Abdulamid Fakoya - May 4

Weather and Climate Systems

Assessment and Comparison of Aerosol Properties from AERONET and MODIS Products in the Southeast Atlantic.

Abdulamid Fakoya

Wednesday, May 4

03:30 PM

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Aerosols lead to radiative forcing due to their interaction with solar radiation and their roles in the alteration of cloud lifetime and properties. Biomass burning (BB) is one of the largest sources of absorbing aerosols globally. Biomass Burning aerosols (BBA) contain Black Carbon (BC), the most absorbing aerosol constituent found in the atmosphere. BC contributes significantly to climate forcing with a value of about 0.4 Wm–2. About 35% of Earth’s BBA emissions come from southern Africa, which makes the region particularly informative for studying aerosol properties and loading. Aerosol Optical Depth (AOD), a key parameter and arguably the most studied optical property of aerosols, is used in radiative transfer calculations and climate change assessment and represents the abundance of aerosol in a vertical column of air. Angstrom Exponent (AE), another key parameter, is a measure of the wavelength dependence of AOD and is relatable to particle sizes.

This study considers the spatiotemporal distribution and seasonal variability of AOD and AE over southern Africa. We present a comparative analysis of satellite-based aerosol measurement and ground-based observation using AOD products obtained from Moderate Resolution Imaging Spectroradiometer (MODIS Terra and MODIS Aqua) Deep Blue (DB) and Dark Target (DT) collection 6.1 and the direct sun algorithm (level 2.0, version 3) of the Aerosol Robotic Network (AERONET) at six sites. The mean annual distribution of AOD at 0.55 µm (AOD550) shows high and low patterns that explain the features of aerosol loading corresponding to the inland and coastal areas, respectively. The interannual variability is mostly governed by the BB season (July – October) and is affected by geography, with AOD550 decreasing away from far inland areas (intense burning regions) towards the coast. Seasonal and monthly climatologies reveal that the highest AOD550 is observed in JJA and SON, with September having the highest mean AOD550 across all regions. It is also observed from AE(440-675) that fine particles are prominent across all months in the inland areas while coarse particles are dominant in the coastal areas except during JJA and SON. The majority of the sites often show a better correlation between AERONET AOD and MODIS Terra than between AERONET AOD and MODIS Aqua. Over land sites, the correlation coefficient (R) between AERONET AOD-MODIS Terra ranges from 0.4 to 0.92, while R between AERONET AOD-MODIS Aqua ranges from 0.53 – 0.89. In the coastal areas, R between AERONET AOD-MODIS Terra and AERONET AOD-MODIS Aqua couplings ranges from 0.41 – 0.43 and 0.2 – 0.49, respectively. In general, MODIS Terra and MODIS Aqua overestimate AOD550 at all coastal sites and underestimate AOD550 in the inland areas. A typical air mass trajectory analysis using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model suggests that aerosol particles originate from inland areas and that source regions of BBA are associated with long-range transport over the Southeast Atlantic Ocean.