Classification and characterization of atmospheric aerosol in Popayán –
Colombia
Clasificación y caracterización de aerosoles atmosféricos en Popayán - Colombia
D. Orozco
(1), D. Alegria
(1,*), S. Gaona J.
(1), A. E. Bastidas G.
(2), E. Rodríguez
(3)1. Grupo de Óptica Aplicada y Didáctica, GIOPAD, Universidad del Cauca, Popayán, Colombia 2. Grupo de Espectroscopia Óptica, Universidad Nacional de Colombia, Sede Medellín, Colombia
3. GOA- Grupo de Óptica Atmosférica, Universidad de Valladolid, España
* Email: [email protected]
Recibido / Received: 20 – Jul – 2007. Versión revisada / Revised version: 30 – Oct – 2007. Aceptado / Accepted: 5 – Nov – 2007.
REFERENCES AND LINKS
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[2] C. Toledano, Climatología de los Aerosoles mediante la Caracterización de Propiedades Opticas y Masas de Aire en la Estación ‘El Arenosillo’ de la Red AERONET, PhD Thesis, Universidad de Valladolid, Spain (2005).
[3] A. Angström, “On the atmospheric transmission of sun radiation an on dust in the air”, Geograf. Ann. Deut.
11, 156-166 (1929).
[4] A. Angström, “The parameters of atmospheric turbidity”, Tellus16, 64-76 (1964).
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[6] B. N. Holben, T. F. Eck, I. Slutsker, D. Tanré, J. P. Buis, A. Setzer, E. F. Vermote, J. A. Reagan, Y. J. Kaufman, T. Nakajima, F. Lavenue, I. Jankowiak, A. Smirnov, “AERONET – A federated instrument network and data archive for aerosol characterization”, Remote Sens. Environ. 66, 1-16(1998).
[7] V. E. Cachorro, P. M. Romero, C. Toledano, E. Cuevas, A. M. de Frutos, “The fictitious diurnal cycle of aerosol optical depth: A new approach for “in situ” calibration and correction of AOD data series”,
Geophys. Res. Lett.31, L12106 (2004).
ABSTRACT:
This paper presents the classification and characterization of atmospheric aerosols in Popayan City, Colombia (2,5°N; 76,5°W, 1700m.a.m.s.l.) measured with a manual sunphotometer built to measure the aerosol optical depth (AOD) and the Angstrom alpha parameter (α). Using these parameters we pretend to study and classify the aerosols in the zone of study. The analysis of dispersion diagram between AOD and alpha is a usual procedure used to identify the type of aerosols according to the characteristics of each region.
Keywords: Aerosol Optical Depth, Angstrom Parameter.
RESUMEN:
Este artículo presenta la clasificación y caracterización de aerosoles atmosféricos en la ciudad de Popayán, Colombia (2,5° N; 76,5° W, 1,700 m.s.n.m) usando un fotómetro solar manual construido para medir las magnitudes espesor óptico de aerosoles (AOD) y el parámetro alfa (α) de Angstrom. Con estos parámetros se pretende estudiar y clasificar los aerosoles en la zona de estudio. El análisis del diagrama de dispersión entre AOD y alfa es un procedimiento habitual para identificar el tipo de aerosoles de acuerdo a las características de cada región.
[8] M. Hess, P. Koepke, I. Schult, “Optical properties of aerosols and clouds: The software package OPAC”,
Bul. Am. Meteor. Soc.79, 831-844 (1998).
1. Introduction
1.1. Climatology of the region of study
Southwestern Colombia is characterized by having an important mountainous system, the Andes mountain range, with middle activity volcanoes, a wide extended tropical forest, and for being close to the Amazonian region and the Pacific Ocean. Here we can locate the cities of Popayan in the Department of Cauca and Cali in the Department of Valle. In the north of the Department of Cauca and in the Valle there is a large industrial zone, mainly the one related with sugar cane production. Previous to the cane harvest, crops are burned to facilitate their manual cut, which waste is used as fuel in the process of obtaining the sugar. The burn of this biomass, along with the gases emission from gasoline and diesel vehicles, gases emission from volcanoes and biogenic contamination of the tropical forest are the main sources of atmospheric pollution that affect air quality, crops, and human health significantly. In addition, it is in this low pressure zone where most of the ozone is produced, as well as where the contamination exchange between North and South America occurs.
In Colombia there are no seasons as in other countries, but relief allows finding lands with different temperatures. The country is located into the Torrid Zone and it should have a tropical climate of high temperatures along all year. However, several geographic factors such as its ranges of mountains, deserts and littorals give it great variety of climates that influence on vegetation and fauna.
Colombian territory is mostly located in north hemisphere of earth and into the Torrid Zone through. Therefore, it is the warmest region in the globe, from a latitude point of view (see Fig. 1).
Fig. 1. Geographic location of Colombia.
Existence of the Andes mountain range on Colombian territory causes a great variety of climates, where it is found from the hottest lands (dry or extremely wet) to glaciers (with snow precipitations), and warm lands, according to the relative altitude above sea level. This is where the temperature zones appear.
Temperature in a place is stable enough all through the year as consequence of tropical location of territory and since its solar radiation is uniform. However, temperature varies sufficient from one point to another, following the higher or lower altitude above sea level, fluctuating between 0°C y 35°C, respectively, in the extreme cases.
Winds are mainly produced because of the pressure and temperature difference. From the different kind of winds, trade winds are the most important for Colombia. The zone where Equator line crosses is the warmest in earth and therefore a low pressure zone. Trade winds are those that blow from high pressure zones to equatorial zone.
As it was mentioned before, in Colombia there exist no seasons, but dry periods and rain periods, mainly influenced by the altitude. The rainiest period is found in March, April, May, September, October and November; in June, July, August, December, January and February, the “summer” or dry periods are presented. First case is due to perpendicular incidence of sun rays over the great water masses in equatorial zone, on which Colombian territory is located. Second case comes out because sun rays do not fall directly over this zone [1].
1.2 About aerosols
Because of their origin, atmospheric aerosols are classified as natural, mainly compound by volcanic ashes, marine salt, desert dust, spores, pollen, etc.; anthropogenic, derived from human activity, such as chimney smoke, and mineral particles from industrial processes. Particles produced by photochemistry from gaseous contaminants are another example of this kind of aerosols.
1.3 Aerosol measurement techniques
Aerosols leave their “signature” in the radiation that comes from the sun and arrives in earth’s surface. The study of atmospheric components, not only aerosols but also ozone, water vapor, oxygen, etc., with radiometric techniques is based in comparison between the direct solar radiation spectrum on Earth’s surface and extraterrestrial solar spectrum [2].
Assuming a single layer plane-parallel atmosphere, attenuation can be expressed by Beer-Bouguer-Lambert law:
(
m)
II = 0exp−τ , (1)
where I is the irradiance measured at ground level, I0 is the extraterrestrial irradiance, τ is the total optical depth of the atmosphere (depth of the entire atmospheric column), and mis the optical mass.
From Beer–Bouguer-Lambert law, the atmos-pheric total optical depth is defined by:
⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − = τ 0 ln 1 I I
m . (2)
The aerosol optical depth (AOD) is the main parameter in the study of the aerosol properties. It is an indicator of the vertical content of amount of aerosols in the atmosphere, and according to its values and by using Mie theory, it can be used the inversion algorithms methodology to obtain the aerosols size distribution. In this particular case, AOD is equally to τ (given by equation 5) since the measuring range does not cover other contributions like Rayleigh and gases [2].
Spectral dependence of Mie scattering is related to the particle size, in particular, by the relationship between particle size and the wavelength. Angstrom proposed the following parameterized expression, valid for a certain spectral range:
α −
βλ =
τa . (3)
where λ is the wavelength (in µm), α is the Angstrom parameter (related with aerosol size) and β is a constant that coincides with optical depth of 1 µm.
Theoretically, alpha parameter can take several values between 0 and 4. Lower values are associated to large particles, in which extinction do not have spectral dependence. For aerosols, it is usually that
α vary between 0 and 2,5 [3,4].
This paper presents the classification and characterization of atmospheric aerosols in Popayan City-Colombia using a manual sunphotometer built
to measure the aerosol optical depth (AOD) and Angstrom alpha parameter (α), magnitudes used in this work to study and classify the type of aerosols in the zone of study.
3. Results
The obtained results were taken with a manual photometer built by authors [5] consisting of three channels, with narrow band interference filters of 400 nm, 550 nm and 650 nm, and bandwidth (FWHM) of 10 nm. The three chosen wavelengths let obtain data for different kinds of aerosols in the range from 0,1 μm to 1 μm, known as aerosols in the accumulation interval, that remain long time in the atmosphere and are very interactive with light [6].
Results, shown as follows, were taken during sun period at the beginning of the year, which is, from January to about the middle of March, and some April days.
As it is observed in figure 2, AOD variation is really sharp due to fictitious diurnal cycle, so it was necessary to do a calibration – correction by means of KCICLO method in order to calculate the optical depth absolute error [7]. In addition, values of AOD initially obtained were too high in relation to those expected for the region of study. Subsequently, data were re-evaluated and the following results were obtained, Table I.
Fig. 2. AOD January – April, 2007. Popayán, Cauca.
Table I.
Statistical analysis of AOD data from January to April
AOD650 AOD550 AOD400 Mean 0,06187 0,08824 0,15935
SD 0,00796 0,00849 0,01053
P25 0,0424 0,07436 0,12793
P75 0,07286 0,10386 0,18241
Minimum 0,00247 0,00561 0,00771
Maximum 0,19408 0,13174 0,29921
According to these measurements we have as representative days:
Fig. 3. AOD January 30, 2007. Popayán, Cauca.
Fig. 4. AOD March 6, 2007. Popayán, Cauca
Fig. 5. AOD April 3, 2007. Popayán, Cauca
Alpha parameter data were obtained among wavelengths; (550/650) nm. These results are shown in Fig 6.
From an analysis of this figure, it is observed that
α values are into a range between 0.25 and 2.6 for
α(550/650) for AOD (550 nm) between 0,095 and 0,131 (Table II), which allows to affirm that analyzed aerosols correspond to continental aerosol, according to what is reported by OPAC aerosol model [8]. In Table II, alpha parameter values are recorded for both wavelength ranges analyzed.
Fig. 6. AOD diagram against Alfa Parameter α(550/650)
Table II
Statistical analysis of Alpha parameter data at (400/550) nm and (400/650) nm.
ALPHA (550/650)
Mean 1,64651
SD 0,15845
P25 0,999
P75 2,03
Minimum -0,151
Maximum 2,557
Median 1,805
4. Conclusions
This classification represents the first step into the development of a reliable database to determine the climatology of the region. Reported data were collected in a three months period, corresponding to the first summer period of the year. It is expected to have data covering the second summer period of the year, which allows monitoring the annual variation of the aerosols presence in the zone.
Aknowledgements
