Multiyear analysis of Terra/Aqua MODIS aerosol optical depth and ground observations over tropical urban region of Hyderabad, India

Remote sensing of global aerosols has constituted a great scientific interest in a variety of applications related to global warming and climate change. The present study uses Level 2 (10 × 10 km) and Level 3 (1° × 1°) Terra/Aqua MODIS (C005) derived aerosol optical depths at 550 nm (AOD550) and compares them with ground-based (MICROTOPS-II, MT) sun photometer measured AOD550 in the period 2002–2008 over Hyderabad, India. The correlation coefficient (R2) between Level 3 Terra/Aqua MODIS and MT AOD550 in all seasons ranges from 0.30 to 0.46. Even lower correlations revealed when the Level 2 MODIS data are used (R2 = 0.16–0.30). The Level 3 MODIS AOD550 underestimates significantly the MT AOD550, while the Level 2 AOD550 values are much larger than those of Level 3. The comparison of the Terra/Aqua MODIS AOD550 at regional scale, and especially over urban/industrial areas with significant aerosol diurnal variation, constitutes a real challenge and may reveal the ability of the two sensors to capture the temporal variation of the aerosol loading within a time interval of ∼3 h. The results show relatively good correlation (R2 ∼ 0.6–0.7) regarding the Level 3 dataset; however, the Level 2 data showed large scatter and very poor correlations. On the other hand, the mean seasonal AOD550 values are similar, while Terra AOD550 is higher than that obtained from Aqua. Both satellite and ground-based measurements show remarkable increasing trends in AOD over Hyderabad, which are attributed to the extension of the urbanized area, the growing of population, motor vehicles and local emissions.

Research highlights
► A pronounced increasing trend in AOD550 is observed during the last 6 years.
► Terra/Aqua MODIS Level-3 datasets underestimated MICROTOPS-II AOD550 values.
► The highest frequency of MICROTOPS-II AOD550 to be observed at 0.4 and 0.5.
► Terra/Aqua MODIS AOD550 data demonstrated a good agreement over Hyderabad.
► Biomass burning and dust events lead to stronger diurnal AOD variation.