Comparison of three functional regression methods on air pollution throughout the first two COVID-19 lockdown phases across 31 Iranian provinces
Abstract
Introduction: Exposure to air pollution heightens respiratory vulnerability, particularly during pandemics. The COVID-19 lockdowns in Iran provided a natural experiment to investigate how reduced human activity influenced air quality across 31 provinces. Understanding these environmental responses is vital for informing sustainable public health and pollution mitigation policies.
Materials and methods: Satellite-derived data on air pollutants and air quality and meteorological variables were obtained for all 31 provinces of Iran from Sentinel-5P, the GLDAS-2 dataset developed by National Aeronautics and Space Administration (NASA), and Google Earth Engine (GEE). The study covered two COVID-19 lockdown periods and their corresponding pre-pandemic periods from the previous year. The evaluated air quality indices consisted of Carbon monoxide (CO), Water Vapor (H₂O,) Nitrogen dioxide (NO₂), Ozone (O₃), Sulfur dioxide (SO₂), Absorbing Aerosol Index (AER), and Atmospheric Formaldehyde (HCHO). Meteorological covariates comprised temperature, pressure, precipitation, and wind speed. Sparse temporal data were reconstructed using FDA and FPCA, representing Functional Data Analysis and Functional Principal Component Analysis, respectively. Three Function-on-Function (FOF) regression models standard, smooth, and principal component based were developed, with and without meteorological adjustments. Model performance was assessed using R², AIC, and BIC, representing the coefficient of determination, Akaike Information Criterion, and Bayesian Information Criterion, respectively.
Results: Air pollutant levels significantly declined during both lockdowns compared with the corresponding pre-pandemic periods, with spatial variations influenced by meteorological and industrial factors. Incorporating meteorological covariates markedly improved model accuracy, particularly for NO₂ and CO. The principal component-based FOF model provided the best fit, explaining over 80% of variance in major pollutants.
Conclusion: COVID-19 lockdowns produced measurable, regionally heterogeneous improvements in air quality across Iran. Integrating meteorological adjustments and advanced functional regression approaches enhances environmental modeling and supports evidence-based air pollution control strategies during health emergencies.
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2. Wu X, Nethery RC, Sabath MB, Braun D, Dominici F. Air pollution and COVID-19 mortality in the United States: Strengths and limitations of an ecological regression analysis. Science advances. 2020;6(45):eabd4049.
3. Xie J, Zhu Y. Association between ambient temperature and COVID-19 infection in 122 cities from China. Science of the Total Environment. 2020;724:138201.
4. Zhu Y, Xie J, Huang F, Cao L. Association between short-term exposure to air pollution and COVID-19 infection: Evidence from China. Science of the total environment. 2020;727:138704.
5. Ma Y, Zhao Y, Liu J, He X, Wang B, Fu S, et al. Effects of temperature variation and humidity on the death of COVID-19 in Wuhan, China. Science of the total environment. 2020;724:138226.
6. Conticini E, Frediani B, Caro D. Can atmospheric pollution be considered a co-factor in extremely high level of SARS-CoV-2 lethality in Northern Italy? Environmental pollution. 2020;261:114465.
7. Ogen Y. Assessing nitrogen dioxide (NO2) levels as a contributing factor to coronavirus (COVID-19) fatality. Science of the Total Environment. 2020;726:138605.
8. Baniasad M, Mofrad MG, Bahmanabadi B, Jamshidi S. COVID-19 in Asia: Transmission factors, re-opening policies, and vaccination simulation. Environmental research. 2021;202:111657.
9. Pansini R, Fornacca D. Early spread of COVID-19 in the air-polluted regions of eight severely affected countries. Atmosphere. 2021;12(6):795.
10. Millett GA, Jones AT, Benkeser D, Baral S, Mercer L, Beyrer C, et al. Assessing differential impacts of COVID-19 on black communities. Annals of epidemiology. 2020;47:37-44.
11. Scafetta N. Distribution of the SARS-CoV-2 pandemic and its monthly forecast based on seasonal climate patterns. International journal of environmental research and public health. 2020;17(10):3493.
12. Shehzad K, Sarfraz M, Shah SGM. The impact of COVID-19 as a necessary evil on air pollution in India during the lockdown. Environmental Pollution. 2020;266:115080.
13. Mahato S, Pal S, Ghosh KG. Effect of lockdown amid COVID-19 pandemic on air quality of the megacity Delhi, India. Science of the total environment. 2020;730:139086.
14. Bashir MF, Ma B, Shahzad L. A brief review of socio-economic and environmental impact of Covid-19. Air Quality, Atmosphere & Health. 2020;13(12):1403-9.
15. Wang P, Chen K, Zhu S, Wang P, Zhang H. Severe air pollution events not avoided by reduced anthropogenic activities during COVID-19 outbreak. Resources, Conservation and Recycling. 2020;158:104814.
16. Lou B, Barbieri DM, Passavanti M, Hui C, Gupta A, Hoff I, et al. Air pollution perception in ten countries during the COVID-19 pandemic. Ambio. 2022;51(3):531-45.
17. Barbieri DM, Lou B, Passavanti M, Hui C, Lessa DA, Maharaj B, et al. Survey data regarding perceived air quality in Australia, Brazil, China, Ghana, India, Iran, Italy, Norway, South Africa, United States before and during Covid-19 restrictions. Data in brief. 2020;32:106169.
18. Filonchyk M, Hurynovich V, Yan H, Gusev A, Shpilevskaya N. Impact assessment of COVID-19 on variations of SO2, NO2, CO and AOD over East China. Aerosol and air quality research. 2020;20(7):1530-40.
19. Bherwani H, Kumar S, Musugu K, Nair M, Gautam S, Gupta A, et al. Assessment and valuation of health impacts of fine particulate matter during COVID-19 lockdown: a comprehensive study of tropical and sub tropical countries. Environmental Science and Pollution Research. 2021;28(32):44522-37.
20. Venter ZS, Aunan K, Chowdhury S, Lelieveld J. COVID-19 lockdowns cause global air pollution declines. Proceedings of the National Academy of Sciences. 2020;117(32):18984-90.
21. Fayaz M. The lock-down effects of COVID-19 on the air pollution indices in Iran and its neighbors. Modeling earth systems and environment. 2022:1-7.
22. Ramsay JO, Silverman B, . Functional Data Analysis: Springer New York, NY; 2005. XIX, 429 p.
23. Xiao L, Li C, Checkley W, Crainiceanu C. Fast covariance estimation for sparse functional data. Statistics and computing. 2018;28:511-22.
24. Crainiceanu CM, Goldsmith J, Leroux A, Cui E. Functional data analysis with R: CRC Press; 2024.
25. Fayaz M, Abadi A, Khodakarim S. The Functional Regression With Reconstructed Functions From Hybrid Principal Components Analysis: With EEG-fMRI Application. Statistics, Optimization & Information Computing. 2022;10(3):890-903.
26. Fayaz M. mohammad-fayaz GitHub profile 2026 [Available from: https://github.com/mohammad-fayaz.
27. Hunsicker ME, Kappel CV, Selkoe KA, Halpern BS, Scarborough C, Mease L, et al. Characterizing driver–response relationships in marine pelagic ecosystems for improved ocean management. Ecological applications. 2016;26(3):651-63.
28. Asna-ashary M, Farzanegan MR, Feizi M, Sadati SM. COVID-19 outbreak and air pollution in Iran: a panel VAR analysis. MAGKS Joint Discussion Paper Series in Economics; 2020.
29. Norouzi N, Asadi Z. Air pollution impact on the Covid-19 mortality in Iran considering the comorbidity (obesity, diabetes, and hypertension) correlations. Environmental Research. 2022;204:112020.
30. Shami S, Ranjgar B, Bian J, Khoshlahjeh Azar M, Moghimi A, Amani M, et al. Trends of CO and NO2 Pollutants in Iran during COVID-19 Pandemic Using Timeseries Sentinel-5 Images in Google Earth Engine. Pollutants. 2022;2(2):156-71.
31. Rad AK, Shariati M, Zarei M. The impact of COVID-19 on air pollution in Iran in the first and second waves with emphasis on the city of Tehran. Journal of Air Pollution and Health. 2020;5(3):181-92.
32. Moazeni M, Maracy MR, Dehdashti B, Ebrahimi A. Spatiotemporal analysis of COVID-19, air pollution, climate, and meteorological conditions in a metropolitan region of Iran. Environmental Science and Pollution Research. 2022;29(17):24911-24.
33. Roshan G, Sarli R, Fitchett JM. Urban heat island and thermal comfort of Esfahan City (Iran) during COVID-19 lockdown. Journal of Cleaner Production. 2022;352:131498.
34. Karimi B, Moradzadeh R, Samadi S. Air pollution and COVID-19 mortality and hospitalization: An ecological study in Iran. Atmospheric Pollution Research. 2022:101463.
35. Keshtkar M, Heidari H, Moazzeni N, Azadi H. Analysis of changes in air pollution quality and impact of COVID-19 on environmental health in Iran: application of interpolation models and spatial autocorrelation. Environmental Science and Pollution Research. 2022;29(25):38505-26.
36. Namdar-Khojasteh D, Yeghaneh B, Maher A, Namdar-Khojasteh F, Tu J. Assessment of the relationship between exposure to air pollutants and COVID-19 pandemic in Tehran city, Iran. Atmospheric Pollution Research. 2022;13(7):101474.
37. Khorsandi B, Farzad K, Tahriri H, Maknoon R. Association between short-term exposure to air pollution and COVID-19 hospital admission/mortality during warm seasons. Environmental Monitoring and Assessment. 2021;193(7):1-6.
38. Hashemi F, Hoepner L, Hamidinejad FS, Abbasi A, Afrashteh S, Hoseini M. A survey on the correlation between PM2. 5 concentration and the incidence of suspected and positive cases of COVID-19 referred to medical centers: A case study of Tehran. Chemosphere. 2022;301:134650.
39. Broomandi P, Crape B, Jahanbakhshi A, Janatian N, Nikfal A, Tamjidi M, et al. Assessment of the association between dust storms and COVID-19 infection rate in southwest Iran. Environmental Science and Pollution Research. 2022;29(24):36392-411.
40. Anbari K, Sicard P, Omidi Khaniabadi Y, Raja Naqvi H, Rashidi R. Assessing the effect of COVID-19 pandemic on air quality change and human health outcomes in a capital city, southwestern Iran. International Journal of Environmental Health Research. 2022:1-12.
41. Rashidi R, Khaniabadi YO, Sicard P, De Marco A, Anbari K. Ambient PM2. 5 and O3 pollution and health impacts in Iranian megacity. Stochastic Environmental Research and Risk Assessment. 2022:1-10.
42. Aboubakri O, Ballester J, Shoraka HR, Karamoozian A, Golchini E. Ambient temperature and Covid-19 transmission: An evidence from a region of Iran based on weather station and satellite data. Environmental Research. 2022;209:112887.
43. Ritchie H, Mathieu E, Rodés-Guirao L, Appel C, Giattino C, Ortiz-Ospina E, et al. Coronavirus pandemic (COVID-19). Our world in data. 2020.
44. Wang H, Tan J, Li X. Global NO 2 dynamics During the COVID-19 pandemic: a comparison between two waves of the coronavirus. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing. 2021;14:4310-20.
45. Bañuelos Gimeno J, Blanco A, Díaz J, Linares C, López J, Navas M, et al. Air pollution and meteorological variables’ effects on COVID-19 first and second waves in Spain. International Journal of Environmental Science and Technology. 2022:1-14.
46. Linares C, Culqui D, Belda F, López-Bueno JA, Luna Y, Sánchez-Martínez G, et al. Impact of environmental factors and Sahara dust intrusions on incidence and severity of COVID-19 disease in Spain. Effect in the first and second pandemic waves. Environmental Science and Pollution Research. 2021;28(37):51948-60.
47. Dutta A, Dutta G. Association of air pollution and meteorological variables with the two waves of COVID-19 pandemic in Delhi: A critical analysis. Heliyon. 2021;7(11):e08468.
48. Boroujeni M, Saberian M, Li J. Environmental impacts of COVID-19 on Victoria, Australia, witnessed two waves of Coronavirus. Environmental Science and Pollution Research. 2021;28(11):14182-91.
49. Aghelpour P, Singh VP, Varshavian V. Time series prediction of seasonal precipitation in Iran, using data-driven models: a comparison under different climatic conditions. Arabian Journal of Geosciences. 2021;14(7):1-14.
2. Wu X, Nethery RC, Sabath MB, Braun D, Dominici F. Air pollution and COVID-19 mortality in the United States: Strengths and limitations of an ecological regression analysis. Science advances. 2020;6(45):eabd4049.
3. Xie J, Zhu Y. Association between ambient temperature and COVID-19 infection in 122 cities from China. Science of the Total Environment. 2020;724:138201.
4. Zhu Y, Xie J, Huang F, Cao L. Association between short-term exposure to air pollution and COVID-19 infection: Evidence from China. Science of the total environment. 2020;727:138704.
5. Ma Y, Zhao Y, Liu J, He X, Wang B, Fu S, et al. Effects of temperature variation and humidity on the death of COVID-19 in Wuhan, China. Science of the total environment. 2020;724:138226.
6. Conticini E, Frediani B, Caro D. Can atmospheric pollution be considered a co-factor in extremely high level of SARS-CoV-2 lethality in Northern Italy? Environmental pollution. 2020;261:114465.
7. Ogen Y. Assessing nitrogen dioxide (NO2) levels as a contributing factor to coronavirus (COVID-19) fatality. Science of the Total Environment. 2020;726:138605.
8. Baniasad M, Mofrad MG, Bahmanabadi B, Jamshidi S. COVID-19 in Asia: Transmission factors, re-opening policies, and vaccination simulation. Environmental research. 2021;202:111657.
9. Pansini R, Fornacca D. Early spread of COVID-19 in the air-polluted regions of eight severely affected countries. Atmosphere. 2021;12(6):795.
10. Millett GA, Jones AT, Benkeser D, Baral S, Mercer L, Beyrer C, et al. Assessing differential impacts of COVID-19 on black communities. Annals of epidemiology. 2020;47:37-44.
11. Scafetta N. Distribution of the SARS-CoV-2 pandemic and its monthly forecast based on seasonal climate patterns. International journal of environmental research and public health. 2020;17(10):3493.
12. Shehzad K, Sarfraz M, Shah SGM. The impact of COVID-19 as a necessary evil on air pollution in India during the lockdown. Environmental Pollution. 2020;266:115080.
13. Mahato S, Pal S, Ghosh KG. Effect of lockdown amid COVID-19 pandemic on air quality of the megacity Delhi, India. Science of the total environment. 2020;730:139086.
14. Bashir MF, Ma B, Shahzad L. A brief review of socio-economic and environmental impact of Covid-19. Air Quality, Atmosphere & Health. 2020;13(12):1403-9.
15. Wang P, Chen K, Zhu S, Wang P, Zhang H. Severe air pollution events not avoided by reduced anthropogenic activities during COVID-19 outbreak. Resources, Conservation and Recycling. 2020;158:104814.
16. Lou B, Barbieri DM, Passavanti M, Hui C, Gupta A, Hoff I, et al. Air pollution perception in ten countries during the COVID-19 pandemic. Ambio. 2022;51(3):531-45.
17. Barbieri DM, Lou B, Passavanti M, Hui C, Lessa DA, Maharaj B, et al. Survey data regarding perceived air quality in Australia, Brazil, China, Ghana, India, Iran, Italy, Norway, South Africa, United States before and during Covid-19 restrictions. Data in brief. 2020;32:106169.
18. Filonchyk M, Hurynovich V, Yan H, Gusev A, Shpilevskaya N. Impact assessment of COVID-19 on variations of SO2, NO2, CO and AOD over East China. Aerosol and air quality research. 2020;20(7):1530-40.
19. Bherwani H, Kumar S, Musugu K, Nair M, Gautam S, Gupta A, et al. Assessment and valuation of health impacts of fine particulate matter during COVID-19 lockdown: a comprehensive study of tropical and sub tropical countries. Environmental Science and Pollution Research. 2021;28(32):44522-37.
20. Venter ZS, Aunan K, Chowdhury S, Lelieveld J. COVID-19 lockdowns cause global air pollution declines. Proceedings of the National Academy of Sciences. 2020;117(32):18984-90.
21. Fayaz M. The lock-down effects of COVID-19 on the air pollution indices in Iran and its neighbors. Modeling earth systems and environment. 2022:1-7.
22. Ramsay JO, Silverman B, . Functional Data Analysis: Springer New York, NY; 2005. XIX, 429 p.
23. Xiao L, Li C, Checkley W, Crainiceanu C. Fast covariance estimation for sparse functional data. Statistics and computing. 2018;28:511-22.
24. Crainiceanu CM, Goldsmith J, Leroux A, Cui E. Functional data analysis with R: CRC Press; 2024.
25. Fayaz M, Abadi A, Khodakarim S. The Functional Regression With Reconstructed Functions From Hybrid Principal Components Analysis: With EEG-fMRI Application. Statistics, Optimization & Information Computing. 2022;10(3):890-903.
26. Fayaz M. mohammad-fayaz GitHub profile 2026 [Available from: https://github.com/mohammad-fayaz.
27. Hunsicker ME, Kappel CV, Selkoe KA, Halpern BS, Scarborough C, Mease L, et al. Characterizing driver–response relationships in marine pelagic ecosystems for improved ocean management. Ecological applications. 2016;26(3):651-63.
28. Asna-ashary M, Farzanegan MR, Feizi M, Sadati SM. COVID-19 outbreak and air pollution in Iran: a panel VAR analysis. MAGKS Joint Discussion Paper Series in Economics; 2020.
29. Norouzi N, Asadi Z. Air pollution impact on the Covid-19 mortality in Iran considering the comorbidity (obesity, diabetes, and hypertension) correlations. Environmental Research. 2022;204:112020.
30. Shami S, Ranjgar B, Bian J, Khoshlahjeh Azar M, Moghimi A, Amani M, et al. Trends of CO and NO2 Pollutants in Iran during COVID-19 Pandemic Using Timeseries Sentinel-5 Images in Google Earth Engine. Pollutants. 2022;2(2):156-71.
31. Rad AK, Shariati M, Zarei M. The impact of COVID-19 on air pollution in Iran in the first and second waves with emphasis on the city of Tehran. Journal of Air Pollution and Health. 2020;5(3):181-92.
32. Moazeni M, Maracy MR, Dehdashti B, Ebrahimi A. Spatiotemporal analysis of COVID-19, air pollution, climate, and meteorological conditions in a metropolitan region of Iran. Environmental Science and Pollution Research. 2022;29(17):24911-24.
33. Roshan G, Sarli R, Fitchett JM. Urban heat island and thermal comfort of Esfahan City (Iran) during COVID-19 lockdown. Journal of Cleaner Production. 2022;352:131498.
34. Karimi B, Moradzadeh R, Samadi S. Air pollution and COVID-19 mortality and hospitalization: An ecological study in Iran. Atmospheric Pollution Research. 2022:101463.
35. Keshtkar M, Heidari H, Moazzeni N, Azadi H. Analysis of changes in air pollution quality and impact of COVID-19 on environmental health in Iran: application of interpolation models and spatial autocorrelation. Environmental Science and Pollution Research. 2022;29(25):38505-26.
36. Namdar-Khojasteh D, Yeghaneh B, Maher A, Namdar-Khojasteh F, Tu J. Assessment of the relationship between exposure to air pollutants and COVID-19 pandemic in Tehran city, Iran. Atmospheric Pollution Research. 2022;13(7):101474.
37. Khorsandi B, Farzad K, Tahriri H, Maknoon R. Association between short-term exposure to air pollution and COVID-19 hospital admission/mortality during warm seasons. Environmental Monitoring and Assessment. 2021;193(7):1-6.
38. Hashemi F, Hoepner L, Hamidinejad FS, Abbasi A, Afrashteh S, Hoseini M. A survey on the correlation between PM2. 5 concentration and the incidence of suspected and positive cases of COVID-19 referred to medical centers: A case study of Tehran. Chemosphere. 2022;301:134650.
39. Broomandi P, Crape B, Jahanbakhshi A, Janatian N, Nikfal A, Tamjidi M, et al. Assessment of the association between dust storms and COVID-19 infection rate in southwest Iran. Environmental Science and Pollution Research. 2022;29(24):36392-411.
40. Anbari K, Sicard P, Omidi Khaniabadi Y, Raja Naqvi H, Rashidi R. Assessing the effect of COVID-19 pandemic on air quality change and human health outcomes in a capital city, southwestern Iran. International Journal of Environmental Health Research. 2022:1-12.
41. Rashidi R, Khaniabadi YO, Sicard P, De Marco A, Anbari K. Ambient PM2. 5 and O3 pollution and health impacts in Iranian megacity. Stochastic Environmental Research and Risk Assessment. 2022:1-10.
42. Aboubakri O, Ballester J, Shoraka HR, Karamoozian A, Golchini E. Ambient temperature and Covid-19 transmission: An evidence from a region of Iran based on weather station and satellite data. Environmental Research. 2022;209:112887.
43. Ritchie H, Mathieu E, Rodés-Guirao L, Appel C, Giattino C, Ortiz-Ospina E, et al. Coronavirus pandemic (COVID-19). Our world in data. 2020.
44. Wang H, Tan J, Li X. Global NO 2 dynamics During the COVID-19 pandemic: a comparison between two waves of the coronavirus. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing. 2021;14:4310-20.
45. Bañuelos Gimeno J, Blanco A, Díaz J, Linares C, López J, Navas M, et al. Air pollution and meteorological variables’ effects on COVID-19 first and second waves in Spain. International Journal of Environmental Science and Technology. 2022:1-14.
46. Linares C, Culqui D, Belda F, López-Bueno JA, Luna Y, Sánchez-Martínez G, et al. Impact of environmental factors and Sahara dust intrusions on incidence and severity of COVID-19 disease in Spain. Effect in the first and second pandemic waves. Environmental Science and Pollution Research. 2021;28(37):51948-60.
47. Dutta A, Dutta G. Association of air pollution and meteorological variables with the two waves of COVID-19 pandemic in Delhi: A critical analysis. Heliyon. 2021;7(11):e08468.
48. Boroujeni M, Saberian M, Li J. Environmental impacts of COVID-19 on Victoria, Australia, witnessed two waves of Coronavirus. Environmental Science and Pollution Research. 2021;28(11):14182-91.
49. Aghelpour P, Singh VP, Varshavian V. Time series prediction of seasonal precipitation in Iran, using data-driven models: a comparison under different climatic conditions. Arabian Journal of Geosciences. 2021;14(7):1-14.
| Files | ||
| Issue | Vol 11 No 2 (2026): Spring 2026 | |
| Section | Original Research | |
| Keywords | ||
| Air pollution; COVID-19 restrictions; Functional principal component analysis (FPCA); Function on function (FOF) regression; Meteorological covariates; Iran | ||
| Rights and permissions | |
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This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License. |
How to Cite
1.
Fayaz M. Comparison of three functional regression methods on air pollution throughout the first two COVID-19 lockdown phases across 31 Iranian provinces. JAPH. 2026;11(2):217-230.
