Aerosol optical depth regime over Megacities of the world
Authors: Papachristopoulou Kyriakoula , Raptis Ioannis-Panagiotis , Gkikas Antonis , Fountoulakis Ilias , Masoom Akriti , Kazadzis Stelios 
Published: September 2022

Abstract. Currently, 55 % of the world’s population resides in urban areas and this number is projected to increase to 70 % by 2050. Urban agglomerations with population over 10 million, characterized as Megacities, are expected to be more than 100 by 2100. Such large concentrations of population could boost creativity and economic progress but also raises several environmental challenges such as air quality degradation. In this study we investigate the spatial and temporal variability of urban aerosol state of 81 cities with population over 5 million, relying on daily satellite-based aerosol optical depth (AOD) retrievals, derived at fine spatial resolution (0.1°x0.1°), over an 18-year period spanning from 2003 to 2020. According to our results, the lowest long-term mean AOD values worldwide were found in European and American cities (from 0.08 to 0.20). For almost all African and Asian cities, mean AOD ranged from 0.25 up to 0.90, but a considerable dust aerosol contribution (up to 70 %) was found for some of them with associated mean Dust Optical Depth (DOD) values reaching up to 0.4. Mostly Chinese and Indian cities tend to have higher mean AOD values in the areas surrounding their centre, while the opposite was found for most of the cities in the rest of the world. High intra-annual AOD variability was revealed for the eastern American cities, while lower values were found in Chinese, eastern Indian and the eastern Mediterranean cities. During the study period, statistically significant negative AOD decadal trends were found for East Asian, European and North American cities with the greatest decrease of -0.1 to -0.3 per decade recorded for the Chinese cities, in which the maximum mean AODs (0.45–0.91) are observed. In most of the US cities, where low mean AOD <0.17 was recorded, considerable declining AOD trends were found (-30 to -50 % per decade). For the rest Asian, African and South American cities statistically significant AOD increase was found, with the greatest values of +0.07 to +0.16 per decade recorded for Indian cities. In Bengaluru (India), it is reported the lowest mean AOD value (0.2) and the maximum AOD increase (+69 %), which may be partially attributed to the population growth over the study period. The agreement of the satellite derived AOD trends against those obtained from ground-based AERONET measurements was examined. For ground-based stations within the geographical limits of the contiguous urban area of the examined cities, a 0.93 correlation for the long-term means of AOD was found and ~75 % of the derived trends agreed in sign. It was found that the spatial homogeneity within the examined satellite domain and the location of the surface station were key factors that determined their agreement. The present study highlights the vital and essential contribution of spaceborne products to monitor aerosol burden over megacities of the planet towards fulfilling the United Nations Sustainable Development Goal “Sustainable cities and communities”, dealing with urban air quality.