Abstract:The differences in haze occurrence frequency identified by two different standards， i.e., the Observation and Forecasting Levels of Haze (QX/T 113-2010) (hereafter referred to as 2010QX) and the Haze Identification for Meteorological Observation (GB/T 36542-2018) (hereafter referred to as 2018GB) are analyzed by employing the hourly meteorological data and particulate matter (PM2.5) mass concentration from March 2016 to February 2020. The results show that the occurrence frequency of haze identified by 2018GB is significantly higher than that of haze identified by 2010QX. However, the numbers of haze days obtained by these two standards are equivalent if we take haze phenomena lasting for 6 h or more as the criterion of haze days. When 80%≤RH<95%, the haze occurrence frequency identified by 2018GB is higher than that of 2010QX. The higher the humidity, the more it increases. Aerosol hygroscopicity parameter has a great influence on the calculation of aerosol extinction coefficient, and the difference of this parameter in different regions should be paid more attention to when using the 2018GB. When PM2.5≤75 μg·m-3, 2018GB can still identify haze, showing the influence of humidity on visibility. The difference of haze occurrence between the two distinguishing methods decreases when the pollution level is moderate-high. The monthly variation of haze occurrence frequency in all cities in Shaanxi Province presents a U-shaped distribution with high in winter and low in summer, and the monthly haze occurrence frequency is highest in January. Except in Yulin and Yan’an in northern Shaanxi and Shangluo in southern Shaanxi, the diurnal variation of hourly haze occurrence frequency shows a single peak in the morning from 09:00 BT to 11:00 BT, while the diurnal variation of hourly haze occurrence frequency in other cities has a bimodal distribution with peaks in the morning from 09:00 BT to 11:00 BT and in the evening from 20:00 BT to 23:00 BT.