Diurnal Variability of the Radiative Impact of Atmospheric Aerosols in Ouagadougou, Burkina Faso: A Seasonal Approach

The objective of this work is to study the diurnal evolution of the radiative impact of atmospheric aerosols in an urban city located in the West African Sahel and the correlations with the main influencing factors of local climate dynamics. The simulation was performed using a treatment chain including the GAME code. In the methodology, the atmosphere is modeled by 33 plane parallel layers and the effects of absorption, multiple scattering by particles and gas are taken account. An hour-by-hour calculation of radiative forcing at the top of the atmosphere, in the atmospheric layer and at the earth’s surface was performed. The data used as input are the monthly averages of optical properties, radiosonde measurements, daily synoptic measurements and surface albedo. The results show a parabolic diurnal course of a negative radiative impact at the top of the atmosphere with an extremum at 12 o'clock. Maximum cooling is observed shortly after sunrise and shortly after sunset. The largest annual deviations are noted between the months of March and December with respective maximum cooling values of −34 W/m 2 and −15.60 W/m 2 . On the earth’s surface, a cooling impact is observed with two diurnal being always observed at around 12 o’clock. An analysis of similar works carried out in urban cities in various locations of the world has shown a relatively good accordance with the values obtained. This study highlights the radiative impact of Saharan desert dust, the effect of the local climate and the succession between dry season (November to May) and the rainy one (July to October), as well as the zenith solar angle and human activity.


Introduction
Ouagadougou is the capital of Burkina Faso, a landlocked country in West Africa bordering in its northern part with the Sahara Desert. With approximately 8.5 million km 2 [1] the Sahara is recognized as the largest desert of the earth and the world's leading source of desert aerosols. Several studies estimate 400 and 700 Mt [2] [3] [4] [5], the contribution of emissions from the Sahara. Ouagadougou, like the entire Sahelian zone, is strongly impacted by these important emissions which significantly affect the optical and microphysical properties of aerosols [6]. This situation directly affects the radiative properties of atmospheric aerosols in the city of Ouagadougou, which is also subject to the effect of emissions linked to human activity. Numerous studies have highlighted the combined effects of the climatic season and human activity on the composition and properties of aerosols in large urban cities [7] [8]. The objective of this work is to study, the monthly evolution of the diurnal radiative forcing in Ouagadougou as well as the influence of the seasonal variability of the West African climate.

Methodology
The calculation methodology is a simulation realized by a treatment chain which includes the GAME code, "Global Atmospheric Model" [9]. The simulation of the aerosol radiative forcing is carried out according to a model of atmosphere divided into 33 plane and parallel layers. The calculation is performed with the line by line method [10], takes into account the effect of shortwave absorbers in the electromagnetic spectrum [11], and deals with multiple scattering problems by the discrete ordinate method [12] as well as interactions between multiple scattering and gas absorption [13]. Specifically, the methodology integrates the effects of all particle-radiation interactions in the atmosphere, namely: • Absorption by gases, H 2 O, O 2 , CO 2 , O 3 .
• The Rayleigh scattering by molecules.
• Absorption by aerosols through the single scattering albedo.
• Diffusion due to aerosols by the asymmetry factor.
• The surface albedo. The treatment chain gives the following output: • Direct radiative forcing at the top of the atmosphere F TOA : • Direct radiative forcing at the surface ΔF BOA : • Radiative forcing in the atmospheric layer ΔF ATM which is a simple difference between radiative forcing at the top of the atmosphere and at the earth's sur- ( ) where T denotes the air temperature, and t the time. ρ is the density of the atmosphere. C p is the specific heat of the air. F (z) is the net flux at altitude z.

Data
The methodology uses several data sets as input. These include the optical properties of aerosols, vertical temperature and humidity profiles, ambient temperatures, surface albedo and solar angle.

Optical Properties
The optical data used in this work are optical thicknesses, Angstrom's exponent, and Single Scattering Albedo (SSA). These data are extracted from the database of photometric measurements and inversion products of "AErosol RObotic NETwork" (AERONET) [14]. In Ouagadougou, the measurements were carried out from 1999 to 2007. All the data were the subject of a qualitative analysis and monthly averages were calculated using the sorted data to better perceive the impact of the climate seasonal dynamics in West Africa.
The optical thicknesses are measured on 4 channels (440 nm, 675 nm, 870 nm and 1020 nm). The Angström exponent is calculated over the wavelength range of 440 µm to 870 µm, covering almost the entire band of atmospheric radiation (Table 1). This coefficient makes it possible to deduce, over all wavelengths of the interval, the optical thickness of the aerosols. Journal of Environmental Protection The single scattering albedo is a key parameter in assessing the radiative impact of an aerosol population as it perfectly describes the scattering character relatively to the aerosol absorbency. The monthly averages used in this study are presented in Table 2.

Vertical Temperature and Humidity Profiles in Ouagadougou
Vertical temperature and humidity profiles were obtained from radiosonde data measured daily. We used the averages of the 2006 measurements because this year corresponds to the period where the photometric data were the most abundant and regular in Ouagadougou.
The vertical temperature profile shows an almost linear tropospheric layer extending from the surface to about 17,000 meters. A climatology of the data has shown that the average gradient of 6˚C/km in the case of a standard atmosphere seems to adequately describe the vertical temperature profile in Ouagadougou although it is necessary to note some slight differences according to the seasons due to the presence of varying degrees of water vapor, clouds, etc. (Figure 1).
The first layer is the most interesting for us because it is at this level that almost all atmospheric aerosols, subject of our study, are concentrated. The upper layers present very little interest in our work.
While the vertical temperature profile is stable over most of the year, the humidity profile fluctuates widely ( Figure 2). For our study, approximations were performed by analyzing the general trend of month-to-month measurements.
Average values were therefore set for each specific altitude band.

Daytime Ambient Temperature and Surface Albedo
The daily variation of ambient temperature is key in evaluating the hourly radiative forcing in the period between sunrise and sunset. In Ouagadougou as in the entire Sahelian zone, in general the ambient temperature during the day has a parabolic form, with a minimum depending on the season between 20˚C and 25˚C at 5 a.m to 6 a.m. in the morning and the maximum at 12 p.m. to 1 p.m.
with mean values of 30˚C and 35˚C (sometimes higher in the hot period of March-April-May). We used in this work, the synoptic data averaged hour by hour.
As for the surface albedo, also called the reflection factor, which expresses the fraction of the radiation reflected by the earth's surface, we estimated it from the MODIS (Moderate Resolution Imaging Spectroradiometer) sensor products which gives the spectral dependence of the albedo surface at seven wavelength bands [15].

Diurnal Evolution of the Radiative Impact
In this paragraph, we make an hour-by-hour estimate of the radiative impact at the top of the atmosphere (TOA), on the earth's surface (BOA) as well as in the atmospheric layer (ATM) for each month of the year. This helps to understand the evolution of the impact of aerosols between sunrise and sunset. The results of the calculations are presented by the curves in Figure 3 below.

At the Surface
The radiative forcing at surface is negative throughout the year. The diurnal evolution of this forcing has the same look throughout the year. The curves show two neighboring peaks shortly after sunrise (between 7 a.m. and 8 a.m.) and shortly before sunset (between 4 p.m. and 5 p.m.). We can logically think of the effect of human activity and the emission peaks it causes in the morning and late afternoon, but also of the effect of the dynamics of air masses and turbulence created by the warming of the surface at sunrise and onset of cooling in the evening. The minimum cooling is observed around 12 o'clock, when the sun is at the zenith and the solar flux is greater.
The results also show that the maximum daily values of cooling range from −104.45 W/m 2 in March to −54.60 W/m 2 calculated in August.

In the Atmospheric Layer
The cooling of the surface is the logical consequence of a warming in the atmospheric layer. In the atmosphere, the calculated positive forcing is the effect of a significant absorption of radiation by aerosols strongly impacted by human activity during the day. On a daily scale, this warming varies increasingly from sunrise until around 9 a.m. From that moment the forcing remains almost constant until the afternoon between 3 p.m. and 4 p.m. before experiencing a decrease until sunset.

Seasonal Analysis of the Diurnal Radiative Impact
The variability of the diurnal radiative impact at the top of the atmosphere, in the atmospheric layer and on the Earth's surface shows a strong correlation with West African climate dynamics. In fact, the climate in West Africa south of the Sahara is punctuated by the succession of monsoon and harmattan flows separated by a front line, the intertropical front whose latitudinal rise and fall brings the rainy season in its North and South part. In the Sahelian zone, the dry season sets in from October to May and gives way to the rainy season which follows it

Discussion
By various methods, and at different space and time scales, radiative forcing calculations have been reported in the literature. All studies show a negative radiative impact at the top of the atmosphere and at the surface, then a positive impact in the atmospheric layer.
Specifically, in the West African region, similar work has been carried out [16] [17] [18]. The analysis of these results shows overall consistency with our study even if some observed differences can be justified in the geographical location, climatic influences, socio-economic activity and methodological approach. In fact, our assessment of the diurnal variation is based on the monthly averages of the input data (optical properties, atmospheric thermodynamic measurements, ambient temperatures, etc.) to better understand the impact of the local climate. An assessment with real-time data would certainly have presented different values at certain times.

Conclusion
This study concerns the evaluation of the diurnal evolution of radiative forcing caused by atmospheric aerosols on the scale of an urban city located in the heart of the Sahel in West Africa. The results of the simulation carried out using the GAME code showed cooling at the top of the atmosphere and at the surface and then warming of the atmospheric layer in accordance with the literature. The study of diurnal variability sheds light on the influence of solar zenith angle and human activity from sunrise to sunset. The seasonal approach of the study clearly shows the influence of the succession of the dry season characterized by a strong suspension of desert dust from the Sahara desert and the rainy season drained by the humid monsoon flows from guinea golf. This result highlights the