Preprints
https://doi.org/10.5194/acp-2022-617
https://doi.org/10.5194/acp-2022-617
 
29 Sep 2022
29 Sep 2022
Status: a revised version of this preprint was accepted for the journal ACP and is expected to appear here in due course.

Aerosol optical depth regime over Megacities of the world

Kyriakoula Papachristopoulou1,2,3,4, Ioannis-Panagiotis Raptis1, Antonis Gkikas2, Ilias Fountoulakis2, Akriti Masoom4, and Stelios Kazadzis4 Kyriakoula Papachristopoulou et al.
  • 1Laboratory of Climatology and Atmospheric Environment, Sector of Geography and Climatology, Department of Geology and Environment, National and Kapodistrian University of Athens, Athens, GR-15784, Greece
  • 2Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, National Observatory of Athens, Athens, GR-15236, Greece
  • 3Department of Physics, ETH Zurich, Zurich, CH-8093, Switzerland
  • 4Physics and Meteorological Observatory Davos, World Radiation Center, Davos, CH-7260, Switzerland

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.

Kyriakoula Papachristopoulou et al.

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-617', Anonymous Referee #1, 21 Oct 2022
    • AC1: 'Reply on RC1', Kyriakoula Papachristopoulou, 14 Nov 2022
  • RC2: 'Comment on acp-2022-617', Anonymous Referee #2, 25 Oct 2022
    • AC2: 'Reply on RC2', Kyriakoula Papachristopoulou, 14 Nov 2022

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-617', Anonymous Referee #1, 21 Oct 2022
    • AC1: 'Reply on RC1', Kyriakoula Papachristopoulou, 14 Nov 2022
  • RC2: 'Comment on acp-2022-617', Anonymous Referee #2, 25 Oct 2022
    • AC2: 'Reply on RC2', Kyriakoula Papachristopoulou, 14 Nov 2022

Kyriakoula Papachristopoulou et al.

Kyriakoula Papachristopoulou et al.

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Short summary
Megacities air quality is determined by atmospheric aerosols. We focus on changes over the last two decades at the 81 largest cities using satellite data. European and American cities have lower aerosol, compared to African and Asian. For European, North American and East Asian cities aerosols are decreasing over time, especially in China and US. At remaining cities, aerosol loads are increasing, particularly in India.
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