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.