都市霾与雾的区分及粤港澳的灰霾天气观测预报预警标准

Distinguishing of Fog or Haze and the Operational Criteria for Observation, Forecasting and Early Warning of Haze in Urban Areas of Guangdong, Hongkong and Macao

都市霾与雾的区分是迫切需要解决的问题。霾的出现有重要的空气质量指示意义,而雾的记录,有明确的天气指示意义。通常在平面时已达到饱和的水汽压,对相当于球面的云雾滴来讲就是未饱和的,云雾滴就会蒸发;在水汽条件不变时,云雾滴由于蒸发而变小,导致它的平衡水汽压升高,就更易蒸发掉。在不饱和大气中小于数μm的云雾滴必然蒸发,而且伴随着蒸发云雾滴尺度会进一步变小,导致曲率越来越大,蒸发速率越来越快。过去错误认为凝结核可以在低相对湿度(RH)情况下产生凝结生成雾滴的观点,是忽视了粒子曲率作用的结果,将实验室大颗粒(常常达mm量级)的吸湿性特征,延用至次μm粒子造成的。降温是达到饱和形成雾滴的重要物理过程,云雾是低温下饱和气块的可见标志。在云雾中必然存在凝结或凝华过程,因而必然伴随着潜热释放,这就使云雾内的温度高于环境,在云雾内必然盛行微弱的上升气流,不可能是下沉气流,这些宏观过程在霾层内是不存在的,因而成为识别雾与霾的重要的宏观动力条件。在对历史资料进行统计时,在排除降水、吹雪、雪暴、沙尘暴、扬沙、浮尘、烟幕等视程障碍现象的情况下,通过调试RH,使雾与轻雾反映自然的年际与年代际气候波动,而霾反映由于人类活动而引起的趋势性变化,其限值大体在90%左右,与美国和英国在讨论霾影响能见度(vis)的长期变化趋势中使用的限值RH<90%相同。20世纪80年代以来大幅增加的霾日,绝大部分是由于人类活动影响的气溶胶细粒子污染造成的。依据上述结果和以前的研究,给出了霾与雾区分的概念模型、霾与雾观测的标准和灰霾天气预警信号发布的标准,并介绍了相关的业务产品。

The Distinguishing of fog or haze in urban areas has become more and more pressing. The occurrence of Haze is an important indicator of air quality, while the observation of fog often implies certain weather. Usually when air is saturated with water vapor over planar surface, it is still unsaturated around spheric surface, thus cloud/fog droplet will evaporate. Given the unchanged water vapor, the droplet will shrink due to evaporation, which increases the balancing water vapor pressure, and hence accelerates the evaporation. In the unsaturated air, fog/cloud droplets smaller than several microns will definitely vaporize. With smaller diameter and larger curvature, evaporation becomes faster. It was a common mistake to think that condensation nuclei can help fog droplet to form even under low relative humidity condition, which ignores the curvature effect of the particle, and simply applies the moist absorption property of large particle （milemeter scale） to fine particle of sub - micron scale. Cooling to saturation is an important process for fog droplet formation and fog is a visible signature of low - temperature saturated air parcel. Since there exists condensation or deposition inside, the accompanying latent heat release warms up the fog layer and produces weak updraft, which can not be found inside the haze layer. These macro dynamics conditions are vital to distinguishing the fog or haze. By statistical analysis of the observational data, excluding the cases of precipitation, snow drift, snowstorm, sand storm, sand dust, and smoke fog etc which cause low visibility, the authors tuned the threshold of relative humidity to identify fog/light fog making it follow natural annual and decadal climate variations, and haze which can reflect the trends linked to human activities. The value of 90% turns out to be the proper threshold, which is close to the one proposed by researchers from America and England when it comes to the impact of haze on visibility. Haze days have dramatically increased since 1980s, most of which are due to the human activity related pollutions of aerosol fine particles. Finally this paper proposes a conceptual model to distinguish fog or haze, standards for observation of fog and haze and the criteria for issuing the haze warning signal as well as some operational product introductions.