Atmospheric aerosols are posing a great threat to the already stressed climate with the effects being felt more on African continent. Their presence and interaction with the clouds contribute to the strongest uncertainty in aerosol characteristics and Earth’s energy budget hence; calling for a long term assessment to be done. The present study analyses long term spatiotemporal microphysical aerosol characteristics (namely: effective radius and surface-area concentration), using AErosolROboticNETwork (AERONET) framework over Kenyan urban atmosphere (Nairobi-1 ° S, 36 ° E), rural atmosphere (ICIPE-Mbita-0 ° S, 34 ° E) and maritime atmosphere (CRPSM-Malindi-2 ° S, 40 ° E). AERONET framework was used due to its availability over the selected sites; it is also located in sites that provided contrasting aerosols type, source and characteristics and due to its syner-gism with other frameworks. The findings indicated a spatial and temporal variability in microphysical properties over CRPSM-Malindi, Nairobi and ICIPE-Mbita. CRPSM-Malindi is dominated with coarse aerosols in all sea-sons while Nairobi with coarse mode in the DJF and MAM seasons. ICIPE-Mbita is on the other hand dominated with fine aerosols in all season. In terms of size distribution, the three AERONET sites displayed a bimodal distribution inflecting at 0.44 μm and fine mode radius of 0.15 μm while CRPSM-Malindi recorded a coarse mode of 3.86 μm and Nairobi and ICIPE-Mbita with 5.06 μm. The coarse aerosols have a higher concentration than the fine aerosols in all AERONET sites because of aerosol coagulation and dominance of certain type of aerosols that are coarse in nature.
Aerosols are tiny (micro and submicron) sized particles (solid or liquid) that are suspended in the atmosphere [
Aerosol variability depends on factors such as dominant anthropogenic emissions, natural loading and atmospheric conditions [
The presence and variability of atmospheric aerosols have strongly contributed to greater uncertainties in aerosol optical, microphysical and chemical characteristics [
The study was undertaken in the region described in detail by Makokha [
The study uses the AErosolROboticNETwork (AERONET) to analyze the spatial and temporal variability of both effective radius and size distribution. This program [
aerosol optical, microphysical and radiative properties for aerosol research and characterization, validation of satellite retrievals and synergism with other database.
Derivation of climate models, visibility and how human health is affected through aerosol characterization requires a better understanding of effective radius ( r e f f ) and surface-area concentration (size distribution). The sensitivity of climate change, visibility and human health caused by natural and anthropogenic variations of the above-listed microphysical properties characterizes the impact of the atmospheric aerosol on climate change; visibility and human health hence need for aerosol characterization.
Effective radius ( r e f f ) is the area weighted mean radius of the aerosol particles. It determines the size of the aerosol particles. The derivation of ( r e f f ) is given by Equation (1) and has been described in details by Dubovik [
r e f f = ∫ 0 ∞ r 3 d N ( r ) d ln r d ln r ∫ 0 ∞ r 2 d N ( r ) d ln r d ln r (1)
where r is the particle radius, d N ( r ) particle distribution. The particles and droplets in aerosols in Equation (1) collide with one another undergoing coalescence leading to change in the aerosol particle size distribution. This increases aerosol diameter as total number of particles decreases, hence size distribution of
the particle volume d V ( r ) d ln r relates with the particle number by the formula [
d V ( r ) d ln r = V ( r ) d N ( r ) d ln r = 4 3 π r 3 d N ( r ) d ln r (2)
The data for the daily averages for both effective radius and size distribution were downloaded from the AERONET inversions (V3) over three AERONET sites in WordPad which was then imported to excel for further manipulation and analysis. These data were regrouped in respective seasons i.e. DJF, MAM, JJA and SON by first deleting the field values and then finding the averages from which, line graphs were plotted for respective seasons throughout the period of study. The variability of seasonal plots over same site were analyzed with time hence a temporal erraticism of aerosol microphysical properties while on the other hand, a spatial variability was achieved by comparing DJF, MAM, JJA and SON of one AERONET site with the seasons from other AERONET sites.
AERONET data of level 1.5 from the three AERONET stations (obtained from January 2004 to December 2017) were considered in this study to investigate both the spatial and temporal variation of r e f f and size distribution over Kenya.
r e f f refers to mean radius of the aerosol particles. It determines the size of aerosol particles hence can be characterized as fine mode (particle size < 0.6 μm) or coarse (particle size > 0.6 μm) mode aerosols [
From
Additionally,
but 2012 season recorded the largest radius as 2.98 μm with smallest as 0.38 μm and average of 0.99 μm. a range and average of 0.48 - 3.49 μm and 1.14 μm respectively were recorded in 2013 MAM and the greatest variability during this season was recorded in 2017, during which the highest average of 1.20 μm and range of 0.45 - 5.43 were recorded. The variability in volume distribution was favored by increased amount of PR in MAM thus increased aerosols wash down [
Also, as noted in
Lastly, during 2008 SON season as indicated in
Equally, r e f f over Nairobi AERONET site during the period of study was also analyzed and the findings represented as in
During the 2006 and 2009 DJF seasons, the r e f f was noted to vary with time as indicated in
Additionally, the analysis of the r e f f over ICIPE-Mbita was done and the findings represented as in
Temporal characteristic of r e f f was equally analyzed over ICIPE-Mbita and it can be noted from
Furthermore, during local wet MAM season as indicated in
slight decrease in average radius and range to 0.72 μm and 0.45 - 1.17 μm respectively. The aerosol
In the JJA season as indicated in
A temporal variability over ICIPE-Mbita during the SON season as indicated in
Spatial variability was similarly noted over the three AERONET sites as indicated in Figures 2-4. CRPSM-Malindi recorded average of
Surface area concentration (Particle size distribution) over AERONET sites dictates the fluctuation in the aerosol radius with time. To start with are the variations in the size distribution over CRPSM-Malindi seasons from 2008 to 2017. It is noted from Figures 5(a)-(e) that there is a temporal variability displayed in a bimodal distribution of aerosol particle size with an inflection at 0.44 µm in all seasons and high concentration in coarse aerosols than fine aerosols. The sizes in
DJF, MAM JJA and SON seasons of 2008 as noted in
Equally, from
The 2012 seasons equally indicated a variation of aerosol size with time. The DJF and MAM both recorded fine aerosol and coarse aerosol modes as 0.15 µm and 2.94 µm respectively while SON season of the same year posted fine mode radius of 0.15 µm and coarse mode radius of 5.06 µm. The 2013 seasons noted a great variability of aerosol sizes distribution with MAM and SON seasons both posting fine mode radius of 0.15 µm and coarse mode radius of 3.86 µm. Additionally, JJA noted much smaller fine mode radius and coarse mode radius of 0.09 µm and 2.24 µm respectively.
Away from size distribution over CRPSM-Malindi, the study also investigated the same property over Nairobi during the same period whose findings were represented as the sunisoidal waves in
Although only two seasons were recorded during the 2007 the study noted a significant seasonal variation as it may be observed from
aerosol fine mode of 0.11 µm and SON a higher value of 0.15 µm. A greater variability was noted in the coarse mode aerosol radius with 2.94 µm as the lowest radius in DJF and 5.06 µm as the highest coarse mode radius in the MAM, JJA and SON seasons [
The study equally investigated size distribution over ICIPE-Mbita during the same period and the findings represented as in
It is noted from
Temporal characteristics may also be noted in 2010 seasons as displayed in
Additionally, the 2013 seasons displayed an interseasonal variation with a bimodal distribution between the fine and coarse mode aerosols, as it may be noted from
Additionally, spatial analysis of size distribution over three AERONET sites was done and noted that CRPSM-Malindi had an average fine and coarse modes of 0.15 µm and 3.48 µm while Nairobi and ICIPE-Mbita both had the average modes of 0.15 µm and 5.06 µm. The greatest variability in the size distribution was note over Nairobi while the lowest variability was over CRPSM-Malindi. The study attributed the low variability over coastal region to the ever present sea salt in the region [
The
The authors wish to acknowledge National Aeronautics and Space Administration (NASA) through PI Holben B. for establishing and maintaining AErosol RObotic NETwork (AERONET) frame work over Kenya. Additionally, we thank the AERONET online data system developed and maintained by the NASA Goddard Earth Science Distributed Active Archive (NASA GES DISC) from which inversion 2 and level 1.5 data was obtained.
The author declares no conflicts of interest regarding the publication of this paper.
Khamala, G.W., Odhiambo, J.O. and Makokha, J.W. (2018) Seasonal Variability in Aerosol Microphysical Properties over Selected Rural, Urban and Maritime Sites in Kenya. Open Access Library Journal, 5: e4821. https://doi.org/10.4236/oalib.1104821