Abstract:

The Pearl River Delta (PRD), one of the three major urban agglomerations in China, contains nine cities extending from the south to central part of Guangdong Province. The O₃ concentration and its corresponding exceedance rate in PRD have shown an upward trend in recent years, making it gradually become the primary pollutant in PRD. The underlying mechanisms driving the increase are still being pursued. Studies have indicated that tropical cyclones may be a significant factor influencing O₃ pollution in PRD. For instance, Hu et al. (2023) found that the average O₃ concentration and exceedance probability in PRD were significantly higher when tropical cyclones originating in western North Pacific moved within 700-4500 km of PRD. Similarly, Pu et al. (2020) found that in Jiangmen, a city in western PRD, 37% of O₃ exceedance days between 2015 and 2017 occurred during weather patterns featuring tropical cyclone activity in the western North Pacific or South China Sea. Shen et al. (2023) further indicated that ~81% of O₃ pollution in Guangdong during summer and autumn was associated with surrounding typhoon activity. Despite the advanced findings, few studies have examined the event evolution from the perspective of integrated atmospheric circulation situation and meteorological conditions. To address this gap, we analyzed multiple tropical cyclone events that evolved in the western North Pacific from 2018 to 2024, with a focus on their impacts on summer and autumn (June to November) O₃ concentration in PRD. The used data sources include: 1) tropical cyclone information from the Best Track Data from the China Meteorological Administration Tropical Cyclone Data Center, 2) meteorological parameters from the Fifth Generation of Global Climate Reanalysis data (ERA5) from European Centre for Medium-Range Weather Forecasts, and 3) the maximum daily 8-hour average (MDA8) of O₃ concentration in 21 cities in Guangdong Province from Chinese Ministry of Ecology and Environment (MEE). We first identified 29 tropical cyclones which remained within 1500 km from Guangzhou (the geographical center of PRD) for over 48 consecutive hours as our research subjects. We set these criteria because previous studies indicate that surface O₃ concentrations typically increase within a range of 400-1500 km from the tropical cyclone center. To clearly illustrate the potential impacts, we selected the week during which a single tropical cyclone exerted the most extensive influence on O₃ concentrations as the complete event record period. Specifically, we defined the date when the PRD MDA8 O₃ reaches its peak under the influence of a tropical cyclone as Day0, and the three days preceding and following this date as Day-3, Day-2, Day-1, Day1, Day2, and Day3, respectively. Subsequent analyses were conducted based on O₃ concentration changes observed during this one-week period. To better assess the impact of tropical cyclones, daily O₃ concentration deviation was adopted as an evaluation metric. It is defined as the difference between the daily MDA8 O₃ on a given date when affected by a tropical cyclone and the monthly average MDA8 O₃ concentration when the date falls within that month. We find that the deviation is mainly positive when tropical cyclone locates within 500-1800 km of Guangzhou. The averaged positive deviation amounts to 30.3 µg∙m⁻³, with the maximum deviation reaching up to 115.7 µg∙m⁻³ which occurs when the tropical cyclone is located in the oceanic area north to Luzon Island, or the oceanic area east and north to Taiwan Island. We also find that under the influence of tropical cyclones, MDA8 O₃ generally exhibited a pattern of upward trend followed by a subsequent decline, though the time of reaching peak levels varied. Subsequently, we classified the 29 tropical cyclones into two categories based on their track profiles, i.e., northwestward and westward paths. Observations revealed that surface O₃ is generally higher during the effects of the northwestward tropical cyclones. Moreover, O₃ exceedances—daily MDA8 O₃ exceeding 120 µg∙m⁻³—lasted longer and affected a greater number of cities under such circumstances. We also find that MDA8 O₃ consistently peaked between Day-1 and Day1, with median value exceeding the summer-autumn averages regardless of the cyclone track path. According to this observation, we designate the period of Day-1 to Day1 as the O₃ episode, and defined Day-3 to Day-2 as pre-episode period and Day2 to Day3 as post-episode period. From the pre-episode to the episode, both types of tropical cyclones promote the evolution of near-surface meteorological elements into conditions favorable for O₃ photochemical reactions but unfavorable for dispersion diffusion and wet cleaning. Notably, these changes were more pronounced under the influence of northwestward tropical cyclones, as the average total cloud cover reduced by 2 during the 10:00-18:00 period, the surface-level downward solar radiation increased by 90.8 W·m⁻², the 2-meter air temperature at 14:00 rose by 1.2 ℃, the daily average relative humidity decreased by 5%, and the daily average wind speed decreased by 0.8 m·s⁻¹. In contrast, meteorological factors conducive to O₃ dissipation typically exhibit more remarkable changes under the influence of the westward tropical cyclones from the episode to the post-episode. This may partly explain the higher near-surface O₃ concentrations observed during northwestward tropical cyclone events. Vertically, changes in meteorological factors which are conducive to O₃ formation and pollutant accumulation were also observed from the pre-episode to the episode, regardless of the tropical cyclone types. More specifically, relative humidity in 1000-800 hPa over Guangdong decreased by 6%-12%, while temperatures generally rose by 0.4-1.2 ℃, as well as diminished ascent or reversed to descent were observed under the influence of northwestward tropical cyclones. These changes exhibited greater magnitude and deeper vertical penetration when compared to westward tropical cyclones. This may represent another potential driver for elevated near-surface O₃ concentration in PRD under the influence of northwestward tropical cyclones. From the perspective of large-scale circulation, we observed that during the episode period influenced by both types of tropical cyclones, the lower troposphere over Guangdong is dominated by northerly winds originating from the inland regions, which facilitates pollution transportation. In the latter phase of the episode, the wind field shifts to southerly or easterly flows from the ocean, with enhanced wind speeds, favoring for pollutant dissipation. Our study enhances the understanding of how tropical cyclones affect near-surface O₃ concentration, and the corresponding findings are expected to provide valuable insights for forecasting and warning systems regarding O₃ pollution during tropical cyclone events.

Key words: tropical cyclones, O₃ concentration, Pearl River Delta, meteorological elements

摘要：

利用2018－2024年热带气旋资料、广东21个地市臭氧（O₃）日最大8 h滑动平均值、欧洲中期天气预报中心第五代全球气候再分析资料（ERA5），分析夏秋季（6－11月）西北移（A路径）和西移（B路径）两类西北太平洋生成热带气旋影响下，珠江三角洲（珠三角）近地面O₃浓度差异，并用天气形势合成方法，分析珠三角近地面O₃浓度差异的气象原因。结果表明：A路径时，广东各地市O₃浓度普遍较B路径更高，O₃质量浓度超过120 µg∙m⁻³地市区域更广、持续时间更长。夏秋季两类路径热带气旋影响下，O₃高值前期至高值期，近地面气象要素、垂直方向湿度与温度分布、垂直运动、大气环流演变均有利于近地面O₃浓度上升。A路径时，10:00－18:00平均总云量平均减少2份、地表向下短波辐射平均增加90.8 W∙m⁻²，14:00的2 m气温平均增加1.2 ℃，日平均相对湿度平均下降5%，日平均风速平均减小0.8 m∙s⁻¹，广东上空1000－800 hPa相对湿度下降6%－12%、气温普遍上升0.4－1.2 ℃，垂直方向的上升运动范围及速度减小或转为下沉运动。近地面气象要素，以及垂直方向气象条件的改变幅度均表现为A路径较B路径更大且垂直层次更深厚，这可能是A路径热带气旋影响下，珠三角近地面O₃浓度更高的原因。

关键词: 热带气旋, 臭氧浓度, 珠三角, 气象要素