Homogenized daily sunshine duration over China from 1961 to 2022

Abstract. Inhomogeneities in the sunshine duration (SSD) observational series, caused by non-climatic factors like China’s widespread transition from manual to automatic SSD recorders in 2019 or station relocations, have hindered accurate estimate of near-surface solar radiation for the analysis of global dimming and brightening as well as related applications, such as solar energy planning and agriculture management. This study compiled raw SSD observational data from 1961 to 2022 at more than 2,200 stations in China and clearly found that the improved precision from 0.1 hour to 1 minute following the instrument update in 2019 led to a sudden reduction in the frequency of zero SSD from 2019 onwards, referred to as the day0-type discontinuity. For the first time, we systematically corrected this known day0-type discontinuity at 378 stations (17 %) in China, resulting in an SSD series with comparable frequencies of zero value before and after 2019. On this base, we constructed a homogenization procedure to detect and adjust discontinuities in both the variance and mean of daily SSD from 1961 to 2022. Results show that a total of 1,363 (60 %) stations experienced breakpoints in SSD, of which ~65 % were confirmed by station relocations and instrument replacements. Compared to the raw SSD, the homogenized SSD is more continuous to the naked eye for various periods, and presents weakened dimming across China from 1961 to 1990 but a non-significant positive trend by a reduction of 60 % in the Tibetan Plateau, suggesting that the homogenized SSD tends to better capture the dimming phenomenon. The northern regions continue dimming from 1991 to 2022 but the southern regions of China brighten slightly. The implementation of the Action Plan for Air Pollution Prevention and Control since 2013 has contributed to a reversal of SSD trend thereafter, which is better reflected in the homogenized SSD with a trend shift from -0.02 to 0.07 hours·day^-1/decade from 2013 to 2022 in China, especially in heavily polluted regions. Besides, the relationships of cloud cover fraction and aerosol optical depth with SSD are intensified in the homogenized dataset. These results highlight the importance of the homogenized SSD in accurately understanding the dimming and brightening phenomena. The homogenized SSD dataset is publicly available for community use at https://doi.org/10.11888/Atmos.tpdc.301478 (He et al., 2024).