Abstract:
Aerosols influence climate by altering the global energy balance via scattering and absorbing
solar radiation (direct effects), and by their effect on the reflectance of clouds and occurrence
of precipitation (indirect effects). Aerosols also influence biogeochemical cycles, lead to
diminished environmental visibility, and harm human health. Aerosols in the urban regions
have a unique character because the amount of emitted aerosol species can vary significantly
in terms of number density, season and location when compared to aerosols in remote
continental and maritime regions. They have a distinct character as they can exhibit significant
seasonal and inter annual variability owing to variations in local emissions and long-range
transport. This thesis focuses on advancing knowledge of critical properties of aerosols,
specifically microphysical and optical properties, which govern the role of aerosols in climatic
and environmental change. The study’s main aim is to investigate the long-term variation of
Aerosol Optical Depth (AOD). It also aims to develop a mathematical model to estimate AOD
using combined meteorological parameters, and to assess the status of air quality over Dire
Dawa, and to identify the possible sources of the aerosols using a HYSPLIT model.
Remote sensing data provide a better understanding of aerosol characteristics at a large scale
and are available in a wide range, while ground-based observation is very limited. In this study,
we used multi-year remote sensing, namely, MODIS-Aqua, MODIS-Terra, MISR, and OMI
satellite retrievals, OPAC aerosol model, and AERONET AOD data, to evaluate and identify
the best and suitable satellite sensor which can retrieve AOD, to explore the trend and
distribution of AOD, and to examine the status of air quality over a semi-arid region Dire Dawa
in the period from 2009 to 2020. The result obtained show that the AOD values were observed
to be higher in summer (June to August) than during the rest of the months due to the
predominance of coarse dust and sea salt particles and possibly also due to the higher water
vapor content of the atmosphere due to high summer temperatures, which encourages the
growth of aerosols. In addition, the air quality status over Dire Dawa during the study period
was moderate as measured by purple sensor and gravimetric method. Generally, AOD values
vary from season to season due to aerosol’s optical and microphysical properties being affected
by anthropogenic sources, natural sources, meteorological conditions and surface albedo. The
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study also strongly recommends to make use of a synergy of remote sensing and ground
installed devices such as sun-photometer to monitor the status of air qualit
