Abstract
Abstract. This study investigated the decadal variation of the direct surface solar radiation (DiSR) and the diffuse surface solar radiation (DfSR) during 1961–2008 in the Shanghai megacity as well as their relationships to Aerosol Optical Depth (AOD) under clear-sky conditions. Three successive periods with unique features of long term variation of DiSR were identified for both clear-sky and all-sky conditions: a "dimming" period from the late 1960s to the mid 1980s, a "stabilization"/"slight brightening" period from the mid 1980s to the mid 1990s, and a "renewed dimming" period thereafter. During the two dimming periods of DiSR, DfSR brightened significantly under clear-sky conditions, indicating that change in atmospheric transparency resulting from aerosol emission has an important role on decadal variation of surface solar radiation (SSR) over this area. The analysis on the relationship between the Moderate-resolution Imaging Spectroradiometer (MODIS) retrieved AOD and the corresponding hourly measurements of DiSR and DfSR under clear-sky conditions clearly revealed that AOD is significantly correlated and anti-correlated with DfSR and DiSR, respectively, both above 99% confidence in all seasons, indicating the great impact of aerosols on SSR through absorption and/or scattering in the atmosphere. In addition, both AOD and the corresponding DiSR and DfSR measured during the satellite passage over Shanghai show obvious weekly cycles. On weekends, AOD is lower than the weekly average, corresponding to higher DiSR and lower DfSR, while the opposite pattern was true for weekdays. Less AOD on weekends due to the reduction of transportation and industrial activities results in enhancement of atmospheric transparency under cloud free conditions so as to increase DiSR and decrease DfSR simultaneously. Results show that aerosol loading from the anthropogenic emissions is an important modulator for the long term variation of SSR in Shanghai.
Highlights
Solar radiation at the Earth’s surface is the ultimate energy source for life on the planet
The goal of this study is to elucidate the role of aerosol on the secular variation of surface solar radiation (SSR) through analysis on Aerosol Optical Depth (AOD), direct surface solar radiation (DiSR) and diffuse surface solar radiation (DfSR) measured in Shanghai, the largest megacity of China, and to investigate a new insight into the origin of dimming and brightening in the east china monsoon region (Wu et al, 2009; Li et al, 2010)
The same procedure is performed for Aqua-AOD and the associated Aqua-DiSR and DfSR in order to study the role of aerosol on SSR more fully and rigorously
Summary
Solar radiation at the Earth’s surface (surface solar radiation, SSR) is the ultimate energy source for life on the planet. It governs diverse surface processes, such as evaporation and associated hydrological cycles. For the period between 1955 and 2000, Qian et al (2006) determined an average decrease of 3.1 W m−2 decade−1 in SSR at 85 sites in China, and Shi et al (2008) found a decreasing trend of 2.5% decade−1 (∼3.8 W m−2 decade−1) with a more pronounced dimming of 4.6% decade−1 for the sub-period of 1961 to 1989
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