Abstract:

Secondary inorganic aerosol (SIA) is an important element of PM_2.5, and its rapid formation is a significant cause of the decline of atmospheric visibility. Sulfate, nitrate and ammonium ions primarily constitute secondary inorganic aerosols, which have a wide range of sources and complex generation pathways, making it challenging to quantify their origins. Recently, stable isotopes (δ³⁴S-SO₄²⁻, δ¹⁵N-NO₃⁻, δ¹⁸O-NO₃⁻, δ¹⁵N-NH₄⁺) have been used to determine the sources of corresponding pollutants through source apportionment. In this study, we collected atmospheric PM_2.5 samples for one year at the Pearl River Delta Heshan Atmospheric Supersite. Based on the analysis of 70 samples, including water-soluble ions, trace metal elements, organic carbon and elemental carbon, we selected 37 samples for further isotopic analysis ( δ³⁴S-SO₄²⁻, δ¹⁵N-NO₃⁻, δ¹⁸O-NO₃⁻ and δ¹⁵N-NH₄⁺ ). We combined stable isotope balance and Bayesian model to quantify the relative contribution of various pollution sources to secondary inorganic aerosols. The results showed that the annual average PM_2.5 mass concentration was 39.8μg•m⁻³, with total SIA accounting for 29.8% of the annual average PM_2.5 mass concentration. Seasonal variation patterns showed that spring> winter>summer>autumn, with a significant decrease in winter compared to that in 2013. The order of contribution of SIA to particle mass concentration was sulfate>nitrate>ammonium. The δ³⁴S-SO₄²⁻ values ranged from 1.8‰ to 9.6‰, with an average of 4.5‰±1.7‰. The results of source apportionment showed that coal combustion, oil combustion and biogenic sulfur accounted for 41.8%, 32.9% and 25.3% respectively of SO₄²⁻ in Heshan. The δ¹⁵N-NO₃⁻ values ranged from 0.8‰ to 8.3‰, with an average of 4.7‰±2.0‰. The Bayesian isotope mixing model revealed that coal combustion was the main source of NO₃⁻ in Heshan, with an average annual contribution rate of 41.6%. The relative contribution of other pollution sources was ranked as biomass burning (BB)>gasoline vehicle>ship emission>biogenic soil emission, with percentages of 20.9%, 15.6%, 13.9% and 8.0%, respectively. The combined results of SIA analysis and source apportionment showed that non-fossil source contributed the most to total SIA (37.2%), followed by coal combustion (35.6%) and oil combustion (27.2%). The contribution of oil combustion to SIA was higher in the clean atmosphere period (up to 48.4%) than in the light and heavy pollution periods. The emission inventory method may have underestimated the contribution of non-fossil sources to SIA, by approximately 10% in the PRD region. Overall, the findings highlight the significance of non-fossil sources and the varying contribution of oil combustion to SIA.

Key words: Pearl River Delta, PM_2.5, secondary inorganic aerosol, nitrogen isotope, sulfur isotope, Bayesian model

摘要：

二次无机气溶胶（SIA，Secondary inorganic aerosol）是PM_2.5中的重要组成部分，其快速形成是导致大气能见度下降的重要原因。二次无机气溶胶主要包括硫酸根、硝酸根和铵根离子，其来源广、生成途径复杂，识别其来源存在一定的挑战。最近，基于稳定同位素（δ³⁴S-SO₄²⁻、δ¹⁵N-NO₃⁻、δ¹⁸O-NO₃⁻、δ¹⁵N-NH₄⁺）的源解析方法被应用到相应污染物的来源解析。选取珠三角鹤山大气超级站作为研究地点，开展了为期1年的大气PM_2.5样品采集，在分析了70个样品的水溶性离子、痕量金属元素、有机碳、无机碳的基础上，选取其中的37个大气PM_2.5样品，分析了相应的δ³⁴S-SO₄²⁻、δ¹⁵N-NO₃⁻和δ¹⁸O-NO₃⁻、δ¹⁵N-NH₄⁺同位素值，结合稳定同位素平衡和优化的贝叶斯模型进行源解析，定量了各污染源对二次无机气溶胶的影响。结果表明，PM_2.5年平均质量浓度为39.8 μg•m⁻³。PM_2.5中SIA占比年均29.8%，呈现春季>冬季>夏季>秋季的季节变化规律，冬季二次污染程度较2013年明显下降；年均各SIA对颗粒质量浓度贡献顺序为硫酸根>硝酸根>铵根。δ³⁴S-SO₄²⁻值范围为1.8‰－9.6‰，平均值为4.5‰±1.7‰。源解析结果显示采样期间燃煤、燃油、生物成因硫排放对鹤山SO₄²⁻分别贡献了41.8%、32.9%、25.3%。δ¹⁵N-NO₃⁻范围为0.8‰－8.3‰，平均值为4.7‰±2.0‰。贝叶斯源解析结果显示，燃煤排放是鹤山NO₃⁻最主要来源，年均贡献率达到41.6%，其余污染源年均贡献率排序为，生物质燃烧 (BB)>汽油车>船舶>土壤微生物，贡献率依次为20.9%、15.6%、13.9%、8.0%。将各SIA浓度与对应的源解析结果合并，结果表明非化石源对总SIA贡献最大，为37.2%，其次为燃煤源及燃油源，分别为35.6%、27.2%。排放清单法可能低估了非化石源对SIA的贡献，在珠三角地区低估值约为10%。清洁大气时期燃油对SIA的贡献显著高于轻度污染与重度污染时期，最高可达48.4%。

关键词: 珠三角, PM_2.5, 二次无机气溶胶, 氮同位素, 硫同位素, 贝叶斯模型