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Reference
Aerosol quantification and caracterization from global and beam irradiance measurements
INEICHEN, Pierre
Abstract
On the basis of the broadband simplified Solis clear sky model, a method is developed to quantify and characterize the atmospheric aerosols content and type from global and/or beam hourly irradiance measurements.
INEICHEN, Pierre. Aerosol quantification and caracterization from global and beam irradiance measurements. 2009, 13 p.
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http://archive-ouverte.unige.ch/unige:39175
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global and beam irradiance measurements
Pierre Ineichen University of Geneva November 2009
Abstract
On the basis of the broadband simplified Solis clear sky model, a method is developed to quantify and caracterize the atmospheric aerosols content and type from global and/
or beam hourly irradiance measurements.
1. Introduction
Anthropogenic activities became an important factor in the climate change and a conti- nuous monitoring of the solar irradiance reaching the ground is essential to understand the impact of such changes on the environment [Cutforth 2007, Stanhill 2001]. Unfor- tunately, the density of the ground measurement network is insufficient, especially on continents like Africa, or countries in the Near East. To circumvent this lack of measured data, the meteorological satellites are of great help and models converting the satellite images into the different radiation components become increasingly
performant.
In this field of solar radiation modelization, the clear sky irradiance component represents the main normalization function [Ineichen 2006]. In order to evaluate it, the atmospheric aerosol and water vapor content have to be known; these two parameters have the greatest influence on the transmittance. If the water vapor can be retrieved from ground or satellite measurements, it is not so easy for the quantity of aerosols in the atmosphere and their type.
The present study describes a method to evaluate and characterize the aerosol content of the atmosphere with the help of two solar irradiance components: global (GHI) and the direct normal irradiance (DNI).
2. Background
The Linke turbidity coefficient TL was the most popular quantification of the turbidity [Linke 1922, Kasten 1996 and Ineichen 2002]. It takes into account both the atmospheric aerosol and water vapor content, but cannot be used as is in the radiative transfer models.
Nevertheless, it can easily be retrieved from DNI with the help of the following equation:
(1) where Io is the solar constant, a the Kasten [Kasten 1996] pyrheliometric formula:
(2) and AM the optical air mass. In this form, it has the disadvantage to be solar elevation dependent, but can be used in its modified form [Ineichen 2002] to circumvent this problem. The Linke turbidity coefficient is still used in models like the ESRA clear sky model [Rigolier 2000, Geiger 2002]
When the water vapor column, the aerosol type and optical depth are known, it becomes possible to evaluate the clear sky with Radiative Transfer (RTM) based models such as
Bird’s model [Bird 1980] or Solis [Müller 2004, Ineichen 2008].
The water vapor column can be evaluated from ground temperature and humidity measurements and the use of an empirical model, like the Atwater model [Atwater 1976]
that was used in the present study.
The broadband simplified version [Ineichen 2008] of the Solis model [Mueller 2004] is used to retrieve the aerosol properties of the atmosphere. It is a physical model, based on radiative transfer calculations (RTM).
3. Ground measurements
Data from 33 ground stations were collected to develop the method. The geographic locations of the stations cover latitudes from the equator to 60°N, altitudes from sea level to 3600 m and a great variety of climates. For the majority of the stations, the beam irradiance component is measured (except for two CIE stations where the beam
Table I List of the stations.
component is calculated from corrected diffuse measurements). For stations part of the SurfRad [Surfrad], BSRN [BSRN] and CIE [CIE 1994] networks, high precision instruments [WMO 2008] such as Kipp and Zonen CM10 and Eppley PSP pyranometers, and Eppley NIP pyrheliometers, are used to acquire the data. For the other stations, it was not possible to determine the exact type of instruments. Also, for the above networks, a stringent calibration, characterization and quality control was applied on all the data by the person in charge of the measurements (following IDMP recommendations [CIE 1994]), the coherence of the data for all the stations was verified by the author and is described in the following section.
Six additionnal stations situated in Africa, Israel and Spain where only the global horizontal irradiance is available are included in the study (shown in grey in Table I).
4. Data quality control
The first step of the quality control is a check of the acquisition time given in the data banks. To point out a possible time shift in the data, the symmetry in solar time of the irradiance for clear days is visually checked. The horizontal global and the normal beam irradiances are plotted versus the sinus of the solar elevation angle, and the morning and afternoon curves should be mingled as visualized on Figure 1a. If this test is positive, a verification can be done by plotting the global clearness index defined as:
(3) for the morning and the afternoon data separately. The upper limit, representative of clear sky conditions, should also be mingled for the morning and the afternoon data as represented on Figure 1b for one year of data acquired at Desert Rock (USA). When these two conditions are fulfilled, the solar geometry can be precisely calculated.
Figure 1a The global horizontal and nor- mal beam irradiences are represented versus the sinus of the solar elevation for one clear
Figure 1b The global clearness index Kt is reprsented separately for the morning and the afternoon data, versus the solar eleva- tion angle for one year in hourly values.
a
d
e c
b
Figure 2 Beam clearness index versus global clearness index for hourly measurements and clear sky model.
a: ground meaurements
b: with 5% of global underestimation c: with 5% of beam underestimation d: clear sky with 1.5 cm underestimation e: clear sky with 2 cm overestimation
The second test consist of the coherence verification between the two irradiance indices, Kt and Kb. The latter defined as:
(4) is plotted versus Kt in Figure 2a. On the same graph, the clear sky data is represented for 4 different values of atmospheric broadband aerosol optical depth (aod), the corresponding Linke turbidity coefficient calculated with euqation (1) at air mass AM = 2 is also given.
A similar representation is illustrated for four different cases: with 5% underestimation in the global or the beam irradiance (Figure 2b and 2c respectively), and for atmospheric water vapor content under and overestimation in the clear sky model (Figure 2d and 2e respectively).
In the case of slight irradiance underestimation, the measurements are not in coherence
with the clear sky model, shifted to the left for a global underestimation, and to low for a beam underestimation. This is typical with calibration uncertainties or uncleaned sensors.
An over or underestimation in the atmospheric water vapor content as input for the clear sky model is not visible on the graphs (i.e. Figure 2d and 2e). When the water vapor is underestimated, it can be seen that high clearness indices are never reached by the measurements.
5. Clear sky model, aerosol optical depth and type determination
The clear sky model used in this study is the broadband simplified version of the Solis model [Mueller 2004, Ineichen 2008a and the updated version in Ineichen 2008b]. It takes into account four different aerosol types: urban, rural, maritime and tropospheric [Shettle 1989]. As the original Solis model does not take into account the circumsolar irradiance (csi), a slight empirical correction was applied based on smarts2 calculations [Gueymard 2004], it has the form:
csi = 0.359 aod7002 + 0.222 aod700 + .0011 (5)
Table II Slope and determination coefficient for the best fit between aod700 retrieved from beam and global irradiance. The second column nb is the number of days involved in the fit.
Figure 3a Beam versus global clearness index for data acquired in Athens in 2001. The corresponding clears sky mo- del is represented for different aerosol optical depths.
Figure 3b Aerosol optical depth versus the day of the year for the four aerosol types.
Figure 3c Beam retrieved aerosol optical depth versus the corresponding global aod. The slope and the determination coefficient is also given.
a
where aod700 is the unitless broadband aerosol optical depth.
The use of this simplified model makes possible the determination of the aod700 from the beam and global measurements separately. Day by day, on an hourly basis, the ir- radiance versus solar time curve is compared to the corresponding clear sky model, and a best fit is done for aod700 varying from 0.005 to 0.5, for the four aerosol types. If the water vapor column is part of the ground measurements, it is used as input to the Solis model; if not, average monthly values obtained from Meteonorm [Meteonorm 2009] and Soda-is [www.soda-is.com] are used. When available, i.e. by clear sky conditions, the lowest root mean square deviation between model and measurements determines the corresponding daily aod700 value. The result consists of four daily global and four daily beam aerosol optical depth values, one for each aerosol type. As the transmittance of the global and the beam irradiance components differs with the different aerosol types, the retrieved aerosol optical depths will also be different, depending on the type and the component. So, the comparison between the values retrieved from the global and the beam irradiances will give an indication on the aerosol type.
Two different graphs are represented on Figure 3b and 3c, the aerosol optical depth ver- sus the day of the year, and the beam versus the global retrieved aod700 for each of the aerosol type, represented for measurements acquired in Athens during the year 2001.
The slope of the diagonal best fit and the determination coefficient R2 are given on the Figures 3b. It can be seen on the different graphs that the best coherence between the global and the beam aerosol optical depths is in this case obtained for urban aerosol type, the slope is near unity. In Figure 3a, the beam versus global clearness indices for data acquired in Athens in 2001 are represented. The corresponding clears sky model is also plotted for different aerosol optical depths and the yearly average ground water vapor column.The results for all stations are given in Table II, all the plots can be down- loaded from
http://www.unige.ch/cuepe/html/biblio/detail.php?id=455&sr=&pr=&mjc=2
For the SurfRad data, the aerosol optical depth is also retrieved from MultiFilter Rotating Shadowband Radiometer (MFRSR) measurements (http://www.srrb.noaa.gov/surfrad/
aod/mfrsr.html). A comparison of the two method is illustrated in Figure 4 for the station of Bondville (USA). It can be seen that the correspondance between the re- sults is satisfactory, even if the dispersion increases for high aod values, due to the instability of the irradiance conditions for higher turbidity. The MFRSR retrieved va- lues are also represented on Figure 5b for the corresponding aerosol type, here rural type for the station of Bondville (USA). A
good correspondance can be seen for the Figure 4 MFRSR versus beam irradiance retrieved aerosol optical depth.
Figure 5a Beam versus global clearness index for data acquired in Bondvilles in 2006. The corresponding clears sky mo- del is represented for different aerosol optical depths.
Figure 5b Aerosol optical depth versus the day of the year for the four aerosol types.
Figure 5c Beam retrieved aerosol optical depth versus the corresponding global aod. The slope and the determination coefficient is also given.
a
lower limit, representative of clear sky conditions.
Monthly aerosol optical depths can be obtained by averaging, month by month, daily values situated between the absolute minimum of the considered month aodmin and ao- dmin + 0.03, as illustrated in Figure 6 for data acquired at the station of Table Mountain for the year 2006. The values are only kept if more than 2 daily values satisfy the above conditions.
The yearly average and the monthly estimated aerosol optical depths obtained from
Table III Type, yearly average and monthly broadband aerosol optical depths (aod700) re- trieved from the global irradiance. In grey, the stations with only the global irradiance.
Figure 6 Daily values kept for the estimation of the monthly average aod values.
Figure 7 Comparison of the yearly aerosol optical depth retrieved from the global and the beam irradiance.
Figure 8 Beam versus global clearness index for data acquired in Hermiston in 1999-2000.
The corresponding clears sky model is represented for different aerosol optical depths. The be- haviour at beam clearness indices of 0.3 is probably due to measurements errors.
the global horizontal irradiance are given on Table III for all the stations. In Figure 7, a yearly values comparison is done between the aod retrieved from the global and the beam irradiance. In the average, both aerosol optical depth values are of the same order of magnitude, except for some specific stations. The high variability in the sky condi- tions can explain the differences for stations like Freiburg, Lerwick or Mt Kenya. For the station of M’Bour, it seems that the beam irradiance is underestimated. For Hermiston, the graph given on Figure 8 suggests that values around Kb = 0.3 are likely traceable to measurements errors.
6. Conclusion
The method developed in the present study shows that the use of hourly irradiance va- lues allows the estimation of the aerosol optical depth with a relatively good precision.
The relatively good agreement between values obtained from the global and the beam components indicate the even if only the global irradiance is known, an aerosol optical depth can be retrieved. The method has been verified with the help of spectral measure- ments issued from the SurfRad network for American stations. Furthermore, if both the global and the beam irradiance components are available, it is possible to have an first estimation of the aerosol type.
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