2000—2020年重庆大气湿沉降化学特征及其来源解析

Chemical characteristics and source apportionment of atmospheric wet deposition in Chongqing during 2000—2020

关键词:  时空变化  大气降水  化学特征  来源解析  驱动因素  重庆

Key words:  spatiotemporal variations  atmospheric precipitation  chemical characteristics  source apportionment  driving factors  Chongqing

摘要:

为了解城市区域“五年规划”带来的生态环境质量持续改善成效，对2000—2020年重庆市城区和近郊监测站点大气降水组分、沉降以及来源变化进行综合分析。结果表明，城区和近郊pH均值分别为4.85和4.49，且两站点SO₄^2-/NO₃^->3，表明重庆仍存在硫酸型酸雨。电导率在2000—2010年波动上升，2010年达到峰值后持续下降。水溶性离子呈现类似趋势，在2000—2010年波动上升，2011年持续降低，城区（616.0 μeq/L）高于近郊（491.4 μeq/L）。离子组成以SO₄^2-（38.41%）和Ca²⁺（22.88%）为主，其次为NH₄⁺（21.16%）和NO₃^-（9.77%），呈现出硫酸-硝酸复合污染特征。SO₄^2-、Ca²⁺和NH₄⁺的沉降通量在2004—2006年达到峰值；后持续下降，而城区和近郊的NO₃^-沉降通量分别在2014年和2010年达到峰值。城区降水离子的沉降通量始终高于近郊，其中SO₄^2-沉降通量最高。两站点NH₄⁺-N/NO₃^--N的均值分别为2.46和2.08，表明重庆大气活性氮主要来源均为农业活动。PMF源解析表明，城区水溶性离子的污染源依次为机动车排放源（31.7%）、燃煤源（19.4%）、海盐及生物质燃烧源（18.7%）、农业源（16.8%）和扬尘源（13.5%）。近郊依次为农业源（27.3%）、海盐及生物质燃烧源（21.9%）、机动车排放源（20.4%）、燃煤源（18.7%）和扬尘源（11.7%）。研究揭示了重庆市城区与近郊大气降水化学组分、沉降过程及污染来源的演变特征与空间差异，可为区域大气污染综合治理与生态环境提质规划实施提供科学参考。

Abstract:

Background, aim, and scope To assess the long-term improvement of eco-environmental quality driven by urban five-year plans, this study comprehensively analyzed the variations in chemical compositions, deposition fluxes and pollution sources of atmospheric precipitation at monitoring sites in urban and suburban areas of Chongqing from 2000 to 2020. This work aims to characterize the spatiotemporal evolution of acid rain and nitrogen deposition in mountainous urban regions, and provide insights into the effectiveness of regional air pollution control strategies. Materials and methods Long-term observational records of wet precipitation chemistry from urban and suburban monitoring stations during 2000—2020 were adopted in this research. Key indices including pH value, electrical conductivity, water-soluble ion concentrations and ion deposition fluxes were calculated. The Positive Matrix Factorization (PMF) receptor model was applied to quantitatively apportion the sources of water-soluble ions in precipitation. Results The mean pH values were 4.85 for the urban site and 4.49 for the suburban site. The SO₄^2-/NO₃^- ratios at both sites exceeded 3, indicating the persistence of sulfuric acid-type acid rain in Chongqing. Electrical conductivity fluctuated upward from 2000 to 2010, peaked in 2010, and then declined continuously afterwards. Water-soluble ions exhibited a comparable temporal pattern: fluctuating increase during 2000—2010 followed by a sustained decrease starting in 2011, with a higher concentration in the urban area (616.0 μeq/L) than in the suburbs (491.4 μeq/L). Ionic composition was dominated by SO₄^2- (38.41%) and Ca²⁺ (22.88%), followed by NH₄⁺ (21.16%) and NO₃^- (9.77%), showing combined sulfuric-nitric acid pollution features. Deposition fluxes of SO₄^2-, Ca²⁺ and NH₄⁺ peaked in 2004—2006 and then decreased steadily. The NO₃^- deposition flux reached its maximum in 2014 at the urban site and in 2010 at the suburban site. The urban area maintained consistently higher ionic deposition fluxes than the suburbs, among which SO₄^2- had the highest flux. The average NH₄⁺-N/NO₃^--N ratios were 2.46 and 2.08 for urban and suburban sites respectively, suggesting agricultural activities constituted the primary source of atmospheric reactive nitrogen. PMF source apportionment revealed that urban precipitation ions were mainly contributed by vehicle emissions (31.7%), coal combustion (19.4%), sea salt and biomass burning (18.7%), agricultural sources (16.8%) and dust (13.5%). For the suburban site, the source contributions ranked as agricultural sources (27.3%), sea salt and biomass burning (21.9%), vehicle emissions (20.4%), coal combustion (18.7%) and dust (11.7%). Discussion The persistent sulfuric acid-type acid rain reflected the legacy influence of historical sulfur emissions, while the concurrent contribution of nitrate indicated the gradual transformation toward compound acid pollution in Chongqing. The divergent peak time of nitrate deposition between urban and suburban regions mirrors the differing temporal trajectories of mobile source emissions and agricultural nitrogen release. The spatial disparity in ion concentration and deposition flux is associated with the differentiated intensity of anthropogenic activities between urban and suburban zones. Varied source apportionment outcomes demonstrate distinct dominant pollution sources across the two regions: traffic emissions prevail in urban core areas, whereas agricultural activities dominate suburban atmospheric wet deposition. Conclusions This study uncovered the temporal evolution and spatial disparity in chemical compositions, deposition processes and pollution sources of atmospheric precipitation between urban and suburban Chongqing. It confirms the effectiveness of emission reduction measures under five-year environmental plans, while highlighting the coexistence of sulfuric acid acid rain and reactive nitrogen pollution across this region. Recommendations and perspectives Differentiated air pollution control strategies should be implemented for urban and suburban areas to strengthen the collaborative mitigation of urban mobile source pollution and suburban agricultural nitrogen emissions. Future research can combine long-term continuous observations and numerical simulations to further explore the eco-environmental effects of wet deposition variation in mountainous areas.