Sergio Cabrera

Variations in UV radiation in Chile

The first observations of UV global radiation in Chile with a portable multichannel radiometer are reported. Four UV wavelengths are included: 305, 320, 340 and 380 nm. Observation latitudes spanning from 18 degrees to 53 degrees S allow an estimation of latitudinal variations in daily maxima for both summer and winter. Measurements over Santiago deviate from a smooth latitudinal profile, probably as a consequence of urban air pollution. The main effect of this is to prevent UV solar radiation from reaching the ground, especially during winter. Altitudinal increments in UV radiation are estimated by comparing observations along the coast and valleys with others on the Andes and one isolated summit. Diurnal variations in the height increment support an increase from morning to evening. The results indicate that in rural areas the altitudinal increment is lower (4%-10% per kilometre) than that reported for Europe, reaching very low magnitudes (2% or less) in the Andean summits of desert regions.

Ozone and UV Radiation over Southern South America: Climatology and Anomalies

Abstract Ozone and UV radiation were analyzed at eight stations from tropical to sub-Antarctic regions in South America. Ground UV irradiances were measured by multichannel radiometers as part of the Inter American Institute for Global Change Radiation network. The irradiance channels used for this study were centered at 305 nm (for UV-B measurements) and 340 nm (for UV-A measurements). Results were presented as daily maximum irradiances, as monthly averaged, daily integrated irradiances and as the ratio of 305 nm to 340 nm. These findings are the first to be based on a long time series of semispectral data from the southern region of South America. As expected, the UV-B channel and total column ozone varied with latitude. The pattern of the UV-A channel was more complex because of local atmospheric conditions. Total column ozone levels of <220 Dobson Units were observed at all sites. Analysis of autocorrelations showed a larger persistence of total column ozone level than irradiance. A decreasing cross-correlation coefficient between 305 and 340 nm and an increasing cross-correlation coefficient between 305 nm and ozone were observed at higher latitudes, indicating that factors such as cloud cover tend to dominate at northern sites and that ozone levels tend to dominate at southern sites. These results highlight the value of long-term monitoring of radiation with multichannel radiometers to determine climatological data and evaluate the combination of factors affecting ground UV radiation.

UV index values and trends in Santiago, Chile (33.5°S) based on ground and satellite data

We report on the surface UV index (UVI) variations in Santiago (Chile) a city with high air pollution and complex surrounding topography. Ground-based UV measurements were continuously carried out between January 1995 and December 2011, by using a multi-channel filter radiometer (PUV-510). Ground-based measurements and satellite-derived data retrieved from the Total Ozone Mapping Spectrometer (TOMS), the Ozone Monitoring Instrument (OMI), and the Scanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY), were compared. We found that satellite-derived UVI products largely overestimate surface UVI. Our ground-based UVI measurements were significantly lower than TOMS-derived UVI data: (46.1±6.3)% (in the period 1997-2003), and OMI-derived UVI data: (47.0±6.3)% (in the period 2005-2007). Clear-sky SCIAMACHY-derived UVI were found to be also nearly systematically greater than ground-based UVI measurements in the period 2002-2011. An exceptionally long period of clear skies between December 2010 and January 2011 was used to test further satellite-derived UVI data; in the whole period, OMI and SCIAMACHY data were 53.1% and 38.3% greater than our ground-based measurements, respectively. These differences are presumably due to aerosol load associated with the local pollution and the complex topography surrounding Santiago. In addition, linear regression allowed us to estimate trends that we use for forecasting. Methodological details are provided below.

Interannual variations of global UV radiation in Santiago, Chile (33.5°S)

Observations with a four-channel UV radiometer in Santiago, Chile (33.5°S, 70.6°W) from January 1992 to December 1998 are presented. Channels are centered at 305, 320, 340 and 380 nm with a 10 nm bandwidth. Measurements were made at one-minute intervals. Hourly mean values at noon for 305 and 340 nm are presented as well as instantaneous irradiances for 60° of solar zenith angle. Their aperiodic variations on a seasonal scale are discussed with respect to the quasi-biennial oscillation (QBO) and ENSO phenomena. A significant positive trend that must be caused by decreasing total ozone is found in 305 nm irradiance. On a seasonal basis, this negative trend appears strong and significant during winter.

Multichannel radiometer calibration: a new approach.

The error in irradiance measured with Sun-calibrated multichannel radiometers may be large when the solar zenith angle (SZA) increases. This could be particularly detrimental in radiometers installed at mid and high latitudes, where SZAs at noon are larger than 50 degrees during part of the year. When a multiregressive methodology, including the total ozone column and SZA, was applied in the calculation of the calibration constant, an important improvement was observed. By combining two different equations, an improvement was obtained at almost all the SZAs in the calibration. An independent test that compared the irradiance of a multichannel instrument and a spectroradiometer installed in Ushuaia, Argentina, was used to confirm the results.

Satellite-derived UV irradiance for a region with complex morphology and meteorology: comparison against ground measurements in Santiago de Chile

Ground-based measurements of ultraviolet (UV) irradiance, carried out by a four-channel UV radiometer in Santiago de Chile from October 2004 to December 2011, have been used to estimate daily values of the UV index (UVI). These ground-based data have been compared with UVI estimates retrieved from the Ozone Measurement Instrument (OMI) on board the Aura spacecraft. Since the widely used OMI-gridded UVI data may not be suitable for the complex local morphology and meteorology, a careful screening of overpass OMI data was applied. Nevertheless, we found that OMI-derived UVI data overestimate ground-based values; depending on cloud-cover conditions, the mean bias (MB) and the root mean square error (RMSE) range from 34.53% to 30.29% and from 35.22% to 43.50%, respectively, with the lowest MB (and the highest RMSE) values occurring under overcast conditions. Moreover, the difference between satellite-derived and ground-based UVI data exhibits a limited seasonality with somewhat larger differences in the fall season. The detected overestimation seems to be linked with the boundary layer aerosol absorption that is not accounted for by the OMI algorithm. Indeed, we found that the difference in UVI increases with the aerosol concentration (which in Santiago shows seasonal variations). Ceilometer profiles of backscatter intensities, directly related to aerosol concentrations, and PM10 concentrations correlate with UVI differences (correlation coefficient r of approximately 0.6 and 0.4, respectively) under cloud-free conditions for time scales ranging from months to years. Additional comparisons were performed between UVI estimates retrieved from our ground-based measurements in Santiago and from the Tropospheric Emission Monitoring Internet Service (TEMIS) Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY). Under cloudless conditions, also TEMIS-derived data overestimate ground-based UVI estimations (by about 31%) and exhibit a small seasonality.

Photosynthetic parameters of the entire euphotic phytoplankton of the Bransfield Strait, Summer 1985

SummaryThe photosynthetic parameters αB, PBm, and Ik were derived from the relationship between primary productivity (P) and irradiance (I) for mixed samples, each mixture consisting of seven samples of phytoplankton collected from different depths, between the surface and 35 m at eight different sampling stations in the Bransfield Strait in January–February 1985. The αB values obtained in this study were considerably higher than those generally reported for phytoplankton from a single depth in polar waters. These higher values may be attributed to the fact that most of the phytoplankton in the mixed samples came from depths with limiting irradiances, which had a greater yield than surface phytoplankton. The PBm value we obtained was lower than that reported by other authors, possibly because the mixed sample phytoplankton have saturation points at low light intensity. Our IK values confirmed the ability of phytoplankton in these latitudes to reach maximum photosynthesis rates with low densities of photonic flux. When treated as independent parameters, the αB and PmBvalues showed a significant correlation for each sampling station. The efficiency of the entire euphotic phytoplankton of the water column, estimated as ∫0euPdz/∫0euBdz or Integral Assimilation Number (IAN), showed a predictable significant correlation with the αB values calculated from the entire euphotic phytoplankton. Areas with high surface biomass were as productive as those with low surface phytoplankton biomass. Areas with low surface phytoplankton biomass showed higher αB values than areas with high surface biomass, which suggests, that, the αB values for the entire photosynthesizing phytoplankton in the water column may have greater predictive value than the αB values for the phytoplankton from a single depth. This finding should be taken into account when satellites are used for studying marine production processes, especially in areas with very different euphotic depths and surface phytoplankton concentrations.

Spatial and temporal photosynthetic compartments during summer in Antarctic Lake Kitiesh

SummaryFour autotrophic compartments were recognised in Lake Kitiesh, King George Island (Southern Shetland) at the beginning of the summer in 1987: snow microalgae, ice bubble communities, phytoplankton in the water column and benthic communities of moss with epiphytes. Chlorophyll a concentration and pigment absorption spectra were obtained in these four compartments before and/or after the thawing of the ice cover. During the ice free period, carbon fixation and biomass was measured in the phytoplankton and in the benthic moss Campyliadelphus polygamus. From these measurements we conclude that the benthic moss is the most significant autotrophic component in this lake in terms of biomass, chlorophyll a content and primary productivity. The integral assimilation number (The ratio of carbon fixation per unit area to biomass per unit area) values were similar for both phytoplankton and the moss, ranging from 3.6 to 5.4 mg C (mg Chl a)−1h−1in phytoplankton and from 4.0 to 6.4 mgC (mg Chl a)−1h−1 in the benthic moss. This approach allows comparisons of carbon fixation efficiency of the chlorophyll a under a unit area between compartments in their different light environments.

Calibration improvement of the IAI Network for the measurement of UVR: multichannel instruments

The IAI Network for the measurement of ultraviolet radiation in Chile, Argentina and Puerto Rico is composed of ten multi-channel radiometers (GUV 511, Bisopherical Instruments Inc.), which are periodically sun calibrated with a traveling reference GUV (RGUV). The RGUV is calibrated under solar light against a SUV100 spectroradiometer. This calibration is then transferred to each instrument in the network through the RGUV. A previous multi-regression model proved to be suitable to derive narrowband irradiance from broadband irradiance, ozone column and solar zenith angles (SZA). A recent modification of the existing multi-regression model improved the multi-channel instrument sun calibration against spectroradiometers. In this approach, the narrowband irradiance is the SUV spectral irradiance and the broadband is the multi-channel GUV irradiance. We included the azimuth angle as a parameter into the multi-regression equation and we applied a non-linear function, instead of a single coefficient, to correct for SZA. In this paper, the new multi-regression approach is applied to both steps of a GUV calibration: SUV - RGUV and RGUV - GUV and the results are compared with traditional calibration methods. Important improvements are observed in the calibration, in particular for SZA larger than 50°.