Abstract:

The governance of coastal ozone pollution in Fujian Province has reached an urgent state, with Quanzhou City facing particularly severe challenges. Located in Southern Fujian, Quanzhou’s extensive industrial parks are distributed across urban, suburban, and county-level areas, resulting in high local emission intensities and exacerbating ozone pollution. This city is now confronted with the most prominent challenge in ozone control among all regions in Fujian, making it a critical focus for air-quality management. A significant regional ozone pollution event occurred across Fujian Province from May 14 to 18, 2024, with Quanzhou experiencing three consecutive days of pollution (May 14-16). During this period, local photochemical production combined with regional transport dominated ozone formation under clear-sky conditions and northwesterly airflow control on May 14-15. However, the event took an exceptional turn on May 16, when an unprecedented nighttime ozone surge caused MDA8 concentrations to exceed the secondary standard at 08:00 Beijing Time, marking a rare non-photochemical period ozone pollution event. Given the absence of prior studies on Quanzhou’s nighttime ozone pollution, this study utilized national environmental monitoring station pollutant measurements, surface meteorological observations, wind profile radar data, atmospheric boundary layer height of aerosol radar, and ERA-5 reanalysis products. Employing statistical analysis and synoptic weather pattern diagnostics, this investigation aimed to systematically explore the meteorological mechanisms underlying the vertical transport-induced nighttime ozone pollution observed on May 15. The key conclusions are as follows. 1) From May 14-16, 2024, Quanzhou City in Fujian Province experienced three consecutive days of ozone pollution accompanied by nocturnal ozone exceedances. This event constituted an extreme climatic anomaly within the five-year observational period (2019-2023), with a historical occurrence probability of 0.64%. On May 15, a cold air mass traversed central and eastern China, exhibiting latitudinal circulation patterns at a height of 500 hPa. This synoptic configuration typically features pronounced trough/ridge oscillations and unstable weather conditions. An analysis of the sea-level pressure field revealed that the cold high-pressure system was positioned slightly behind an upper-level trough. Fujian Province was under the control of this cold high, resulting in dominant subsidence airflow. Prevailing winds from the north persisted throughout the lower and middle troposphere (below 850 hPa). The upper stratosphere is influenced by northwest-to-westward winds. Except for Zhangzhou, where the pattern was less pronounced, other coastal cities in Fujian exhibited a single hourly ozone peak during the night of May 15, with no significant delay between the upstream and downstream peaks, indicating vertical transport as the primary cause of the elevated ozone concentration. Meanwhile, Quanzhou’s environmental monitoring stations recorded hourly ozone peaks ranging from 193 to 202 μg·m⁻³ (average 196.8 μg·m⁻³), which is the highest value among all cities in Fujian Province. The duration of hourly ozone concentrations exceeding 160 μg·m⁻³ was 5 h, making it the longest duration in the province. Therefore, in terms of pollution severity and persistence, Quanzhou was the most severely affected area during this nighttime regional ozone pollution event. 2) Cold highs represent a key synoptic pattern affecting ozone pollution in Fujian Province. Their influence enables high concentrations of ozone in the upper troposphere to be transported to the ground through intense vertical subsidence currents. The exceptional nighttime ozone surge between May 15 and 16 caused the MDA8 concentrations to exceed the secondary standard at 08:00 Beijing Time on May 16, fulfilling the ozone pollution day criteria. This event typified nighttime ozone pollution driven by vertical transport mechanisms. A comprehensive synoptic analysis identified two distinct meteorological phases that influenced the vertical transport process. 3) Phase I (May 15 22:00, May16 May, 03:00 Beijing Time) coincided with the passage of a 500 hPa cold vortex system and an upper-level trough, introducing cold air advection over the region. Surface wind speeds exhibited dramatic increases (from 3.2 m·s^-1 on May 14 to 12.4 m·s⁻¹ on May 15), reaching a Beaufort scale 6 intensity. Wind profiler radar data revealed ground gusts exceeding 18.3 m·s⁻¹ (Beaufort scale 8), indicating pronounced low-level wind shear associated with the breakdown of the boundary layer inversion. Wind fields play an important role in ozone transport, and the magnitude of wind speed reflects the efficiency of pollutant transportation. Wind fields play a crucial role in ozone transport, with wind speed serving as an indicator of pollutant transport efficiency. This dynamic forcing mechanism facilitated turbulent mixing between the residual and surface boundary layers, enabling the subsidence of ozone-rich residual air masses into the near-surface environment. Spatially distributed monitoring stations recorded ozone concentration peaks between 193 and 202 μg·m⁻³, along with elevated levels of secondary pollutants, including PM_2.5, PM₁₀, CO, and SO₂. 4. Phase II (May 16, 03:00-05:00 Beijing Time) was characterized by the persistent evaluation of the atmospheric boundary layer height, which reached a maximum height of 1.463 km. The 23.3% decline in CO concentrations observed between 05:00 and 06:00 Beijing Time provided critical insights into atmospheric transport dynamics. Given CO’s primary association of CO with near-surface combustion processes and its inverse relationship with the free convection level (FCL), this decrease indicates the invasion of air masses originating above the FCL. Meteorological analysis further revealed that elevated mixing layer heights during this phase promoted the vertical descent of ozone-enriched free tropospheric air, introducing relatively dry and cold air masses with higher ozone gradients. However, these intruding air masses exhibited significantly reduced concentrations of PM_2.5, PM₁₀, CO, and SO₂, resulting in coincident decreases in surface temperature (T) and relative humidity (RH), with the exception of ozone. This is a hallmark of the multi-pollutant dilution effects associated with free tropospheric intrusion. In conclusion, the meteorological causes of Quanzhou’s nighttime ozone pollution events were determined to be the combined effects of cold front-induced dynamic lifting (Phase I) and post-frontal cold air advection (Phase II), representing a typical case of large-scale synoptic weather system-driven vertical transport influencing local non-photochemical period ozone pollution. This study provides new insights into the physical mechanisms governing these events, emphasizing the critical role of synoptic-scale meteorological patterns in coastal urban areas. These findings provide scientific support for developing targeted control strategies to mitigate episodic ozone pollution caused by vertical transport processes. This pollution event demonstrated that the increase or decrease in near-surface pollutant concentrations depends on the difference between the pollutant levels in the incoming air mass and the original residual air mass.

Key words: nocturnal O₃ pollution, vertical transport, wind profile radar, boundary layer turbulence, southern Fujian

摘要：

2024年5月14－18日福建省连续5 d出现区域性O₃污染过程，闽南地区代表城市泉州在14－16日维持3 d O₃污染，历史出现概率0.64%。14－15日在晴热天气与偏北气流控制下，本地生成和区域输送是导致O₃污染的主要原因，但15日夜间－16日凌晨泉州近地面O₃浓度异常升高导致16日08:00即出现O₃-MDA8超二级标准限值。利用环境国控点污染物监测、气象地面观测、风廓线雷达探测等多源地基遥感数据及ERA-5再分析资料，采用统计分析、天气学诊断等方法，探究该过程泉州近地面受垂直传输影响出现O₃污染的天气学成因。结果表明，此次O₃夜间污染主要分为两个影响阶段：第一阶段15日22:00－16日03:00，风矢量垂直廓线显示近地面风速陡升至18.3 m·s⁻¹，风切变使垂直方向产生湍流，大气残留层的高浓度污染气团随冷空气大风侵入地面，导致地面O₃迅速升高，各评价点的O₃浓度峰值为193－202 μg·m⁻³且PM_2.5、PM₁₀、CO、SO₂等其他污染物浓度也均有上升；第二阶段16日03:00－05:00，边界层高度稳定维持在1.2 km以上，混合层升高导致自由对流层中高浓度O₃气团垂直下沉向地面扩散，该气团较老气团更加干冷且富含O₃，但其他污染物浓度则较低，入侵影响地面后气温（t）、相对湿度（RH）、CO、PM_2.5、PM₁₀、SO₂等要素同步下降。

关键词: O₃夜间污染, 垂直传输, 风廓线雷达, 边界层湍流, 闽南地区