Solar dimming and brightening over Thessaloniki, Greece, and Beijing, China

Abstract

This work presents evidence that ultraviolet (UV)‐A solar irradiances show increasing trends at Thessaloniki, Greece, where air quality has been improving because of air pollution abatement strategies. In contrast, over Beijing, China, where air quality measures were taken later, solar brightening was delayed. It is shown that until the early 1990s, UV‐A irradiances over Thessaloniki show a downward trend of –0.5% yr−1, which reverses sign and becomes positive in the last decade (+0.8% yr−1). This brightening is related to a decreasing trend in local aerosol amounts. Both the negative rate of change (dimming) and the positive rate of change (brightening) are amplified in the UV‐A solar irradiances, compared with the total solar irradiance, by a factor of 2.6. Satellite derived short‐wave radiation over Beijing showed negative changes of –0.4% (1984–1991) and –0.1% yr−1 during 1994–2006. The negative trend in solar radiation continued even during 2000–2006. Satellite‐derived aerosol optical depth (AOD) increased by +1.0% yr−1 during 2000–2006, in agreement with in situ measurements of increasing AOD. Therefore, a statistically significant change from dimming to brightening in Beijing could not be seen in the last decade, but it is expected to occur in the near future.

Solar dimming over the tropical urban region of Hyderabad, India: Effect of increased cloudiness and increased anthropogenic aerosols

“…Both trends are found to be statistically significant at the 95% confidence level, underlying a significant increase in aerosol load over Hyderabad in the last 6 years, mainly attributed to the economic growth and industrialization, population growth, increased vehicular emissions and expansion of the city. This increasing trend is opposite to that observed in the AOD over Mediterranean [Papadimas et al, 2008] and Thessaloniki, Greece (decreasing rate of −4.5% yr −1 ), but it is in agreement with the increasing AOD trend (1.0% yr −1 ) over Beijing between 2000 and 2008 [Zerefos et al, 2009]. Such a large increase in AOD over an urban area with surrounding industrial activities may be a significant reason for the solar dimming over Hyderabad observed in the recent years, even under clear (without clouds) atmospheres (Figure 1c).…”

“…A recent study over China and Japan [Norris and Wild, 2009] has proved that the variations in cloudiness and cloud properties (optical depth, albedo) are most responsible for the changes in solar radiation reaching the ground. However, large decadal changes in solar radiation are not found only for all-sky conditions (including clouds), but also for cloudless atmospheres [e.g., Ohvril et al, 2009;Zerefos et al, 2009] indicative of anthropogenic contributions through changes in aerosol emissions governed by economic development.…”

“…Moreover, as experience in European Union has shown [e.g., Zerefos et al, 2002], legislative measures to reduce air pollution take a decade or so to be transformed into observable improvements in air quality. Zerefos et al [2009] have shown that where stringent air-quality abatement efforts started over regions before 20 year or so ago, global brightening is expected to have already started. Thus, a significant part of the brightening is related to the reduction of the local aerosol emissions in recent years, a fact that is far away from today's situation in India [e.g., Padma Kumari et al, 2007].…”

“…During 1980s, the reduction in total solar radiation under cloudless conditions was established by several studies [Stanhill and Cohen, 2001;Stanhill and Moreshet, 1992;Liepert, 2002;Romanou et al, 2007]; this phenomenon showed a great variability from region to region and was mainly attributed to the increase in anthropogenic aerosols especially in the developing and the densely populated countries. On the other hand, other studies Romanou et al, 2007;Zerefos et al, 2009] have shown that the negative tendencies in the solar radiation over Europe and North America have reversed sign in the last decade, a phenomenon that was attributed to the control of emissions and the improvement of air quality.…”

“…[3] A number of studies conducted over various worldwide locations have suggested significant variations in solar radiation intensity considered as "solar dimming and brightening phenomenon" [e.g., Liepert, 2002;Wild et al, 2004Wild et al, , 2005Norris and Wild, 2007;Ohmura, 2009;Zerefos et al, 2009]. The majority of these studies have been included in the review study by Wild [2009], and nearly all conclude to a decrease in solar radiation during the period 1960-1990 (solar dimming) and a recovery (solar brightening) at some locations in the 2000s.…”