广州大夫山雨季林内外空气TSP和PM2.5浓度及水溶性离子特征

采用平行同步采样法,于2012年雨季,对广州市大夫山森林公园林内外空气的总悬浮颗粒物(TSP)和细颗粒物(PM_2.5)样品进行了24 h收集,测定了TSP和PM_2.5的质量浓度并分析了样品中水溶性无机离子成分。结果表明:林内外PM_2.5的质量浓度平均值分别为(40.18±10.47)和(55.79±13.01) g/cm³;林内外TSP的质量浓度分别为(101.32 ± 33.19)和(116.61±35.36) g/cm³。林内与林外比,PM_2.5和TSP平均质量浓度都显著减少(P < 0.05),表明森林能显著改善空气环境质量。TSP和PM_2.5中SO₄^2-、Na⁺、NH₄⁺和NO₃^-为水溶性无机离子主要成分,占总离子质量的80%以上,林外这些离子的浓度高于林内(NH₄⁺除外)。这4种离子雨季在空气中的主要存在方式为NaCl、Na₂SO₄、NH₄HSO₄和NH₄NO₃。计算表明,采样期间海盐对大夫山空气TSP和PM_2.5的水溶性组分中Na⁺和Cl^-贡献最大,其它元素主要源自陆地源。林内外TSP和PM_2.5的c(NO₃^-)/c(SO₄^2-)比值在0.3以下,表明固定源是大夫山森林公园空气主要污染贡献者,TSP中c(NO₃^-)/c(SO₄^2-)的比值大于PM_2.5的比值,说明移动源对TSP的贡献大于PM_2.5。

Comparison of TSP, PM2.5 and their water-soluble ions from both inside and outside of Dafushan forest park in Guangzhou during rainy season

There is national research interest in the dynamics of airborne particulate matter (PM) due to its detrimental effects on air quality and human health. Both regional surveys and locally intensive monitoring of PM, particularly particles less than 2.5 micrometers in diameter (PM_2.5), are now being conducted throughout China. The main research emphases include the spatial distribution, chemical characteristics, and main sources of airborne PM_2.5 in many urban areas, as well as industrial sites.The Pearl River Delta (PRD) is one of the highly urbanized and industrialized regions in China. Unfortunately, accompanying the rapid eco-social developments, the PRD has also deteriorated in terms of air, soil and water quality during the last few decades. Air pollution has been a serious environmental problem for many years now and frequent haze episodes in this region were often attributed to high concentrations of airborne PM_2.5.Forests play an important role in reducing airborne particulate matter and in maintaining air quality. In order to improve the urban environment and to serve the public health, many urban forests have been constructed to scavenge the huge amounts of dust through the forest canopy. However, the influence of the forest canopy on particulate matter water-soluble ions has seldom been studied to date. In the present study, we regularly monitored concentrations of the total suspended particulates (TSP), PM_2.5 and their water-soluble ions (Na⁺, NH₄⁺, Ca²⁺, K⁺, Mg²⁺, SO₄^2-, NO₃^-, NO₂^-, Cl^- and F^-) from both inside and outside a suburban forest in Guangzhou, during the rainy season of 2012. The objective of this study was to reveal the importance of tree canopies in scavenging airborne particles and their water-soluble ions.The results showed that the forest canopy significantly reduced the concentrations of PM_2.5 and TSP (P < 0.05). The concentrations of diurnal PM_2.5 and TSP inside the forest were (40.2±10.5) g/cm³ and (101.3±33.2) g/cm³ respectively, increasing to (55.8±13.0) g/cm³ and (116.6±35.4) g/cm³ outside the forest, respectively. SO₄^2-, Na⁺, NH₄⁺ and NO₃^- were the most abundant ions in both TSP and PM_2.5, accounting for more than 80% of total ions. The concentrations of SO₄^2-, Na⁺ and NO₃^- outside the forest were significantly higher than those inside, with the exception of NH₄⁺. Correlation analysis on the ions revealed that during the study period, the principal water-soluble ions existed in the form of NaCl, Na₂SO₄, NH₄HSO₄ and NH₄NO₃. Na⁺ and Cl^- in the water-soluble fraction of PM_2.5 and TSP were mainly sourced from sea salt, while the presence of other ions was predominantly influenced by anthropogenic sources. The ratios of c(NO₃^-)/c(SO₄^2-) in TSP and PM_2.5, both inside and outside the forest were less than 0.30, indicating that the stationary sources, e.g. coal burning, power plants and other industrial emissions, might be the main contributors to aerosols at the study site. The ratios of c(NO₃^-)/c(SO₄^2-) in TSP were higher than those in PM_2.5, implying that the mobile sources contributed more to TSP than to PM_2.5. Our results demonstrated that tree canopies could filter PM_2.5 and reduce concentrations of all water-soluble ion species, with the exception of NH₄⁺, suggesting that urban forests can improve air quality.