Temporal variability of solar ultraviolet radiation in low and mid-latitude South American sites.

Abstract

Solar Ultraviolet Radiation (UV), consisting of electromagnetic waves from 100–400 nm, accounts for about 5% of the sun’s emitted energy. UV exposure has both risks and benefits, impacting human health, ecosystems, materials, and biogeochemical cycles. Climate change, influenced by land use changes and other factors, alters UV intensity based on location, seasons, and atmospheric composition. The UV Index, a dimensionless indicator, helps people assess UV levels and adopt protective measures. However, in middle-income countries like Brazil and Argentina, UV monitoring networks are sparse and underfunded, limiting reliable UV data. Satellite data become crucial, although they are also subject to instrument errors and algorithmic uncertainties. This research investigates the diurnal and seasonal variability of UV levels at five sites across low and mid-latitude South America, under both all-sky and clear-sky conditions, utilizing ground-based and satellite-derived data. The study spans from 2005 to 2022, varying according to each dataset. The sites are: Santarém in the Brazilian Amazon (2.25°S; 54.44°W, 18 m, Apogee SU-100-ss UV sensor), Natal in Northeast Brazil (05.78°S; 35.21°W, 30 m, Davis 6490 UV sensor), Itajubá in Southeast Brazil (22.41°S, 45.44°W, 885 m, Davis 6490 UV sensor), Santa Maria in the south of Brazil (29.4°S, 53.8°W, 476 m, Brewer Spectrophotometer MKIII #167), and Buenos Aires in Argentina (34.58°S, 58.48°W, 25 m, Solar Light UV Biometer – Radiometer model 501). Additionally, the study compares ground-based and satellite-derived UV Index levels from OMI at noon and overpass times, and GOME-2 at noontime during clear sky conditions, which are assessed using LER (Lambertian Equivalent Reflectivity) and METAR cloud cover data. Santarém and Natal, tropical low-latitude cities, displayed a semi-annual cycle with two maxima and consistently higher year-round UV index levels. In contrast, Itajubá, Santa Maria, and Buenos Aires exhibited an annual cycle, with UV index peaking during austral summer. Cloud cover played a dual role in UV variability. In Santarém, UV levels under all-sky conditions exceeded clear-sky levels during certain rainy months, while in Natal, noontime all-sky UV index were 6.8% higher during periods of prevalent broken clouds. Extreme UV index were observed from 10 AM to 2 PM in Natal and Santa Maria and from 11 AM to 2 PM in Buenos Aires. Satellite data comparisons showed that UV index derived from OMI had a lower bias than those from GOME-2, though both datasets exhibited low overall errors. At Natal, the LER method identified significantly more clear-sky days than METAR, with the combined dataset (LER & METAR) providing the most reliable results for overpass time. Data from Itajubá exhibited the largest discrepancies and highest errors, while Santa Maria and Buenos Aires showed better agreement, with Buenos Aires presenting the most accurate results.This thesis makes an important contribution to understanding the variability of UV radiation in South America. The methodologies applied here also hold potential for expanding UV and cloud cover research to other underrepresented areas. This work confirms the reliability of instruments and datasets, providing a foundation for environmental and health studies in the complex field of UV radiation, its interactions, and its effects, addressing a critical knowledge gap in atmospheric and environmental sciences.