Direct Radiative Forcing Due to Aerosol Properties at the Peruvian Antarctic Station And Metropolitan Huancayo Area

Descrevemos os resultados do estudo da profundidade otica do aerossol (POA) e do Forcamento Radiativo Direto (FRD) no topo da atmosfera (TOA), obtidos durante a campanha de medicao e monitoramento, XXI Expedicao Antartica do Peru, entre os meses de janeiro e fevereiro de 2013, e na area metropolitana de Huancayo (AMH) entre os meses de junho e julho de 2019. Na Estacao Antartica Peruana Machu Picchu utilizou-se um fotometro solar SP02-L. Tal instrumento possui 4 canais: 412 nm, 500 nm, 675 nm e 862 nm, permitindo realizar medicoes diretas do espectro de radiacao visivel. Na AMH usamos o sensor BF5, que mede a radiacao direta, difusa e global em comprimento de onda curta. Os calculos de AOD em latitudes polares variaram entre 0,0646 e 0,1061. Na AMH apresenta valor maximo de 0,58 (11 de junho) e minimo de 0,19 (12 de junho). Determinou-se o coeficiente de Angstrom variando de 0 a 0,07, esses valores indicam a presenca de particulas grandes. Na AMH varia de 0 a 1,8, que indica a presenca de  aerossois de fonte de queima de biomassa e industrial. As propriedades oticas observadas foram usadas para estimar a forcante radiativa direta por aerossois (FRDA) no topo da atmosfera. Os resultados indicam que no King George Island, o FRDA, esta entre -2 e 4 W/m 2 ; ja para a AMH a forcante radiativa direta de aerossol esta entre 0 e 20 W/m 2 .


INTRODUCTION
As the direct solar radiation passes through the atmosphere, it is attenuated by two main physical processes: scattering (angular redistribution of energy) and absorption (conversion of energy into either heat or photochemical change). Both effects are known to be wavelength dependent (El-Shobokshy & Al-Saedi, 1993). Traditionally, the tool most commonly used for monitoring the aerosol optical depth of the atmosphere has been the sun photometer.
Essentially, the instrument which is small in physical dimensions, is sighted at the sun to measure the direct solar irradiance in some selected narrow spectral intervals (Volz, 1959).
Aerosol particles affect the climate system via the following physical mechanisms: First, they scatter and can absorb solar radiation. Second, they can scatter, absorb and emit thermal radiation. Third, aerosol particles act as cloud condensation nuclei (CCN) and ice nuclei (IN).
The first two mechanisms are referred to as direct effects and are not subject of this paper but are discussed in detail in e.g., Haywood & Boucher (2000). The last one is referred to as indirect effect. It will be the subject of this review together with other atmospheric properties influenced by aerosols (e.g. semi-direct effect, suppression of convection).
Even though the semi-direct effect is a consequence of the direct effect of absorbing aerosols, it changes cloud properties in response to these aerosols and therefore is part of this review on aerosol-cloud-interactions.
Radiative Forcing (RF) is defined as the change in net irradiance at the tropopause due to an applied perturbation holding all atmospheric variables fixed, once stratospheric temperatures have been allowed to adjust to equilibrium. The concept of RF was first developed for one-dimensional (1-D) radiative convective models (e.g., Intergovernmental Panel

Site description
The Scientific Peruvian Station at Antarctic "Machu Picchu" (referred as ECAMP, 62º05'30" S, 58º28'16" W and 6 meter above sea level (masl)) is located at King George Island, the largest of the South Shetland Islands, on the northern tip of the Antarctic peninsula ( Figure 1). About 90% of King George Island extension is covered by ice (Simoes et al., 1999). The Peruvian station is located very close to the open ocean so the climate regime is  The study was also conducted in the MHA located at coordinates 12°4´12.03´´S, 75°12´43.55´´W with altitude of 3300 masl, it is part of the central Andean region of Peru, as seen in Figure 2, located in South America and east of the Pacific Ocean. It is one of the 10 most populous provinces of Peru, whose annual population growth rate is 1.6%, with more than half a million inhabitants (Instituto Nacional de Estadistica e Informatica, 2007.
The MHA belongs to the Mantaro Valley, it occupies an area of 319.4 km 2 . Its topography is quite complex with rock formations and altitudes ranging between 3000-5000 masl, this range of altitudes is generally due to the presence of mountains. For the corresponding measurements of solar radiation in the Province of Huancayo, was used the BF5 sensor, located at 12°4′0″ S and 75°13′0″ W, in the Metropolitan Huancayo Area. This records data with a frequency interval every minute, of the following variables: global radiation and diffuse incident radiation since June 4, 2019. With this information, the atmospheric clarity index was determined, and the temporal variability was analyzed direct, diffuse and global solar radiation. The BF5 Sensor is a patented design. It uses a series of photodiodes with a unique computer-generated hatch pattern to measure incident solar radiation. A microprocessor calculates the Global and Diffuse components of solar radiation. A built-in heater keeps the BF5 free from dew, ice, and snow down to -20 °C.

Aerosol Optical Depth
The direct solar irradiance measured with this sun-photometer is used to describe the spectral values of the aerosol optical depth (AOD) associated to each wavelength that are determined based on the well known law of Lambert-Bouguer-Beer. (1) Where is the solar intensity recorded at each wavelength , is the value of solar radiation at the top of the atmosphere (TOA), R is the solar distance expressed as astronomical units, m is the air optical mass and is the total optical depth dependent on wavelength.
This last term can be described as the sum of the different constituents of the atmosphere.
Where is the aerosol optical depth, is the Rayleigh-Scattering optical depth, and is the ozone optical depth (Liou, 2007).
It should be noted that equations 1and 2 are used in the estimation of AOD for ECAMP.
For AMH, if the model established by Iqbal (1983) is used, the optical aerosol thickness can be estimated using the IQC model:

Angstrom exponent
Another important optical property of aerosols is the Ångström exponent (Ångström, 1964). It permits to quantify the spectral dependence of the AOD related with size and the vertical profile (Tomasi et al., 1983). Ångström exponent is the slope of the lines that passes

Aerosol Optical Depth
Records of AOD at polar latitudes shows the lowest values of the world with higher values at Arctic than Antarctic. Figure 3A shows The comparison between the ECAMP and the other station offers the evidence of the main and important differences of the optical properties of the aerosols. Polar sites has a relatively very clean atmosphere, but they have a strong influence of particles, very small, mainly of marine source and eventually from anthropogenic sources and to the turbidity conditions that is usually present in summer and fall because the strong prevailing winds that also transport haze and dust (Shaw, 1982).

Angstrom coefficient
Angstrom coefficient α is useful to compare and characterize the wavelength dependence of AOD and columnar aerosol size distribution (Eck et al., 1999;Cachorro et al., 2001).
Smaller values represents bigger particles, for example dust. On the other hand higher values represent smaller particles like smoke and/or burning particles (Shifrin, 1995). One way to 114 discriminate if the aerosols are mainly composed by particles of medium -small radius, smaller than 1 mm, or higher is to calculate the Ångström for the evaluated days. Values of α that are in the range of 0.12 and 0.4 indicates the presence of particles of big size (Otero et al., 2006), as it is shown in figure 3A from for the Antarctic site. During the year of campaign, the mean value for Angstrom coefficient ( ) varied from 0 to 0.07 that represents a low variability that can be both to instrumental and atmospheric properties but also indicates the dominance of big particles probably coming from maritime source. Also to the MHA presents the mean value for Angstrom coefficient ( ) varied from 0 to 1.8, that indicated the presence the aerosols types biomass burning and industrial.

Aerosol Top Of Atmosphere (TOA) Direct Aerosol Radiative Forcing
The Aerosol TOA Direct aerosol radiative forcing (DARF) is strongly dependent of AOD (τ a ) and of single scattering albedo (SSA, ω 0 ), that it is a measure of scattering and absorption processes of solar light caused by aerosols becoming a key variable for DARF determination.
Comparing the forcing estimates with AOD values, we find that the radiative forcing is primarily governed by the magnitude of AODs which varied from a low value of 0.04 to high values above 0.065 at 0.5 um during the field campaigns.
For evaluating and estimating the DARF it was used the median of AOD (at 500 nm) as it is the most representative value due to this non-parametric distribution. Our estimation for the King George Island site suggests that based on the equation 5 the direct aerosol radiative forcing is between [-2 4] w/m 2 . The figure 5BÑ shows the variation of the DARF based on the optical properties determined in the MHA, also the direct aerosol radiative forcing is between [0 20] w/m 2