Urbanization effect on sunshine duration during global dimming and brightening periods in China

Abstract. There is an ongoing debate on whether the observed decadal variations in surface solar radiation, known as "dimming and brightening", are a global or just local phenomenon. We investigated this issue using a comprehensive set of long-term sunshine duration records from China, which experienced a rapid growth in urbanization during past decades. 172 pairs of urban and nearby rural stations were analyzed over the period 1960–1989 ("dimming phase") and 1990–2013 ("brightening phase"). There is a large overlap in urban and rural sunshine duration trends for both dimming (≈ 86 %) and brightening (≈ 84 %) phases. This indicates that rather than urban dimming or rural brightening, the global dimming and brightening phenomena are more of national/regional scale in China. In the dimming phase, sunshine duration significantly declined in both urban and rural areas at an average rate of −0.20 h d⁻¹ decade⁻¹ and −0.14 h d⁻¹ decade⁻¹ respectively, i.e. rural dimming has been around two-thirds of urban dimming. This ratio generally increases from a minimum of 0.39 to a maximum of 0.87 with increasing indices of urbanization, reaching saturation when the urbanization level exceeds 50 %, or the urban population exceeds 20 million persons, or the population density becomes higher than 250 person km⁻². Urbanization can be treated as a useful indicator for anthropogenic air pollution in studying pollution-driven changes in sunshine duration during the dimming phase when pollution control and monitoring were largely absent. After the transition into the brightening phase, the increasing number of environment-related laws and regulations as well as investments in the abatement of environmental pollution might have helped in counteracting air pollutants generated during the urbanization process. Therefore, in the brightening phase, urbanization no longer simply indicates an increase in air pollution and its effect on sunshine duration becomes insignificant. In conclusion, urbanization can give a general indication of pollution-driven sunshine dimming until pollution regulations become effective.