Е-Публикации
Технически университет - София

Abstract: This study evaluates whether the number of registered passenger cars (motorization rate) can be used as a standalone proxy indicator for local climate-related variables. Using official datasets for Bulgaria and its capital city, Sofia, over the period 2004–2022, the analysis examines the relationship between vehicle registration counts and two aggregated climate indicators: average annual air temperature and average annual precipitation. Correlation and regression analyses reveal weak and statistically non-significant associations with temperature, along with negligible explanatory power for precipitation. These findings indicate that vehicle registration counts alone are insufficient for explaining or monitoring variation in aggregated climate-related variables. Building on this negative result, the study outlines a methodological framework for future transport–climate research based on activity- and composition-oriented indicators, including vehicle-kilometres travelled, fleet structure, and higher temporal resolution data.

References

1. ACEA (2024), The Automobile Industry Pocket Guide 2024, Available online: https://www.acea.auto/publication/the-automobile-industry-pocket-guide-2024-2025/ (accessed March 2026).
2. European Environment Agency (EEA), (2024.). National emissions reported to the UNFCCC and to the EU under the Governance Regulation. EEA. https://doi.org/10.2909/6331f651-8863-4656-a911-669f2a332a1e.
3. European Environment Agency (EEA). Annual European Union greenhouse gas inventory 1990–2022 and inventory report 2024. EEA Report No 05/2024. European Environment Agency, Copenhagen, 2024. https://doi.org/10.2800/400289.
4. Allister S., Chen J., Fernandez-Pello C., Fundamentals of Combustion Processes, Springer (2011), https://doi.org/10.1007/978-1-4419-7943-8
5. Barkenbus, J.N., 2020. Prospects for Electric Vehicles. Sustainability, 12, 5813. https://doi.org/10.3390/su12145813.
6. Barlow, T.J., Latham, S.; McCrae, I.S.; Boulter, P.G., 2026. A Reference Book of Driving Cycles for Use in the Measurement of Road Vehicle Emissions. TRL Report PPR354; TRL Limited: Wokingham, UK, (2009). Available online: https://assets.publishing.service.gov.uk/media/5a7984f440f0b642860d8c2d/ppr-354.pdf (accessed on 01 March 2026).
7. Baumgarten C., Mixture Formation in Internal Combustion Engines, Springer (2006), https://doi.org/10.1007/3-540-30836-9
8. Berkowicz, R., Winther, M., M. Ketzel, Traffic pollution modelling and emission data, Environ. Modell. Software, Volume 21, Issue 4 (2006), 454–460, ISSN 1364-8152, https://doi.org/10.1016/j.envsoft.2004.06.013.
9. Bhardwaj, A., Iyer, S.R., Ramesh, S., White, J., Subramanian, L., 2023. Understanding sudden traffic jams: From emergence to impact, Develop. Eng., 8, 100105, ISSN 2352-7285,.
10. J. Carey, The other benefit of electric vehicles, Proc. Natl. Acad. Sci. U.S.A. 120 (3) e2220923120, https://doi.org/10.1073/pnas.2220923120 (2023).
11. European Commission, 2026. CO2emission performance standards for cars and vans. Available online: https://climate.ec.europa.eu/eu-action/transport-decarbonisation/road-transport/cars-and-vans_en (accessed March 2026).
12. Council Directive 91/441/EEC of 26 June 1991 amending Directive 70/220/EEC on the approximation of the laws of the Member States relating to measures to be taken against air pollution by emissions from motor vehicles, Available online: http://data.europa.eu/eli/dir/1991/441/oj (accessed March 2026).
13. Dimitrov, E., Tashev, A., 2022. CNG effect on performance parameters of diesel engine operated on dual-fuel mode. AIP Conf. Proc., 2449 (1): 050009. https://doi.org/10.1063/5.0091441.
14. Directive 2014/45/EU of the European Parliament and of the Council of 3 April 2014 on periodic roadworthiness tests for motor vehicles and their trailers and repealing Directive 2009/40/EC Text with EEA relevance, Available online: http://data.europa.eu/eli/dir/2014/45/oj (accessed March 2026).
15. D.J. Easterbrook, Chapter 9 - Greenhouse Gases, Evidence-Based Climate Science (Second Edition), Elsevier, 2016, ISBN 9780128045886, https://doi.org/10.1016/B978-0-12-804588-6.00009-4.
16. Edenhofer, Ottmar, ed. (2015). Climate change 2014: mitigation of climate change. Vol. 3. Cambridge University Press https://doi.org/10.1017/CBO9781107415416.
17. El-Mahallawy, F., S. E-Din Habik, 2002. Fundamentals and Technology of Combustion, Elsevier, ISBN: 978-0-08-044106-1, eBook ISBN: 9780080532189.
18. European Environment Agency (EEA), (2026). EMEP/EEA. Air pollutant emission inventory guidebook 2019. Luxembourg: Publications Office of the European Union, 2019. Available online: https://www.eea.europa.eu/en/analysis/publications/emep-eea-guidebook-2019 (accessed March 2026).
19. European Environment Agency (EEA), 2026. Passenger cars per thousand inhabitants. Dataset. https://doi.org/10.2908/ROAD_EQS_CARHAB. Last update: 12 March 2025 (accessed March 2026).
20. European Environment Agency (EEA), 2026. Greenhouse gas emissions by source sector. Available online: https://www.eea.europa.eu/data-and-maps/data/data-viewers/greenhouse-gases-viewer (accessed March 2026).
21. Executive Environment Agency (EEA Bulgaria), (2026). Bulgaria’s Informative Inventory Report 2022 (IIR). Available online: https://eea.government.bg/bg/dokladi/BG_IIR_submission2022_2020_15March2022.pdf (accessed March 2026).
22. Executive Environment Agency (EEA Bulgaria), 2026. National Inventory Report 2022: Greenhouse Gas Emissions in Bulgaria 1988–2020. Available online: https://old.eea.government.bg/bg/dokladi/BG_NIR_15April_2022.pdf/view (accessed March 2026).
23. Mikalai Filonchyk, Michael P. Peterson, Lifeng Zhang, Volha Hurynovich, Yi He, Greenhouse gases emissions and global climate change: Examining the influence of CO2, CH4, and N2O, Science of The Total Environment, Volume 935 (2024), https://doi.org/10.1016/j.scitotenv.2024.173359.
24. Funazaki, A., Taneda, K., Tahara, K., Inaba, A., 2003. Automobile life cycle assessment issues at end-of-life and recycling, JSAE Review, 24, 4 Pages 381-386, ISSN 0389-4304, https://doi.org/10.1016/S0389-4304(03)00081-X.
25. Gu, H.; Hu, Q.; Zhu, D.; Diao, J.; Liu, Y.; Fang, M. Research on Outdoor Thermal Comfort of Children’s Activity Space in High-Density Urban Residential Areas of Chongqing in Summer. Atmosphere (2022), 13, 2016. https://doi.org/10.3390/atmos13122016.
26. Houghton J (2009) Global warming: the complete briefing. Cambridge University Press, Cambridge, https://doi.org/10.1017/CBO9780511841590.
27. Huang, Y., Surawski, N., Organ, B., Zhou J., Tang O., Chan E., Fuel consumption and emissions performance under real driving: comparison between hybrid and conventional vehicles, Sci. Total Environ., 659 (2019), Pages 275-282, ISSN 0048-9697, https://doi.org/10.1016/j.scitotenv.2018.12.349.
28. IEA. Global EV Outlook 2024. Paris: International Energy Agency, 2024. Available online: https://www.iea.org/reports/global-ev-outlook-2024, Licence: CC BY 4.0.
29. Intergovernmental Panel on Climate Change (IPCC). Climate Change 2021 – The Physical Science Basis: Working Group I Contribution to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, 2023,.
30. Kawamoto, R.; Mochizuki, H.; Moriguchi, Y.; Nakano, T.; Motohashi, M.; Sakai, Y.; Inaba, A. Estimation of CO2 Emissions of Internal Combustion Engine Vehicle and Battery Electric Vehicle Using LCA. Sustainability (2019), 11, 2690. https://doi.org/10.3390/su11092690.
31. Kent, J.L., 2014. Driving to save time or saving time to drive? The enduring appeal of the private car, Transportation Research Part A: Policy and Practice, 65, 103–115, ISSN 0965-8564, https://doi.org/10.1016/j.tra.2014.04.009.
32. Kirkpatrick, A., 2020. Internal Combustion Engines: Applied Thermosciences, Online ISBN:9781119454564, September, https://doi.org/10.1002/9781119454564.ch1.
33. Kosai, S., Matsui, K., Matsubae, K., Yamasue, E., Nagasaka, T., 2021. Natural resource use of gasoline, hybrid, electric and fuel cell vehicles considering land disturbances, Resour., Conservat. Recycl. 166 (2021), ISSN 0921-3449, https://doi.org/10.1016/j.resconrec.2020.105256.
34. Koźlak, A.; Wach, D. Causes of traffic congestion in urban areas: Case of Poland. SHS Web Conf. (2018), 57, 01019. https://doi.org/10.1051/shsconf/20185701019.
35. Leach, F., Gautam Kalghatgi, Richard Stone, Paul Miles, 2020. The scope for improving the efficiency and environmental impact of internal combustion engines, Transport. Eng., 1 100005, ISSN 2666-691X, https://doi.org/10.1016/j.treng.2020.100005.
36. C. Linares, J. Díaz, Impact of high temperatures on hospital admissions: comparative analysis with previous studies about mortality (Madrid), European Journal of Public Health, Volume 18, Issue 3, June (2008), Pages 317–322, https://doi.org/10.1093/eurpub/ckm108.
37. Litman, T. Congestion Costing and Congestion Pricing. Transportation Research Part A: Policy and Practice, Volume 136 (2020), Pages 1–14. https://doi.org/10.1016/j.tra.2020.03.012
38. Mamalis, A.G., Spentzas, K.N. & Mamali, A.A. The impact of automotive industry and its supply chain to climate change: Somme techno-economic aspects. Eur. Transp. Res. Rev. 5, 1–10 (2013). https://doi.org/10.1007/s12544-013-0089-x.
39. Meng, Y., Wang J., Han C., Feng Z., Cao S., 2023. Investigation of heat stress on urban roadways for commuting children and mitigation strategies from the perspective of urban design, Urban Climate, Volume 49, ISSN 2212-0955, https://doi.org/10.1016/j.uclim.2023.101564.
40. Michel, A., Robert, J., Robert, V., Patrick, T., Pascal, P., Real-world European driving cycles, for measuring pollutant emissions from high- and low-powered cars, Atmos. Environ., 40, 31, (2006), Pages 5944-5953, ISSN 1352-2310, https://doi.org/10.1016/j.atmosenv.2005.12.057.
41. Michel, A., 2004. The ARTEMIS European driving cycles for measuring car pollutant emissions, Sci. Total Environ., 334–335, Pages 73-84, ISSN 0048-9697, https://doi.org/10.1016/j.scitotenv.2004.04.070.
42. Sofia Municipality, 2026. Final Greenhouse Gas Emission Inventory for the Territory of Sofia Municipality for 2020. Sofia, 2022. Available online: https://www.sofia.bg/climat (accessed March 2026).
43. National Statistical Institute (NSI), 2026, Bulgaria. Statistical Yearbook 2023. Available online: https://www.nsi.bg/sites/default/files/files/publications/God2023.pdf (accessed March 2026).
44. National Statistical Institute (NSI), Bulgaria, 2026. Population and vehicle statistics database. Available online: https://www.nsi.bg/bg (accessed March 2026).
45. National Statistical Institute (NSI), 2026. Bulgaria. Statistical Yearbook 2014. Available online: https://www.nsi.bg/sites/default/files/files/publications/god2014.pdf (accessed March 2026).
46. ndre, M., Hickman, A. J., Hassel, D., & Joumard, R. (1995). Driving Cycles for Emission Measurements Under European Conditions. SAE Transactions, 104, 562–574. https://doi.org/10.4271/950926
47. Francis Ostermeijer, Hans R A Koster, Jos van Ommeren, Victor Mayland Nielsen, Automobiles and urban density, Journal of Economic Geography, Volume 22, Issue 5, September (2022), Pages 1073–1095, https://doi.org/10.1093/jeg/lbab047.
48. Paramita, P.D.P.; Daniarta, S.; Imre, A.R.; Kolasiński, P. Techno-Economic Analysis of Waste Heat Recovery in Automotive Manufacturing Plants. Appl. Sci. (2025), 15, 569. https://doi.org/10.3390/app15020569.
49. Pavlínek, P. Transition of the automotive industry towards electric vehicle production in the east European integrated periphery. Empirica 50, 35–73 (2023). https://doi.org/10.1007/s10663-022-09554-9.
50. Petronijević, V.; Đorđević, A.; Stefanović, M.; Arsovski, S.; Krivokapić, Z.; Mišić, M. Energy Recovery through End-of-Life Vehicles Recycling in Developing Countries. Sustainability (2020), 12, 8764. https://doi.org/10.3390/su12218764.
51. Ravi, S.S.; Osipov, S.; Turner, J.W.G. Impact of Modern Vehicular Technologies and Emission Regulations on Improving Global Air Quality. Atmosphere (2023), 14, 1164. https://doi.org/10.3390/atmos14071164.
52. Regulation (EC) No 715/2007 of the European Parliament and of the Council of 20 June 2007 on type approval of motor vehicles with respect to emissions from light passenger and commercial vehicles (Euro 5 and Euro 6) and on access to vehicle repair and maintenance information, Available online: http://data.europa.eu/eli/reg/2007/715/oj. (accessed March 2026).
53. Regulation (EU) 2024/1257 of the European Parliament and of the Council of 24 April 2024 on type-approval of motor vehicles and engines and of systems, components and separate technical units intended for such vehicles, with respect to their emissions and battery durability (Euro 7), amending Regulation (EU) 2018/858 of the European Parliament and of the Council and repealing Regulations (EC) No 715/2007 and (EC) No 595/2009 of the European Parliament and of the Council, Commission Regulation (EU) No 582/2011, Commission Regulation (EU) 2017/1151, Commission Regulation (EU) 2017/2400 and Commission Implementing Regulation (EU) 2022/1362, http://data.europa.eu/eli/reg/2024/1257/oj.
54. Ren, C.; Yu, C.W.; Cao, S.-J. Development of urban air environmental control policies and measures. Indoor Built Environ (2023), 32(2), 299–304. https://doi.org/10.1177/1420326X221120380.
55. Richards, P., Barker, J. (2023). Automotive Fuels Reference Book, 4th ed., SAE International: Warrendale, PA, USA, https://doi.org/10.4271/9781468605792.
56. Romanello, Marina et al., The 2024 report of the Lancet Countdown on health and climate change: facing record-breaking threats from delayed action, The Lancet, Volume 404, Issue 10465, 1847 - 1896. https://doi.org/10.1016/S0140-6736(24)01822-1
57. Sato, F.E.K.; Nakata, T. Energy Consumption Analysis for Vehicle Production through a Material Flow Approach. Energies (2020), 13, 2396. https://doi.org/10.3390/en13092396
58. Sharma, R.; Kumar, R.; Singh, P.K.; Raboaca, M.S.; Felseghi, R.-A. A Systematic Study on the Analysis of the Emission of CO, CO2 and HC for Four-Wheelers and Its Impact on the Sustainable Ecosystem. Sustainability (2020), 12, 6707. https://doi.org/10.3390/su12176707.
59. Silva, J.A.M., Flórez-Orrego, D., Oliveira, S., 2014. An exergy based approach to determine production cost and CO2 allocation for petroleum derived fuels, Energy, 67 490–495, ISSN 0360-5442, https://doi.org/10.1016/j.energy.2014.02.022.
60. Tashev, A.; Dimitrov, E. Investigation of the impact of compressed natural gas on heat release rate in dual-fuel operation of a D3900 diesel engine at mid loads. ETR (2024), 1, 368–372. https://doi.org/10.17770/etr2024vol1.7946.
61. Todorov, G.; Kralov, I.; Koprev, I.; Vasilev, H.; Naydenova, I. Coal Share Reduction Options for Power Generation during the Energy Transition: A Bulgarian Perspective Energies (2024), 17, 929. https://doi.org/10.3390/en17040929
62. UNFCCC, 2026. Bulgaria 2021 Common Reporting Format (CRF) Table. Available online: https://unfccc.int/documents/273452 (accessed March 2026).
63. Wang, L., Yu, Y., Kai Huang, Zhiqi Zhang, Xi Li, 2020. The inharmonious mechanism of CO2, NOx, SO2, and PM2.5electric vehicle emission reductions in Northern China, J. Environ. Manage., 274, ISSN 0301-4797, https://doi.org/10.1016/j.jenvman.2020.111236.
64. Wang, A., Ren, C., Wang, J., Feng, Z., Kumar, P., Haghighat F., Cao S., Health assessment and mitigating solutions to heat-pollution induced by urban traffic, J. Clean. Product., 434 (2024), ISSN 0959-6526, https://doi.org/10.1016/j.jclepro.2023.140097.
65. Yagi, M., Managi, S., Demographic determinants of car ownership in Japan, Transport Policy, Volume 50, (2016), Pages 37-53, ISSN 0967-070X, https://doi.org/10.1016/j.tranpol.2016.05.011.
66. Yang, Z., Jia, P., Liu, W., Yin, H., Car ownership and urban development in Chinese cities: A panel data analysis, Journal of Transport Geography, Volume 58, (2017), Pages 127-134, ISSN 0966-6923, https://doi.org/10.1016/j.jtrangeo.2016.11.015.
67. Yu, X., Sandhu, N.S., Zhenyi Yang, Ming Zheng, Suitability of energy sources for automotive application A review, Appl. Energy, 271 (2020), 115169, ISSN 0306-2619, https://doi.org/10.1016/j.apenergy.2020.115169.
68. Zhang, Z.; Chai, H.; Wu, L.; Zhang, N.; Wu, F. Automobile-Demand Forecasting Based on Trend Extrapolation and Causality Analysis. Electronics (2024), 13, 3294, https://doi.org/10.3390/electronics13163294.
69. Zhu, R., Wong, M.S., Guilbert, É. et al. Understanding heat patterns produced by vehicular flows in urban areas. Sci Rep 7, 16309 (2017). https://doi.org/10.1038/s41598-017-15869-6.
70. Zimakowska-Laskowska M, Laskowski P. Comparison of pollutant emissions from various types of vehicles. Combustion Engines, (2024);197(2):139-145. https://doi.org/10.19206/CE-181193.

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