Clean sky at Southern Patagonia

The presence of atmospheric aerosols decreases the solar radiation that reaches the ground. If their concentration increases significantly, there could be a potential loss of incoming solar radiation. Due to the fact that particles of different size absorb light in different ways, atmospheric aerosols can be characterized by the extinction they produce. Aerosol optical depth is indicative of the radiative significance of aerosol particles and can be calculated by measuring direct solar spectral irradiance.

There is seldom aerosol data of southern hemisphere and none of southern Patagonia. Since May 1997, the atenuation of solar light is being measured at Río Gallegos ( 51,63S; 69,15 W ). Data is obtained with a handheld NOAA Serie J sunphotometer for 380 and 500 nm with a 2,5º field of view and reduced by the Langley Plot slope method. To assure stable atmospheric conditions, data is only taken into account when the residual standard deviation is less than 0.002 with correlation coefficients grater than 0.9990.

It was found that aerosol optical depth background values for 380 nm are mostly arround 0.02, like Mauna Loa or Antartic background values. When Armstrong turbidity factors a and b are calculated, most of background b values are less than 0.005, which can be associated to clean sky. There are some a negative values, which can be due to an aerosol particle size distribution different than the one proposed by Armstrong or that particles are so few that the equipment is not sensitive to their light attenuation. With a 2 channel sunphotometer is not possible to assure the first possibility.