The influence of vegetation on the horizontal and ...

Science of the Total Environment 443 (2013) 287–298

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

The influence of vegetation on the horizontal and vertical distribution of pollutants in a street canyon J.A. Salmond a,⁎, D.E. Williams b, G. Laing c, S. Kingham d, K. Dirks e, I. Longley f, G.S. Henshaw c a

School of Environment, The University of Auckland, Auckland, New Zealand Department of Chemistry, The University of Auckland, Auckland, New Zealand Aeroqual Ltd, 109 Valley Rd, Mt Eden, Auckland, New Zealand d Department of Geography, The University of Canterbury, Christchurch, New Zealand e Faculty of Medical and Health Sciences, The University of Auckland, Auckland, New Zealand f National Institute of Water and Atmospheric Research, Auckland 1149, New Zealand b c

H I G H L I G H T S ► ► ► ►

Vegetation had a marked influence on the vertical and horizontal concentrations of NO2 within the street canyon. Vegetation led to an increase in the storage of pollutants within the canopy space. Vegetation reduced upwards transport of vehicle emissions and downward penetration of clean air. Some evidence to suggest vegetation changed the chemistry of the atmosphere within the leaf canopy.

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Article history: Received 26 August 2012 Received in revised form 28 October 2012 Accepted 30 October 2012 Available online 28 November 2012 Keywords: Vegetation Street canyon Nitrogen dioxide Trees Air quality

a b s t r a c t Space constraints in cities mean that there are only limited opportunities for increasing tree density within existing urban fabric and it is unclear whether the net effect of increased vegetation in street canyons is beneficial or detrimental to urban air quality at local scales. This paper presents data from a field study undertaken in Auckland, New Zealand designed to determine the local impact of a deciduous tree canopy on the distribution of the oxides of nitrogen within a street canyon. The results showed that the presence of leaves on the trees had a marked impact on the transport of pollutants and led to a net accumulation of pollutants in the canyon below the tree tops. The incidence and magnitude of temporally localised spikes in pollutant concentration were reduced within the tree canopy itself. A significant difference in pollutant concentrations with height was not observed when leaves were absent. Analysis of the trends in concentration associated with different wind directions showed a smaller difference between windward and leeward sides when leaves were on the trees. A small relative increase in concentrations on the leeward side was observed during leaf-on relative to leaf-off conditions as predicted by previous modelling studies. However the expected reduction in concentrations on the windward side was not observed. The results suggest that the presence of leaves on the trees reduces the upwards transport of fresh vehicle emissions, increases the storage of pollutants within the canopy space and reduces the penetration of clean air downwards from aloft. Differences observed between NO and NO2 concentrations could not be accounted for by dispersion processes alone, suggesting that there may also be some changes in the chemistry of the atmosphere associated with the presence of leaves on the trees. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The strategic placement of vegetation in urban areas has the potential to make a significant contribution to improving the quality ⁎ Corresponding author at: School of Environment, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand. Tel.: +64 9 3737599x88650; fax: +64 9 3737434. E-mail addresses: [email protected] (J.A. Salmond), [email protected] (D.E. Williams), [email protected] (G. Laing), [email protected] (S. Kingham), [email protected] (K. Dirks), [email protected] (I. Longley), [email protected] (G.S. Henshaw). 0048-9697/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.scitotenv.2012.10.101

and sustainability of urban environments, especially in warmer climates. In these environments, vegetation can reduce energy consumption (due to cooling from shade and evapotranspiration processes (Rosenfeld et al., 1995; Akbari, 2002)), minimise storm water runoff and noise pollution, and increase local biodiversity in urban areas. However, in many urban areas, space constraints mean that there are only limited opportunities for increasing vegetation density (especially tree density) within the existing urban fabric. These are largely limited to small garden areas or kerbside locations within street canyons.

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J.A. Salmond et al. / Science of the Total Environment 443 (2013) 287–298

Vegetation within street canyons, especially trees, has the potential to affect local scale air quality in a number of different ways. For example, changes in horizontal and vertical pollutant dispersion processes may lead to improved or reduced air quality in areas where the vegetation acts as a barrier to pollutant transport (Khan and Abbasi, 2001). Leaves have large surface areas which not only increase the aerodynamic roughness (slowing air movement) but also provide an effective surface for scavenging pollutants (Bealey et al., 2007). Pollutants may also be removed from the atmosphere through wet and dry deposition (Litschke and Kuttler, 2008), and adsorption and absorption processes on the leaf surfaces (Wellburn, 1998; Fowler et al., 1998). Conversely, depending on the species and climate, plants may release volatile organic compounds and other pollutants, such as the oxides of nitrogen, into the atmosphere (Corchnoy et al., 1992; Benjamin and Winer, 1998; Leung et al., 2011). This can lead to changes to the local photochemistry and potentially increased concentrations of harmful pollutants. Local reductions in temperature resulting from increased shade and evapotranspiration may change the rate of chemical reactions within the atmosphere, leading to reduced concentrations of other pollutants such as ozone (O3) (Cardelino and Chameides, 1990; Nowak et al., 1998). Thus, at local scales, it not clear what impact increased vegetation in street canyons may have on the local air quality. Given the high levels of population exposure to locally emitted traffic pollutants within street canyons, it is important to ensure that any changes to urban vegetation designed to reduce energy consumption and improve regional air quality do not have undesirable impacts on the local air quality. Despite the plethora of modelling and field studies which have considered the controls on pollutant concentrations in street canyons, there have only been a limited number of studies which directly consider the impacts of urban vegetation on local scale pollution concentrations within the street canyon (Gromke and Ruck, 2007; Bealey et al., 2007; Gromke and Ruck, 2012). These studies have generally been limited to numerical modelling and wind tunnel studies (such as Ries and Eichhorn, 2001; Gromke and Ruck, 2007; Gromke et al., 2008; Buccolieri et al., 2009; Gromke and Ruck, 2009; Balczó et al., 2009; Salim et al., 2011a; Buccolieri et al., 2011). These models consistently predict that, on average, at pedestrian height, higher pollutant concentrations will be observed within vegetated canyons, with notable increases in concentration observed on the leeward wall offsetting small reductions on the windward side (Gromke and Ruck, 2007, 2008; Buccolieri et al., 2009; Salim et al., 2011b). This is consistent with aerodynamical models which demonstrate that the presence of trees within a street canyon reduces wind speeds and increases turbulence (Mochida and Lun, 2008; Buccolieri et al., 2009). However, such studies have typically been limited to simple geometries and do not consider the complexity of ‘real-world’ scenarios. One of the few modelling studies to consider the impact of vegetation at ‘real’ intersections, and evaluate the results based on data from a surface air quality monitoring station, concludes that the failure of models to include the effects of vegetation in street canyons can result in ‘non-negligible’ errors (Buccolieri et al., 2011). Unlike regional scale studies of the impacts of vegetation on pollutant concentrations, local scale modelling studies are designed primarily to consider the impacts of trees on dispersion processes, and generally other effects of vegetation on pollutant concentrations such as atmospheric chemistry and deposition processes, are neglected. To date, there has also been no evaluation of the impact of vegetation on the distribution of pollutants with height, which has important consequences not only for local air quality and indoor air quality (especially where windows from multiple storey buildings open onto the street canyon), but also regional scale air quality. Further, there has been no systematic evaluation of the impact of vegetation health, height, species, density, distance from the road, or leaf area index (ratio of leaf area to ground area) on the synergistic processes determining pollutant dispersion, deposition and atmospheric chemistry.

As a consequence, there remains considerable controversy as to whether increased vegetation within urban areas has the potential to provide a positive or negative contribution to local and regional scale air quality. The complexity of the problem, and the paucity of data available regarding the variables which determine whether vegetation acts to increase or decrease pollution within a street canyon, has hindered a systematic cost–benefit analysis of the value of vegetation within urban areas at a variety of scales (Cavanagh and Clemons, 2006). Ultimately, the role of vegetation is likely to be determined by the complex inter-relations between local factors such as vegetation type and density, climatic conditions, street geometry, pollutant characteristics and emission rates. This paper presents data from a field study in Auckland, New Zealand, designed to determine the local impact of the presence/ absence of leaves from a tree canopy on the horizontal and vertical distribution of the oxides of nitrogen within a street canyon. Such an investigation facilitates the study of the air pollutant system as a whole to determine the net impact of processes such as dispersion, deposition and chemistry on pollutant concentrations. The ability to generalise the results from this study is limited by the complexity of the local urban geometry and the site-specific characteristics of pollutant emissions, climate and vegetation. Nevertheless, it makes an important contribution to developing understanding of the impacts of vegetation on local air quality within a street canyon. 2. Methodology 2.1. Site description Auckland is a subtropical city with a population of approximately 1.4 million people located on the North Island of New Zealand (36°52′ S, 174°45′ E). Situated on a narrow isthmus, between the Tasman Sea to the west and Pacific Ocean to the east, Auckland has a mild maritime climate with warm humid summers and mild damp winters. Nitrogen dioxide is considered to be a major pollutant in the Auckland Region and, although concentrations are generally below the National Emissions Standard at many sites, they occasionally breach the 1-hour National Environmental Standard of 200 μg m−3 at kerbside sites (Auckland Regional Council, 2007). In Auckland, 83% of the oxides of nitrogen are emitted from vehicle exhausts, 13% from industry, 3% from biogenic sources and 1% from domestic heating (Metcalfe et al., 2006). Generally, 92–97% of the NOx emitted from older vehicles is in the form of NO (Harrison and Shi, 1996). Although in newer vehicles this figure may be much higher when averaged over the whole fleet, the percentage of primary NO2 emissions is only 20–30% (Carslaw et al., 2007). As a consequence, concentrations of NO are typically much higher than those of NO2 in areas dominated by fresh emissions. NO2 can also be formed as a secondary pollutant by the oxidation of NO. This takes time and may be limited by the presence of oxidants such as ozone in the atmosphere. Thus, generally, a high ratio of NO2:NOx indicates an aged air mass whilst a low ratio indicates high levels of NO and thus fresh local emissions. Previous studies have suggested that secondary NO2 formation in Auckland is limited by the availability of ozone (O3), which may account for the higher concentrations of NO2 observed in the winter as a result of generally higher O3 concentrations at this time (Gimson, 2005). The concentrations of NO2 in the Auckland region tend to be highest in the CBD (Auckland Regional Council, 2007), but are spatially variable and strongly dependent on proximity to traffic sources. This study is focussed on measurements of concentrations of the oxides of nitrogen made at different heights on Symonds Street in Auckland's Central Business District (CBD). This location is shown in the wider context of the air quality monitoring stations operated by Auckland Council which are relevant to this paper and discussed in Section 2.2 (Fig. 1). Symonds Street is a busy bus corridor linking the suburbs to the South of the city with the CBD. It is oriented NE–


SW and consists of two lanes of traffic in each direction, one of which is a bus lane. The average traffic flow is 18,000 vehicles per day (7-day average March 2006) (Auckland Transport, 2012). The road is lined with London Plane trees (broadleaf deciduous) on both sides with wide pavements (Fig. 2). The road plus pavement is approximately 30 m wide and is flanked by buildings 4–8 storeys in height and, at the measurement point, has a height-to-width ratio of approximately 0.5. Measurements were made on the SE facing side of the street. 2.2. Measurements Three chemiluminescence monitors (API 200e) were used to measure concentrations of NO and NO2 in the study area. These were professionally calibrated (by QEMS Ltd) in situ before and during the experiment and showed little drift. Two monitors were located in the science building on the SE side of Symonds Street measuring concentrations at 2 m (ground floor) and 6 m (first floor) above the street respectively. A third instrument was located on top of the adjacent eight-storey building to measure the urban background concentrations (approximately 31 m above ground level). Due to the height of this building, the absence of local pollutant sources and good exposure at the inlet, the measurements from the top of this building were assumed to be typical of background urban boundary layer concentrations. Uniform lengths of teflon sampling tubes shielded by PVC pipe to prevent reactions in the tubes were used at all sites. Measurements were recorded every minute with 30 min and hourly averages calculated following standard data quality control checks. Instruments were periodically swapped between locations to ensure that instrument bias was not affecting the results.

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Limited periods of additional sampling at the second floor level immediately above the ground and first floor sites on Symonds Street (12 m above ground and above the top of the tree canopy) was also under taken using a prototype instrument for measuring nitrogen dioxide produced by Aeroqual. Designed to be a low-cost instrument suitable for large monitoring networks, this instrument has a response time of about 10 min and was calibrated alongside the chemiluminescence instrument prior to deployment. An approximately 2 m long teflon sampling line was used and air drawn down the line by a small internal pump. Results showed that this instrument had a standard error of ± 1.5 ppb over the range of 0–20 ppb (Williams et al., 2009). Air quality monitoring data from other urban sites (City — Queen Street, Takapuna, Henderson and Musick Point, see Fig. 1) were provided by the former Auckland Regional Council. These sites are all part of the Council air quality monitoring network. The Henderson site is located in the suburbs of Auckland, in an open area away from major roads and surrounded by buildings and mixed deciduous and evergreen vegetation (Auckland Regional Council, 2005). The Queen Street monitor is located in a non-vegetated street canyon (20,738 vehicles per day, 7-day average (2004) Queen Street north of Wellesley Street) in the central business district at a height of 3 m (Auckland Regional Council, 2005). The Musick Point monitor is located at a height of 15 m on a peninsula surrounded by open vegetation (golf course) 12 km to the east of Auckland away from immediate sources of pollution. The site is exposed to Auckland's ‘urban plume’ with prevailing wind flows (Auckland Regional Council, 2005). The Takapuna monitoring site is located at 3 m above open playing fields surrounded by mixed vegetation. It is located 60 m east

Fig. 1. Map of Symonds Street field site in Auckland and the Auckland Council monitoring locations: Takapuna (North) Henderson (West) City — Queen Street (Central) and Musick Point (East) and Symonds Street measurement site.

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Fig. 2. The measurement site on the SE side of Symonds Street. Note the complex geometry, presence of trees and broad pavements. Photo taken in spring when leaves are developing (not full canopy) so that the measurement sites can be clearly seen.

of State Highway 1 (109,680 vehicles per day, 7-day average (2005) SH1 Tristram Ave to Northcote Rd) (Auckland Regional Council, 2005). Given the challenges associated with measuring representative urban background concentrations in complex urban environments, peripheral urban monitoring locations are routinely used in air quality studies to provide an estimate of the concentration of pollutants above the surface. However measurements of air quality taken from well above the urban surface (where the effects of individual point sources can be expected to be blended) may provide a more representative measurement of mean background concentration, especially when secondary chemical formation processes are important. Comparisons of the measurements taken at Musick Point, a peripheral urban site, with those at the elevated urban boundary layer site in Symonds Street reveal that there are marked differences in the diurnal cycle between the two locations. Most notably, although the general importance of secondary formation processes are well represented at both sites, concentrations at the peripheral location are much lower. Furthermore the efficiency of vertical mixing processes and the time taken for the horizontal transport of pollutants to Musick Point and the direction of the prevailing winds are reflected in the local timing of peaks and troughs in the two data sets. For this reason, the elevated site is used as indicative of the urban background concentration in later discussions.

data available for the site (not shown). Average diurnal temperature range during both periods was approximately 5 °C (Fig. 3). The autumn period was chosen rather than the summer for this study as the meteorological conditions in March and April were very similar to those in June and July. A small decrease in mean ambient temperature (4 °C) was observed between the two periods which is likely to have resulted in some increase in domestic heating. However, given the small contribution of this source to overall NO2 concentrations in the region, this will have had little effect on the data. Wind speed and direction statistics were very similar throughout the two measurement periods (Fig. 3), with a slight tendency towards E and SE directions during the day in the winter period. Therefore it is unlikely that differences in meteorological conditions alone could account for the observed changes in dispersion characteristics between the two observation periods. 2.4. Statistical analyses Statistical analyses consist of comparisons between the mean concentrations during the ‘leaf on’ and ‘leaf off periods and also between sites and heights (ground floor and first floor). These were carried out using Bonferroni-adjusted multiple t-tests on daily average data after subjecting them to a square root transformation. 3. Results

2.3. Field campaign 3.1. Temporal and spatial variability of NO2 in the Auckland region Measurements were made from 1st March 2009 to 30th July 2009. Leaf fall commenced in early May and took approximately 3 weeks. For the purposes of this paper, data from March 1st to April 30th 2009 were considered representative of the autumn ‘leaf-on’ period whilst data from June 1st to July 30th 2009 were considered representative of the ‘leaf-off’ period. Due to equipment failure, onsite measurements of meteorological parameters were not available for the entire period. Meteorological data were therefore obtained from the NIWA CliFlo Climate Database for the Onehunga site. This residential site, located 8 km south of the city centre, had a near-complete data set of mean hourly values of wind speed, temperature, wet bulb and relative humidity and showed good correlation with the

NO2 concentrations show strong diurnal variability with low concentrations observed during the night (minima observed pre-dawn) and higher concentrations during the daytime period throughout the Auckland region (Fig. 4). During both measurement periods, a marked peak in concentrations is reported at all measurement sites between 0700 and 0900 New Zealand Standard Time (NZST), consistent with the peak in emissions associated with the morning rush hour. A peak in the primary pollutant NO is also observed at this time (Fig. 5). A second peak in NO2 concentrations is observed during the evening period. This occurs between 1800 and 2000 during the leaf-off period (Fig. 4b), but is less pronounced and more temporally


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Fig. 3. Average diurnal temperature range, wind speed and wind direction during leaf-on (black) and leaf-off (grey) periods.

variable throughout the network during the leaf-on period. For example, NO2 concentrations peak at 1600 in the central city locations of Queen Street and Symonds Street, whilst concentrations in the suburbs peak 2–3 h later at 1900 (Fig. 4a). Furthermore, although small coincident peaks in NO are observed in the city centre, they are not observed at the surburban locations. This may reflect the fact that the central city monitors are located within street canyons which limit dispersion compared to the more open suburban locations. Given that traffic patterns are unlikely to have changed significantly in the two periods, the changes in the evening peak in NO and NO2 concentrations observed at all sites between leaf-on and leaf-off periods may relate to changes in stability. For example the increased stability

of the atmosphere (due to earlier sunset) during the winter (leaf-off) period would trap emissions in a shallower boundary layer. However, it is interesting to note that outside of the central business district, an evening peak in NO concentrations coincident with the evening rush hour is not observed. Instead, NO concentrations peak later in the evening. Given that we would expect traffic-related sources of NO and NO2 to peak at the same time, this suggests that the pronounced evening peak in NO2 concentrations during the leaf-off period may also be associated with secondary formation of NO2 in the atmosphere from the oxidation of NO. This is consistent with previous studies which have shown increased oxidation capacity of the Auckland atmosphere during the winter compared to the summer period (Gimson,

Fig. 4. Diurnal concentrations of NO2 observed at locations throughout the Auckland region during a) autumn leaf-on conditions (March 1st–April 30th 2009) and b) leaf-off conditions (June 1st–July 30th 2009).

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Fig. 5. Diurnal concentrations of NO observed at locations throughout the Auckland Region during a) autumn leaf-on conditions (March 1st–April 30th 2009) and b) winter time leaf-off conditions (June 1st–July 30th 2009).

2005). It is unlikely that non-traffic related emission sources of NO2 could account for these trends as the emission inventory suggests that domestic heating emissions in the region are negligible and industrial sources are also unlikely to peak at this time (Auckland Regional Council, 2005). During both seasons, the highest concentrations were reported for Queen Street, a heavily trafficked central urban street, flanked by buildings on both sides and with no vegetation near the monitoring location. As expected, concentrations of both NO2 and NO were lowest at the peripheral urban site (Musick Point) throughout most of the diurnal cycle in both seasons, reflecting the generally clean air surrounding the Auckland region. A lag in peak concentrations is also observed between the peak concentrations in the central city and Musick Point especially in the winter when it may be by as much as 1 h, reflecting the time taken for horizontal and vertical transport processes to disperse the pollutants to these sites. Lag times of up to 1 h have also been observed elsewhere as pollutants are transported from areas with high densities of emissions to background locations (Moreno et al., 2009). 3.2. Role of vegetation in determining the variability of NO2 in a street canyon at ground level Throughout the field study, concentrations of nitrogen dioxide (NO2) observed at the main study site at ground level on Symonds Street showed the same trends as those reported by the Auckland Regional Council monitoring network (Fig. 4). However, despite similar traffic flow patterns, concentrations at ground level in Symonds Street were lower than those reported at Queen Street (the other central urban monitoring location) , possibly reflecting the fact that the Symonds Street canyon is wider than the Queen Street canyon at the points of monitoring. Interestingly, during the leaf-off period, concentrations near the surface in Symonds Street more closely

resemble those at Takapuna (a more open suburban site but close to Highway 1 which has traffic flows five times greater than those reported in Symonds Street). Since the atmosphere is a dynamic system, and both emission patterns and dispersion trends are variable with time, it is not possible to determine the influence of leaf-fall on concentrations by directly comparing the NO and NO2 concentrations at one site between two time periods (leaf-on and leaf-off). Indeed, statistical analysis of the NO and NO2 concentration data reveals that concentrations are significantly higher during the leaf-off period compared to the leaf-on period at nearly all the ground level monitoring sites regardless of the presence or absence of deciduous trees in the immediate area, based on Bonferroni-adjusted multiple comparison t-tests (for NO2, p b 0.01 for all sites and for NO, p b 0.01 for all sites except the two distant sites Musick Point (t = 2.06, p = 0.20) and Urban Background (t = 2.5, p = 0.08)). However, it is reasonable to assume that in the absence of marked changes in meteorological conditions which could favour one site over another (which in this case is a fair assumption), the system as a whole will, on average, respond in a similar pattern between the two monitoring periods to regional changes in conditions. Thus, we can expect the ratio in concentrations between sites in the network to remain relatively constant over time. For example there is no significant change to the ratio of NO2 concentration between Queen Street and Takapuna during the leaf-on and leaf-off periods based on a t-test (t = 1.72, p = 0.09). However, examination of the ratio of NO2 concentrations between the ground floor Symonds Street site and the Queen Street site shows a significant difference between the two monitoring periods ( t = 5.36, p b 0.001) (see also Fig. 6). The largest difference in the ratio occurs during the daytime period (1300–1700), when concentrations of NO2 are higher at the Symonds Street location during the leaf-on period, resulting in a ratio closer to unity (Fig. 6a). Similarly, between 0500 and 0700 concentrations are markedly higher at the Symonds Street site during


the leaf-on period compared to the leaf-off period. A similar trend is observed in the NO data. This suggests that the presence of the leaf canopy is leading to increased storage of both NO and NO2 within the street canyon during periods when emissions are high. However the presence of vegetation in the canyon has less influence over surface concentrations during the nocturnal period, and the ratio between the two sites remains constant between the leaf-off and leaf-on periods at this time. This suggests that the presence/absence of leaves on the trees in the Symonds Street canyon (the only obvious change in local conditions between the two seasons) has a significant impact on NO2 concentrations. 3.3. The impact of vegetation on the vertical distribution of NO2 in an urban street canyon at a diurnal scale During the leaf-on period, concentrations of both NO2 and NO observed at the first floor level in Symonds Street were higher than those observed at the ground floor throughout much of the diurnal cycle (Figs. 4 and 5). However, during the leaf-off period, concentrations between the two heights were very similar. In the absence of any changes in vegetation, we could expect the ratio of the NO2 concentrations at the ground floor and first floor levels in Symonds Street to remain constant between the two averaging periods. Thus this change is likely to be due to changes resulting from the presence of leaves on the trees. Interestingly, unlike all the ground level monitoring sites, there was no statistically significant difference between the concentrations

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observed at the first floor site during the leaf-off and leaf-on periods for either NO (t = 0.26, p = 0.80) or NO2 (t = 0.63, p = 0.10). This supports the hypothesis that the presence of the leaf canopy slows the horizontal and vertical transport of NO and NO2 and further indicates that the resulting storage of pollutants in the canopy space was sufficient to offset the regional increase in pollutant concentrations observed in the later months. Previous modelling and measurement studies in non-vegetated street canyons consistently show a marked reduction in most pollutant concentrations with height (Vardoulakis et al., 2002; Xie et al., 2009) and more generally in the urban atmosphere (Restrepo et al., 2004; McAdam et al., 2011). However, for the specific case of NO2 and NO, the reported trends in vertical distribution are more complex. The oxidation of NO may occur very fast (time scales of the order of tens of seconds), especially with the penetration of ozone rich air into the canopy, and thus take place whilst the pollutants remain in the canyon (Vardoulakis et al., 2003; Baker et al., 2004; Grawe et al., 2007). As a result, although concentrations of NOx typically decrease with height, the concentration of NO2 has been shown, on occasion, to increase with height due to increased oxidation of NO (Vakeva et al., 1999). Further studies have shown that the vertical ratio of NO2:NOX is dependent on the advection of plumes of aged or fresh emissions into the street canyon (Janhall et al., 2003). In this case, since concentrations of both NO and NO2 are higher at the first floor level during the leaf-on period, it is most likely that there is increased storage of these pollutants in the leaf canopy space as discussed earlier. The difference in concentrations between

Fig. 6. The ratio of ambient concentrations between Symonds Street and Queen Street during the leaf-on (black) and leaf-off (grey) monitoring periods for a) nitrogen monoxide and b) nitrogen dioxide.

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the two heights also suggests that the two heights were poorly coupled by vertical transport processes during the leaf-on period. In comparison, during the leaf-off period, there were no significant differences in concentration of either NO or NO2 between the ground and first floor sites in Symonds Street suggesting that the two heights are efficiently coupled by atmospheric mixing processes and pollutant concentrations are essentially homogeneous with height within the canyon. NO2 concentrations at the first floor level also showed decreased diurnal variability and less sensitivity to traffic flows compared to the ground floor site (Fig. 4), further supporting the notion of reduced vertical exchange between the surface and aloft. For example, there is an absence of a marked peak in concentrations during the morning rush hour, with a single peak in concentration observed between 1600 and 1800 LST, well after the morning peaks observed at surface sites elsewhere in the network. Interestingly, similar patterns are not observed in the NO time series (Fig. 5). Although concentrations of NO are higher at the elevated site during the leaf-on period, a lag in concentrations is not observed compared to the rest of the network; the morning peak in concentration is discrete and temporally localised and a decrease in concentrations (albeit at a slower rate than other sites in the network) is observed during the day (Fig. 5). This would suggest that processes other than changes in dispersion may also account in part for the observed trends in NO2 concentration. This is especially true during the daytime period when, unlike all the other sites in the network, concentrations at the first floor site remain high and continue to increase (by a small amount throughout the afternoon period) following the morning peak. Given that there are no other primary sources of NO2 within the leaf canopy space, this suggests that there may be an increased rate of chemical conversion of NO to NO2 at this height. Concentrations of NO observed between 0700 and 1300 do decrease by a slightly larger amount in the tree canopy compared to those in the trunk space beneath (Fig. 5), probably due to reduced horizontal and vertical advection of clean air into the canopy air space, which further supports this hypothesis. Changes in atmospheric chemistry resulting from vegetation have been observed elsewhere and have been attributed to the increase in volatile organic compounds (VOCs) from vegetation (Leung et al., 2011). However, the chemistry of nitrogen species is very complex in urban environments. It is therefore not possible to identify clear evidence for chemical pathways given the instrumentation used. Further fieldwork is required to develop a better understanding of the influence of vegetation on local chemistry in determining NO2 concentrations. For a limited time during the main field campaign, measurements of NO2 were also made above the tree canopy near the top of the street canyon using inexpensive portable monitors. Again, marked differences could be observed between the leaf-on and leaf-off periods. During the leaf-on period, concentrations within the street canyon, above the tree canopy at the second floor were very similar to those reported at the urban background site six floors above (Fig. 7). This suggests that very little of the surface emissions penetrated through the top of the tree canopy, and that the leaves were effectively acting as a lid, trapping emissions at street level (Fig. 7). In comparison, when the leaves were off the trees, concentrations at this second floor level were higher than the urban background value throughout the diurnal cycle. Further, during the leaf-off period, concentrations were even higher than those reported at the first and ground floor levels during the nocturnal period. In the absence of leaves, concentrations of NO2 are much higher above the tree canopy, indicating more efficient vertical coupling during the day, and either local increases in secondary NO2 production or stagnant air flows at the upper level at night. Further study is required to elucidate the underlying mechanisms. The results do indicate however that the presence of the leaves makes a significant difference to the processes determining local air quality within the canyon.

3.4. Effects of leaves on the vertical distribution of NO2 at a high temporal resolution Detailed analysis of one 24-hour period shows that there is also a lag between the peaks observed at the ground floor and at the first floor sites (Fig. 8). This supports the hypothesis that the leaved tree canopy is acting to slow the vertical transport of vehicle emissions upwards. However, both the length of the lag and the role of the storage/production term appear variable with time. Further work, including more detailed measurements of the 3-dimensional wind field, is required to fully support this hypothesis. During the night, the concentrations of NO2 at the first floor level were also consistently higher than the minima observed at ground level, suggesting a storage effect on the time scale of hours as clean air does not seem to penetrate into the canopy space to flush out the pollutants. (An alternative explanation could be the enhanced photochemical production of NO2). Further work is required to elucidate between the underlying causes. Thus, at the first floor level, concentrations appear to be responding to processes operating over longer time periods. This is important from a health perspective as there is increasing evidence that transient exposure to elevated levels of pollution (consistent with spikes in concentration) can have significant health effects on individuals already affected by debilitating heart or lung disease (Moreno et al., 2009). Thus, although mean concentrations are higher at the first floor level, exposure to short-term peaks in concentration is reduced (Fig. 8). 3.5. Influence of vegetation on mean concentrations of pollutants near surface within the street canyon Concentrations of NO2 on Symonds Street (normalised by the urban background value to remove the seasonal influence) show a small increase when leaves are on the trees, with mean concentrations of 213% of the urban background level during the leaf-on period and 190% during the leaf-off period. However, the difference in NO concentrations between the two periods was much larger, with mean concentrations of NO of 471% the urban background value during leaf-on and only 373% during leaf-off. This supports the modelling studies and suggests that the trees do lead to an increase in near surface concentrations. This is especially true as during the winter period we could expect near surface concentrations to be higher due to increased cold starts of vehicles which is likely to result in an increase in near surface conditions, thereby making it difficult to detect the decrease in concentration associated with the absence of leaves on the trees. 3.6. Influence of vegetation on horizontal flows within the street canyon In the absence of vegetation, previous studies have demonstrated the importance of vehicle flows, prevailing wind velocities and street geometry in determining pollutant concentrations within street canyons (see Vardoulakis et al. (2003) and Salmond and McKendry (2009) for further discussion). If wind flows are within 60° of perpendicular to a street canyon, a spiral or helical type flow may develop. As a result, in the absence of vegetation, modelling and field studies consistently report that on the leeward side, pollutant concentrations are 2–3 or more times those on the windward side of the street canyon (Vardoulakis et al., 2002; Xie et al., 2009). Models which take into consideration the effects of vegetation suggest that on average, higher pollutant concentrations at pedestrian height will be observed within vegetated canyons, with notable increases in concentration observed on the leeward wall offsetting small reductions on the windward side (Gromke and Ruck, 2007, 2008; Buccolieri et al., 2009; Salim et al., 2011b). In order to analyse the concentrations by wind flow pattern, the mean concentrations in Symonds Street were split into leeward flows (NW sector 225–15°) and windward flows (SE sector 75–195°). As


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Fig. 7. Changes in NO2 concentration with height within the Symonds Street canyon during a) leaf-on and b) leaf-off monitoring periods.

expected, increased concentrations of NO2 were observed associated with leeward flows compared to windward flows during both the leaf-on and leaf-off monitoring periods (Fig. 9). However, the difference in concentration observed between the leeward and windward flows was dampened by the presence of leaves on the trees, especially during

the morning and evening rush hour periods (Fig. 9a) when emissions are high. Data were then normalised by the urban background concentration in order to determine whether a net increase or decrease could be observed in the windward and leeward flows. This removes the

Fig. 8. Concentrations of NO2 observed at 1-minute time intervals at the first floor and ground floor sites during the a) leaf-on period (March 27th–April 6th 2009) and b) the leaf-off period (June 4th–14th 2009).

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Fig. 9. The effect of wind direction on NO2 concentrations for NW (leeward) flows (grey) and SE (windward) flows (black) for a) leaf-on and b) leaf-off monitoring periods for the ground floor Symonds Street site.

effect of any bias in concentration associated with wind direction and regional-scale transportation, as well as the seasonal change in emissions. Using this approach, a small increase in concentrations can be seen on the leeward side, as predicted by previous modelling studies (e.g. Gromke and Ruck, 2007) during the leaf-on period, but a corresponding decrease in concentrations on the windward side is not observed (Fig. 10). Given the complex flow patterns observed in field studies (generated by uneven canyon geometry, traffic induced turbulence and the temporal variability of wind flow patterns) it is not surprising that a smaller difference between the windward and leeward conditions is observed compared to model output. However the results do suggest that the presence of the leaves limits clean (potentially ozone-rich) air entering the canyon.

4. Conclusions In summary, the presence of leaves on the trees has a marked influence on the concentration and vertical distribution of NO2 and NO within the street canyon. During the leaf-on period, mean concentrations were higher below tree top (which includes the canopy and trunk space) within the canyon. At the first floor level a marked reduction in variability was observed. During the leaf-off period, no significant changes in concentration with height were observed. The presence of leaves also reduced the difference between windward and leeward concentrations, suggesting weakened circulatory dispersion patterns within the canyon. Although a small increase in concentrations was reported on the leeward side as anticipated by modelling

Fig. 10. The effect of wind direction on NO2 concentrations normalised by urban background concentrations to minimise the effect of regional and seasonal changes of concentrations for NW (leeward) flows (grey) and SE (windward) flows (black) for a) leaf-on and b) leaf-off monitoring periods for the ground floor Symonds Street site.


studies a corresponding decrease in concentrations on the windward side was not observed. The results provide evidence to suggest that the presence of leaves on the trees reduces the upwards transport of fresh vehicle emissions, increases the storage of pollutants within the canopy space and reduces the penetration of clean air downwards from aloft. Further work is required to determine whether the transport of ozone-rich air into the canyon is also limited by the presence of the tree canopy. Although there is some evidence to suggest changes in the chemistry of the atmosphere within the leaf canopy (for example the formation of secondary NO2 appears to be promoted during the day and reduced at night), these are hard to distinguish in this study from dispersion patterns and organic emissions which were not measured in detail. Further study is therefore required to fully elucidate the underlying processes responsible for the trends observed. These results show that vegetation does have a marked impact on the mean and vertical distribution of pollutants within a street canyon. Although these measurements broadly support the modelling results for hypothetical scenarios (which do not take into consideration the complexity of real world dispersion patterns, complex geometries, organic emissions or pollutant chemistry), they indicate the need for further study aimed at improving our understanding of the collective impact of these processes on air quality. In many urban areas, space constraints mean that there are only limited opportunities for increasing vegetation density (especially tree density) within the existing urban fabric. These are largely limited to small garden areas or kerbside locations within street canyons. Thus although studies using regional-scale models indicate that increased vegetation in urban areas may improve regional scale air quality (in Auckland for example, regional scale modelling studies suggest that trees may be responsible for the removal of approximately 2500 t of NO2 annually (Cavanagh and Clemons, 2006)), given the complexity of processes operating in a vegetated street canyon, it is important that careful cost–benefit analyses are undertaken at local scales before vegetation is introduced to kerbside locations. Acknowledgements This work was funded by NZ Ministry of Business, Innovation and Employment funding to D. Williams and J. Salmond (project UOAX0709) and a Faculty Research Development Fund grant from The University of Auckland to J. Salmond and further supported by Aeroqual Ltd. Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version, at http://dx.doi.org/10.1016/j.scitotenv.2012.10. 101. These data include Google maps of the most important areas described in this article. References Akbari H. Shade trees reduce building energy use and CO2 emissions from power plants. Environ Pollut 2002;116:S119–26. Auckland Regional Council. The ambient air quality monitoring network in the Auckland region, July 2005 TP296 Auckland Regional Council Technical Publication No. 296; 2005 [ISSN 1175-205X_ ISBN 1877416-266]. Auckland Regional Council. Nitrogen dioxide in air in the Auckland region — passive sampling results Auckland Regional Council Technical Publication No. 346; 2007 [ISBN-13: 978-1-877416-86-6 ISBN-10: 1-877416-86-X]. Auckland Transport. Traffic count data accessed May 2012. from http://www. aucklandtransport.govt.nz/improving-transport/maintenance/Road/Pages/ Traffic-Counts.aspx2012. Baker J, Walker HL, Cai X-M. A study of the dispersion and transport of reactive pollutants in and above street canyons — a large eddy simulation. Atmos Environ 2004;38: 6883–92. Balczó M, Gromke C, Ruck B. Numerical modeling of flow and pollutant dispersion in street canyons with tree planting. Meteorol Z 2009;18(2):197–206.

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