Aerosol optical depth regime over megacities of the world
Authors: Papachristopoulou Kyriakoula , Raptis Ioannis-Panagiotis , Gkikas Antonis , Fountoulakis Ilias , Masoom Akriti , Kazadzis Stelios 
Published: December 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 a 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 a population over 5 million, relying on daily satellite-based aerosol optical depth
(AOD) retrievals, derived at fine spatial resolution
(0.1∘×0.1∘), 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 center, while the opposite was found for most of
the cities in the rest of the world. High intraannual 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 of
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 of
“sustainable cities and communities”, dealing with urban air quality.