Impacts of urban roadside forest patches on NO2 concentrations

doi:10.1016/j.atmosenv.2020.117584

Atmospheric Environment

Volume 232, 1 July 2020, 117584

https://doi.org/10.1016/j.atmosenv.2020.117584 Get rights and content

Highlights

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We studied if tree canopy cover can increase NO₂ levels below the canopy.
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Tree canopies did not cause higher or lower NO₂ levels underneath canopies.
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NO₂ levels at the below-canopy height did not differ from adjacent treeless areas.
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This may be due to the tree species and the small forest patch size in the study.

Abstract

Although it is commonly believed that trees can improve air quality, recent studies have shown that such pollution mitigation can be negligible – or that tree canopies can even increase pollutant concentrations near their sources compared to adjacent treeless areas. We explored the impacts of urban roadside forest patches on the concentrations of nitrogen dioxide in summer and winter in the Helsinki Metropolitan Area, Finland, and especially investigated if canopy cover can result in increased concentrations of NO₂ below the canopy. Our results, however, did not show significantly higher – or lower – NO₂ concentrations underneath tree canopies compared to levels above canopies. Neither did NO₂ levels at the below-canopy sampling height differ significantly between forest patches and adjacent open, treeless areas. The lack of a canopy effect may derive from the rather small size of the forest patches, and – compared to previous studies with similar design – divergent tree species composition forming a dense canopy structure. Our results corroborate previous studies that the potential ecosystem services offered by urban near-road forests are more likely due to benefits other than those related to the removal of air pollutants.

Introduction

Air pollution continues to be among the largest urban environmental problems worldwide. In many urban areas, concentrations of, for instance, nitrogen dioxide (NO₂) that originates from road traffic, energy production and industry, exceed safe levels for people and ecosystems (EEA, 2016). Traffic-related combustion is often the main source of nitrogen oxides (NO_x = NO + NO₂) of which the majority is emitted primarily as NO that is rapidly oxidized by ozone (O₃) to nitrogen dioxide (Anttila et al., 2011). Ground-level O₃ is formed when volatile organic compounds (VOCs) react with NO_x under sunlight; thus the concentrations of O₃ are dependent on NO_x and VOC emissions (e.g. Calfapietra et al., 2013; Chameides et al., 1992). VOCs are emitted from anthropogenic sources (e.g. traffic and industry) and natural sources (biogenic VOCs from e.g. trees). If local VOC concentrations are high, O₃ concentrations are very sensitive to NO_x concentrations. Furthermore, when NO_x concentrations are low, NO_x is a precursor to ozone formation, but at higher concentrations, NO_x catalyzes ozone destruction, resulting in O₃ depletion (e.g. Rodes and Holland, 1981). These complex interactions thus, in part, determine eventual NO₂ concentrations in urban environments. Elevated NO₂ levels may cause infections and respiratory symptoms in children and persons with asthma, in particular, as well as allergic and atopic symptoms (Krämer et al., 2000; Kampa and Castanas, 2008).

The influence of vegetation on urban air pollutant levels, and especially the potential of urban green to purify polluted urban air, has in recent years raised a considerable amount of research interests worldwide. Although the primary action for improving the quality of air ought to be the cutting of emissions (Duncan et al., 2016), it has been proposed – based on laboratory (e.g. Chaparro-Suarez et al., 2011; Hu et al., 2016) and modeling studies (e.g. Hirabayashi et al., 2012; Selmi et al., 2016) – that especially trees in urbanised settings capture air pollutants. For instance, absorption of, e.g. NO₂ into plant leaves (Rondón and Granat, 1994; Takahashi et al., 2005; Chaparro-Suarez et al., 2011) should improve urban air quality and provide a valuable ecosystem service or nature-based solution to the air pollution problem.

On the other hand, recent field studies have shown variable and often contradictory results on plant purification effects, the efficacy depending, e.g. on the studied air pollutant, climatic conditions and vegetation type and structure (e.g. Yin et al., 2011; Pataki et al., 2013; Setälä et al., 2013; Brantley et al., 2014; Fantozzi et al., 2015; Irga et al., 2015; Tong et al., 2015; Xing and Brimblecombe, 2019). Interestingly, recent studies by our research team and others have shown that concentrations of, for instance NO₂ (Harris and Manning, 2010; Yli-Pelkonen et al., 2017c; Viippola et al., 2018) in near-road environments and polycyclic aromatic hydrocarbons (PAHs) in near-road and park environments (Viippola et al., 2016; Yli-Pelkonen et al., 2018), can actually be higher under tree canopies than in open areas without trees. Such “negative” vegetation effects on local air quality can be considered an ecosystem disservice (Escobedo et al., 2011).

Reasons for the high concentration of pollution under tree canopies are not clear but we have suggested that they are likely due to polluted air being “trapped” under tree canopies due to reduced ventilation (Viippola et al., 2016, 2018; Yli-Pelkonen et al., 2017c). Although trees can absorb NO₂ from the ambient air to some extent (Rondón and Granat, 1994; Chaparro-Suarez et al., 2011), we have suggested that the “trapping effect” may be high enough to mask uptake so that the net outcome is worse – or at least not better – air quality within tree canopies than in adjacent areas without trees (Yli-Pelkonen et al., 2017a, c; Viippola et al., 2018).

There are several mechanisms that can affect the dispersion of NO₂ from the pollution source, such as road traffic. NO₂ concentrations typically decline rather rapidly when moving further downwind from a road, however the decay curve is also dependent on traffic volume (Viippola et al., 2018; Yli-Pelkonen et al., 2017c; Xing and Brimblecombe, 2019). In near-road environments, solid barriers, such as noise walls or buildings, and semi-porous barriers, such as greenbelts or forests, can obstruct air movement and thus both horizontal and vertical pollutant transport and dispersion downwind from the pollution source (Abihijith et al., 2017; Baldauf, 2017). It has been suggested that air pollutant concentrations can increase between roads and solid barriers (Baldauf et al., 2008; Hagler et al., 2011) or greenbelts (Al-Dabbous and Kumar, 2014; Tong et al., 2016; Yli-Pelkonen et al., 2017c) due to the formation of a recirculation zone of air in front of a barrier. As shown by Yli-Pelkonen et al. (2017c), increasing density of trees can result in higher NO₂ concentrations between the road and the front edge of a forest.

Furthermore, vertical transport of NO₂ can be affected by barriers, such as greenbelts, due to increased turbulence in front of the barrier that can, depending on barrier height, elevate the air pollutant plume higher and over the barrier (Baldauf, 2017; Ghamesian et al., 2017). Empirical field studies focusing on the vertical distribution of NO₂ concentrations spanning over 50 m downwind from the pollution source are practically non-existent, but computational fluid dynamic simulations predicting general contours of pollutant concentrations for scenarios with or without barriers exist (Ghasemian et al., 2017).

Building on our aforementioned research on the topic, our current study aimed to further explore the canopy trapping effect by studying NO₂ levels in roadside forest patches under summer (dense foliage) and winter (thin, leafless, pervious canopy) conditions in Finland – and this time at two heights: below and above the canopy. Based on our earlier studies, our hypothesis is that (1) the levels of NO₂ below tree canopies are higher than (a) those above canopies, and (b) those measured at the same height from the ground and distance from the pollution source in adjacent treeless areas. Our expectation is that (2) the levels of NO₂ above tree canopies are indifferent compared to concentrations at the same height and distance from the pollution source in treeless areas. Furthermore, we expect that (3) the potential trapping effect is greater during summer (with leaves) than winter (without leaves).

Section snippets

Study design and sampling

We studied the concentrations of NO₂ by measurements with diffusive passive samplers on the northern side of east-west oriented roads with large traffic volumes in the Helsinki Metropolitan Area (60°10′15″N, 24°56′15″E), southern Finland (Table 1). Altogether nine sampling sites were set up in three cities: five in Helsinki, three in Vantaa and one in Espoo (Fig. 1 and Appendix A). Each site included one forest patch area and one open area without trees. No biasing roads or intersections were

Results

Mean NO₂ concentrations did not differ between forest and open areas or between the two sampling heights – representing the height levels below and above the canopy. The concentrations of NO₂ were significantly higher in winter than in summer (Model A: Table 2, Fig. 3). When data from two seasons (summer and winter) were analyzed separately (model B), mean NO₂ levels did not differ between forest and open areas, or between the two sampling heights in winter. However, sampling height did have an

Discussion and conclusions

Contradicting our first hypothesis, concentrations of traffic-derived NO₂ were not statistically significantly higher below than above the tree canopies of mainly broadleaved forest patches. Neither did NO₂ levels at the lower sampling height differ significantly between forest patches and the adjacent open, treeless areas. However, that NO₂ concentrations above tree canopies (at a mean height of about 5 m) did not differ significantly between forested and open areas supports our second

CRediT authorship contribution statement

Vesa Yli-Pelkonen: Conceptualization, Methodology, Investigation, Data curation, Formal analysis, Visualization, Writing - original draft, Project administration, Funding acquisition. Viljami Viippola: Conceptualization, Methodology, Investigation, Writing - review & editing, Funding acquisition. D. Johan Kotze: Formal analysis, Visualization, Writing - review & editing. Heikki Setälä: Conceptualization, Methodology, Investigation, Writing - review & editing, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was funded by the City of Helsinki, the Maj and Tor Nessling Foundation and the Helsinki Metropolitan Region Urban Research Program (EKO-HYÖTY project). We acknowledge Taru Hämäläinen and Ronja Jokirinne for field work assistance and the anonymous reviewers for their constructive feedback and suggestions to improve the paper.

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Impacts of urban roadside forest patches on NO2 concentrations

Highlights

Abstract

Introduction

Section snippets

Study design and sampling

Results

Discussion and conclusions

CRediT authorship contribution statement

Declaration of competing interest

Acknowledgements

Atmospheric Environment

Atmos. Environ.

Atmos. Environ.

Transport. Res. Part D

Sci. Total Environ.

Environ. Pollut.

Atmos. Environ.

Environ. Pollut.

Urban Clim.

Environ. Pollut.

Environ. Pollut.

Atmospheric Environment

Environ. Pollut.

Environ. Pollut.

Atmos. Environ.

Atmos. Environ.

Environ. Pollut.

Sci. Total Environ.

Environ. Pollut.

Urban For. Urban Green.

Atmos. Environ.

Atmos. Environ.

Urban For. Urban Green.

Environ. Pollut.

Ecosyst. Serv.

Chemosphere

Environ. Pollut.

Impacts of urban roadside forest patches on NO₂ concentrations