Title: Natural experiments from Earth Hour reveal urban night sky being drastically lit up by few decorative buildings Show Chinese title

Title: 从地球一小时的自然实验中发现，城市夜空被少数装饰性建筑大幅照亮

Abstract: Light pollution, a typically underrecognized environmental issue, has gained attention in recent years. While controlling light pollution requires sustained efforts, Earth Hour offers a unique natural experimental setting to assess temporary lights-out measures. Using photometric and spectroscopic sensors, we observed up to 50% night sky darkening during Earth Hour from 2011 to 2024 in Hong Kong, primarily as a result of a small but critical number of lights-out instances in central business districts, as evidenced by crowd-sourced photography records. Weekend lighting pattern in the city during Earth Hour remained unaffected. The emission reductions mostly occurred in the 445-500, 500-540, and 615-650 nm spectral ranges-corresponding to peak emissions from LED billboard screens- and in the 585-595 nm range, associated with metal halide floodlights used for facades and billboards. Our study identifies these sources as major contributors to urban light pollution. By combining multimodal observations, we offer a comprehensive assessment of light pollution sources and the potential benefits of sustainable lighting practices in urban environments. This research highlights the importance of targeted light pollution mitigation efforts and provides critical insights for policymakers to enhance urban sustainability and human well-being. Show Chinese abstract

Abstract: 光污染是一种通常未被充分认识的环境问题，近年来引起了关注。 虽然控制光污染需要持续的努力，但地球一小时为评估临时熄灯措施提供了一个独特的自然实验环境。 使用光度和光谱传感器，我们在2011年至2024年期间观察到香港在地球一小时期间夜空变暗多达50%，这主要是由于商业中心区少量但关键的熄灯实例所致，这由众包摄影记录所证实。 地球一小时期间城市周末的照明模式未受影响。 排放减少主要发生在445-500、500-540和615-650纳米的光谱范围——对应于LED广告牌屏幕的主要发射峰——以及585-595纳米范围，与用于外墙和广告牌的金属卤化物泛光灯有关。 我们的研究将这些来源确定为城市光污染的主要贡献者。 通过结合多模态观测，我们对光污染源和可持续照明实践在城市环境中的潜在益处进行了全面评估。 这项研究突出了有针对性的光污染缓解措施的重要性，并为政策制定者提供了关键的见解，以提高城市可持续性和人类福祉。