021. An eco-score system incorporating driving behavior, vehicle characteristics, and traffic conditions. Transportation Research Part D, 95: 102866. https://doi.org/10.1016/j.trd.2021.102866
Wang, A., R. Tu, J. Xu, Z. Zhai, M. Hatzopoulou. 2021. A novel model emission modelling approach and its application with on-road emission measurements. Applied Energy, 306 (PA), 117967. https://doi.org/10.1016/j.apenergy.2021.117967
Talaat, H., J. Xu, M. Hatzopoulou, H. Abdelgawad. 2021. Mobile monitoring and spatial prediction of black carbon in Cairo, Egypt. Environmental Monitoring and Assessment, 193 (9). https://doi.org/10.1007/s10661-021-09351-0
Wang, A., J. Xu, R. Tu, M. Zhang, M. Adams, M. Hatzopoulou. 2021. Near-road air quality modelling that incorporates input variability and model uncertainty. Environmental Pollution, 284: 117145. DOI: 10.1016/j.envpol.2021.117145
Tu, R., J. Xu, A. Wang, Z. Zhai, M. Hatzopoulou. 2021. Effects of ambient temperature and cold starts on excess NOx emissions in a gasoline direct injection vehicle. Science of The Total Environment, 143402. https://doi.org/10.1016/j.scitotenv.2020.143402
2020
Xu, J., A. Wang, N. Schmidt, M. Adams, M. Hatzopoulou. 2020. A gradient boost approach for predicting near-road ultrafine particle concentrations using detailed traffic characterization. Environmental Pollution, 265: 114777. https://doi.org/10.1016/j.envpol.2020.114777
Xu, J., R. Tu, A. Wang, Z. Zhai, M. Hatzopoulou. 2020. Generation of spikes in ultrafine particle emissions from a gasoline direct injection vehicle during on-road emission tests. Environmental Pollution, 267: 115695. https://doi.org/10.1016/j.envpol.2020.115695
Wang, A., J. Xu, R. Tu, M. Saleh, M. Hatzopoulou. 2020. Potential of machine learning for prediction of traffic related air pollution. Transportation Research Part D, 88: 102599. https://doi.org/10.1016/j.trd.2020.102599
Zhai, Z., R. Tu, J. Xu, A. Wang, M. Hatzopoulou. 2020. Capturing the variability in instantaneous vehicle emissions based on field test data. Atmosphere (Basel), 11. https://doi.org/10.3390/atmos11070765
Xu, J., M. Saleh, M. Hatzopoulou. 2020. A machine learning approach capturing the effects of driving behaviour and driver characteristics on trip-level emissions. Atmospheric Environment, 224: 117311. https://doi.org/10.1016/j.atmosenv.2020.117311