UV index climatology over the United States and Canada from ground‐based and satellite estimates

Abstract

[1] Long-term monthly mean UV index values for Canada and the United States were calculated using information from two sources: from noon erythemal UV estimated from Total Ozone Mapping Spectrometer (TOMS) total ozone and reflectivity data and from UV index values derived from observations of global solar radiation, total ozone, dew point, and snow cover. The results are presented as monthly maps of mean noon UV index values. Mean UV index values in summer range from 1.5 in the Arctic to 11.5 over southern Texas. Both climatologies were validated against spectral UV irradiance measurements made by Brewer spectrophotometers. With snow on the ground the TOMSbased data underestimate UV by up to 60% with respect to Brewer measurements and UV derived from global solar radiation and other parameters. In summer, TOMS UV index climatology values are from 10 to 30% higher than those derived from global solar radiation and other parameters. The difference is probably related to aerosol absorption and pollution effects in the lower troposphere that are not currently detected from space. For 21 of 28 midlatitude Brewer sites, long-term mean summer UV measured values and UV derived from global solar radiation and other parameters agree to within +5 to À7%. The remaining seven sites are located in ''clean'' environments where TOMS estimates agree with Brewer measurements while UV derived from global solar radiation and other parameters is 10-13% lower. Brewer data also demonstrate that clean and ''typical'' sites can be as little as 70-120 km apart.

Short- and long-term variability of spectral solar UV irradiance at Thessaloniki, Greece: effects of changes in aerosols, total ozone and clouds

“…Signs from the onset of ozone recovery since the late 1990s on surface UV-B irradiance have been mainly detected over the northern high latitudes; recent studies indicate that UV-B has been declining during the last 2 decades (Bernhard, 2011;Eleftheratos et al, 2014). However, during the same period, both the UV-B and the UV-A irradiance are increasing over many locations in the Northern Hemisphere mid-latitudes, mainly due to the negative trends in the amount of clouds and aerosols (De Bock et al, 2014;Fitzka et al, 2012;Román et al, 2015;Smedley et al, 2012;Zerefos et al, 2012). In a recent study, Fragkos et al (2015) showed that in Thessaloniki, even under extreme high (low) total ozone column (TOC) conditions, the erythemal irradiance can be lower (higher) than its climatological values due to the dominant effect of aerosols.…”

“…However, during the same period, both the UV-B and the UV-A irradiance are increasing over many locations in the Northern Hemisphere mid-latitudes, mainly due to the negative trends in the amount of clouds and aerosols (De Bock et al, 2014;Fitzka et al, 2012;Román et al, 2015;Smedley et al, 2012;Zerefos et al, 2012). In a recent study, Fragkos et al (2015) showed that in Thessaloniki, even under extreme high (low) total ozone column (TOC) conditions, the erythemal irradiance can be lower (higher) than its climatological values due to the dominant effect of aerosols. Zerefos et al (2012) suggest that over Canada, Europe and Japan there is a statistically significant evidence of a slowdown or even a turning point in the upward UV-B trends after 2006, which is an indication that since then the negative trends of aerosols are no more the main driver of the long-term changes in UV-B irradiance.…”

“…Additionally, reliable estimations of the trends of spectral surface UV irradiance provide useful information for assessing these impacts and for adopting proper measures (Morgenstern et al, 2008;Newman and McKenzie, 2011;van Dijk et al, 2013). Climatologies and trends of surface UV irradiance (spectral or broadband) can be derived either directly from ground-based measurements (Fitzka et al, 2012;Glandorf et al, 2005;Zerefos, 2002), or indirectly from measurements of surface reflectivity, ozone, aerosols and cloudiness derived either from satellites (Damiani et al, 2014;Fioletov et al, 2004;Li et al, 2000), or from ground-based instruments (Antón et al, 2011;Román et al, 2015;Walker, 2009). The uncertainties of these parameters and the applied methodologies increase the uncertainty of the indirectly derived UV irradiance, when compared to measurements (Cordero et al, 2013;Weihs and Webb, 1997).…”

Estimation of photolysis frequencies from TOMS satellite measurements and routine meteorological observations

“…These estimates are affected by both instrumental errors and modelling uncertainties . Individual comparisons of satellite estimates with GB measurements has shown mainly a positive bias of TOMS, especially during summertime (Herman et al, 1999;Chubarova et al, 2002;Kazantzidis et al, 2005) and in some cases a negative bias in winter (Kalliskota et al, 2000;Fioletov et al, 2004). According to Herman et al (1999) and McKenzie et al (2001), TOMS produces systematically higher UV irradiance values at the northern midlatitudes, while in the Southern Hemisphere a better agreement with surface measurements has been found.…”

Solar radiation and human health

“…They concluded that TOMS UV retrievals have a positive bias between 1.4% and 12.5% at these sites. Fioletov et al [86] derived UV index climatologies for Canada and the USA using TOMS data for 1980-1990 and compared with Brewer measurements from several sites. They found that in summer the TOMS UV estimates were 10-30% higher than the Brewer measurements, most likely caused by air pollution and absorbing pollution.…”