Exploring systematic offsets between aerosol products from the two MODIS sensors.

Area of Science:

• Atmospheric Science
• Climate Science
• Remote Sensing

Background:

• Long-term global aerosol loading and optical properties are crucial for climate change assessments.
• Moderate-resolution Imaging Spectroradiometer (MODIS) sensors on Terra (MODIS-T) and Aqua (MODIS-A) satellites provide aerosol data.
• The dark-target (DT) algorithm derives aerosol optical depth (AOD) and Angstrom Exponent (AE) products (MOD04 and MYD04).

Purpose of the Study:

• To analyze the overlapping time series of Collection 6 (C6) MODIS aerosol products (MOD04 and MYD04) since mid-2002.
• To investigate the observed differences between morning (Terra) and afternoon (Aqua) aerosol retrievals.
• To assess potential corrections for sensor-specific issues and their impact on aerosol product consistency.

Main Methods:

• Analysis of global monthly mean AOD and AE from MODIS-T and MODIS-A C6 products.
• Comparison with simulated aerosol fields from the Goddard Earth Observing System (GEOS-5) model.
• Application of correction methods (C6+ and convolution) to input reflectance data to address sensor calibration issues.

Main Results:

• Global monthly mean AOD from MODIS-T is consistently higher than MODIS-A by ~13% (~0.02 over land, ~0.015 over ocean).
• Offsets exhibit seasonal and long-term variability; MODIS-T shows higher AOD and lower AE over ocean.
• Correction methods reduced land AOD offset but did not significantly reduce ocean AOD offset; AE offset was reduced.

Conclusions:

• Global differences between Terra and Aqua aerosol products should not be interpreted as a true diurnal aerosol signal.
• The MODIS-A product may be more suitable for some applications due to a smaller bias compared to ground-truth data.
• Given the small AOD offset (~0.02), both MODIS products may be adequate for most applications.