Objective   Tropospheric ozone is an important greenhouse gas and a pollutant harmful to organisms. It not only affects the radiation balance of the ground-atmosphere system, but also seriously endangers the ecological environment. When the near-ground ozone concentration exceeds a certain threshold, it will cause a series of adverse effects on human health and the growth of animals and plants. Since the near-ground ozone concentration depends largely on the physical conditions of the upper layer atmosphere, it is of great significance to carry out vertical ozone detection and study the ozone distribution characteristics for the source analysis and pollution prevention of ozone. In recent years, the near-ground ozone concentration in Weifang has been increasing, especially in summer, which has replaced fine particles as the main pollutant. Therefore, the temporal and spatial distribution characteristics of ozone in Weifang during summer are analyzed in this paper.

  Methods  The differences of ozone distribution under two different weather conditions of fine day and rainy day are studied through analyzing typical cases in this paper. In addition, in order to study the differences in ozone distribution at different times of the day, the time of a day was divided into four periods, namely morning transition, daytime, evening transition and nighttime, and the vertical distribution characteristics of ozone at each period were statistically analyzed. The low-altitude ozone concentration data used for the analysis was detected by the GBQ L-04 ozone lidar (Fig.1) produced by Hefei Zhongke Guangbo Quantum Technology limited company. The monitoring period is from June 1, 2020 to August 31, 2020. The monitoring location is located in Weifang Environmental Monitoring Center Station (119.15°E, 36.70°N).

  Results and Discussions  The daily variation of low-altitude ozone concentration on sunny days is distinct, while on rainy days it varies with the time of the day when precipitation occurs. Ozone pollution will not be too strong when precipitation occurs in the evening because of good production conditions and weak diffusion conditions for ozone during the day due to cloud cover and atmospheric convective motion. Conversely, if precipitation occurs in the morning and the sky clears after rain, daytime ozone pollution is less influenced by precipitation. When meteorological conditions such as radiation, temperature and humidity are similar, strong winds will significantly reduce the ozone concentration. Before rainfall, strong convective movement and gale will make the convective ozone layer thicker and the ozone concentration lower (Fig.2). Many interfering bright blocks appear on the ozone distribution map detected by radar during precipitation. This indicates that the results of ozone concentration detected by ozone lidar during precipitation are not reliable. On non-precipitation days, the convective ozone layer is mainly distributed below 1500 m, showing the characteristics of diurnal variation of high in the day and low at night, and the high concentration value often appears at 12-18 o'clock (Fig.3). This may be closely related to the photochemical process of near-ground and the atmospheric vertical diffusion. The convective ozone layer on non-precipitation days can be divided into several layers from up to down (Fig.4). This may be related to the thermal vertical structure of the atmosphere.

  Conclusions  The distribution characteristics of ozone under typical weather conditions and the statistical characteristics of ozone on non-precipitation days in Weifang are analyzed with data detected by a differential absorption lidar in this paper. The research shows that meteorological conditions have a great impact on ozone distribution. On non-precipitation days, the convective ozone layer is mainly distributed below 1500 m. It increases with height between 300-500 m , and reaches a maximum near 500 m, which is basically consistent with the boundary layer height detected by the Mie-scattering lidar. The ozone concentration of each period of a day at 1500 m tends to be consistent, and there is no obvious diurnal variation up from this height. The ozone concentration in this layer can be used as the atmospheric background value in ozone forecast.