关键词: 对流层NO2柱浓度, 时空分布, TROPOMI卫星, 安徽地区

Abstract: Objective　Nitrogen dioxide (NO2) is a critical atmospheric pollutant closely associated with anthropogenic emissions and complex atmospheric photochemistry. Although ground-based observation networks can provide precise point-source measurements, their spatial coverage and temporal continuity are limited. Satellite remote sensing, especially the Tropospheric Monitoring Instrument (TROPOMI) aboard the Sentinel-5P satellite, offers high spatial resolution and wide coverage, overcoming the limitations of ground-based observation networks in regional atmospheric studies. Anhui Province, located in the Yangtze River Delta region of eastern China, has experienced substantial NO2 pollution during the rapid industrialization and urbanization process. However, high-precision, long-term investigations focusing on the fine-scale spatiotemporal dynamics and multi-driver mechanisms of NO2 in this specific transition zone remain scarce. Therefore, this study utilizes TROPOMI tropospheric NO2 vertical column density (NO2 VCD) products from 2019 to 2023 to systematically quantify the spatiotemporal evolution of NO2 across Anhui Province and investigate its influencing factors, including meteorological conditions, regional transport and the control measures during the COVID-19 epidemic, thereby providing a scientific support for regional air quality management and emission reduction policies. Methods　This study evaluated the tropospheric NO2 VCD data across Anhui Province from TROPOMI sensor (spatial resolution: 3.5 km × 7 km) spanning 2019 to 2023. Meteorological parameters, including temperature and precipitation, were extracted from the ERA5 reanalysis dataset. And ground-level NO2 concentrations from Online Air Quality Monitoring and Analysis Platform of China were used to validate satellite-retrieved NO2 VCDs. To track atmospheric transport pathways and identify regional pollution sources, 48-hour backward trajectories at the altitude of 500 m in the study area were analyzed using the HYSPLIT model based on GDAS meteorological data. The Potential Source Contribution Function (PSCF) method was then applied to quantify the relative contribution of different source areas to NO2 in the study area. Pearson correlation analysis was used to assess the relationships between NO2 VCD and meteorological variables, as well as inter-city correlations of NO2 pollution across Anhui. Additionally, year-on-year (YoY) and month-on-month (MoM) changes in NO2 VCD were calculated to quantitatively evaluate the impact of COVID-19 control measures on NO2 levels in early 2020. Results and Discussion　Comparison between ground observations and TROPOMI NO2 VCD in Hefei shows strong spatiotemporal consistency, with a monthly average Pearson correlation coefficient of 0.81, confirming the reliability of the satellite data over the study area. Spatially, NO2 VCD in Anhui Province exhibits a clear distribution pattern of high in the central region and low in remote mountainous areas. High-value regions are concentrated across the industrial corridor encompassing Ma'anshan, Hefei, and Wuhu, with a peak value of 7.19 × 1016 molecule·cm−2 in the northeast of Ma'anshan. Conversely, low-value areas are primarily located in south of Anhui (such as Huangshan, Anqing), where NO2 VCD values remain consistently below 4.0 × 1016 molecule·cm−2, which is mainly due to high forest coverage and low industrial intensity. Correlation analysis shows that the monthly average NO2 VCD in Hefei, Ma'anshan, and Wuhu are highly correlated with the provincial average, with correlation coefficients of 0.93, 0.96, and 0.97, respectively. Temporally, the monthly average NO2 VCD exhibits a maximum of 10.04 × 1016 molecule·cm−2 in January and a minimum of 1.63 × 1016 molecule·cm−2 in August. Pronounced seasonal variations were observed, with the highest in winter (6.40 × 1016 molecule·cm−2), followed by autumn and spring, and the lowest in summer (1.80 × 1016 molecule·cm−2). This trend does not conform to the seasonal pattern of low NO2 concentration in winter and high in summer under natural conditions, indicating that the NO2 concentration changes in this region are mainly influenced by anthropogenic emissions. Among meteorological factors, NO2 VCD exhibits a strong negative correlation with temperature (R = −0.88) and a moderate negative correlation with precipitation (R = −0.48), indicating that temperature plays a more dominant role. Regarding regional transport, backward trajectory analysis shows that winter air masses affecting Hefei, Ma'anshan, and Wuhu predominantly originate from the northwest. PSCF results indicate that the corresponding high-contribution potential source regions are concentrated in nearby industrial and urban areas. Following the onset of COVID-19 control measures in February 2020, NO2 VCD in Anhui decreased sharply to 3.96 × 1016 molecule·cm−2. The month-on-month reduction rates across all cities exceeded the corresponding year-on-year rates. Hefei recorded the largest year-on-year reduction of 73.73%, providing direct quantitative evidence for the immediate air quality benefits resulting from the reductions of vehicular traffic and heavy industrial operations during the epidemic. Conclusions　TROPOMI satellite data demonstrated excellent reliability in monitoring tropospheric NO2 VCD of Anhui Province, showing strong consistency with ground-based observations. From 2019 to 2023, NO2 VCD exhibited distinct spatiotemporal patterns: spatially, high-density pollution clusters were concentrated in central industrial hubs (Hefei, Ma'anshan, and Wuhu), contrasting sharply with the lower baseline values observed across southern mountainous regions (e. g., Huangshan); temporally, a pronounced seasonal cycle occurred, peaking in winter and reaching a minimum in summer. Hefei, Ma'anshan, and Wuhu were identified as the primary areas of high NO2 pollution, strongly correlated with NO2 pollution in the entire Anhui region. Among meteorological drivers, temperature exerts a stronger influence on NO2 VCD than precipitation. In terms of potential NO2 source, long-range atmospheric transport in winter is primarily governed by northwesterly air masses. Furthermore, a marked reduction in NO2 levels in Anhui was found to be associated with the 2020 COVID-19 control, indicating that NO2 pollution is mainly influenced by human activities and industrial production.

Key words: tropospheric NO2 column concentration, spatiotemporal distribution, TROPOMI satellite, Anhui Province