Polycyclic aromatic hydrocarbons (PAHs) are naturally occurring combustion byproducts of coal, petroleum, and gasoline. PAHs are emitted readily into the environment and have been associated with negative health outcomes including respiratory disease [1
], cardiovascular disease [1
] and cancer [5
]. Of particular concern in urban environments is exposure to PAHs from pervasive outdoor sources including automobile, diesel fuel, and heating oil emissions [8
]. A better understanding of how exposure to PAHs may vary by location, specifically within short distances from dominant sources, is critical to informing PAH exposure models based on locations of and within apartment buildings in urban environments.
Important differences in spatial distribution of outdoor ambient PAHs have been described [10
]. For example, Jaward et al
. reported large-scale PAH spatial variations in air samples collected on a ship traveling from The Netherlands to South Africa [12
]. Lee et al.
also reported highest PAH concentrations measured at a major intersection with heavy traffic, followed by urban and rural locations in Taiwan [11
]. Within a city concentrations of outdoor airborne PAHs can also vary significantly [13
]. For example, Nielsen described higher ambient PAH concentrations measured along a busy street in Copenhagen compared to concentrations measured in a park, several meters away [13
]. Similarly, a recent study observed a clear horizontal concentration gradient of PAHs within 150 meters of a highway in New Jersey [16
]. Our group also described a vertical gradient in ambient PAH concentrations whereby the concentrations of outdoor PAHs measured from an apartment building window on the 6th floor or higher were lower than those measured at lower floors [17
]. These findings can be explained by differences in proximity to roadways and traffic emission sources on a relatively large scale. However, variability of PAH concentrations in different structural environments (e.g., open space vs.
semi-closed spaces) within short distances from traffic emission sources, have not been well-studied.
PAHs have two types of anthropogenic sources: pyrogenic (i.e.
, incomplete combustion of organic materials such as traffic emissions and heating oil) and petrogenic (i.e.
, unburned fossil organic materials such as direct evaporation from petroleum products). Petrogenic vs.
pyrogenic emission ratios of semivolatile (i.e.
, low molecular weight) vs.
, high molecular weight) PAHs often differ [18
]. For example, methylphenanthrenes, semivolatile PAHs, are emitted more abundantly from petrogenic sources [19
], whereas the predominant source of nonvolatile PAHs is from traffic emissions, a pyrogenic source [20
]. In addition, studies support a distinction between semivolatile and nonvolatile PAHs in atmospheric behaviors [20
] and by season [23
] as well as exposure-related health outcomes. For example, we reported asthma was linked to exposure to semivolatile PAHs [4
] while obesity was linked to exposure to nonvolatile PAHs [24
]. Therefore, a greater understanding of the spatial characteristics of PAHs relies on considerations of nonvolatile vs.
semivolatile PAHs concentrations.
It is well known that ambient PAHs from predominately outdoor sources often vary across different rooms within homes [25
]. However, it is unknown if PAH concentrations measured from a window at the front of a building (in open space, adjacent to a street) varies from concentrations measured from a window at the side or back of a building (semi-closed space, in an alley), limiting accurate personal exposure assessments. Therefore, our objective was to characterize nonvolatile and semivolatile outdoor ambient PAHs measured at the front, open side compared with alley, semi-closed side of a building in an urban neighborhood in New York City (NYC). We hypothesized that even across a short distance relative to street traffic, the dominant emission source, there would be a difference in PAH concentrations, especially the nonvolatile PAHs. In addition, we hypothesized that the difference in PAH concentrations would remain apparent between frontage vs.
alley sides of buildings throughout the urban NYC neighborhood.
Outdoor ambient PAH concentrations are variable throughout NYC. Here we have illustrated that differences exist in concentrations measured in open vs. semi-closed spaces, even when both locations are within short distance relative to traffic emission sources and outside the same apartment. In our sample of 14 simultaneous monitoring periods, nonvolatile ambient PAHs were significantly higher when measured from a window adjacent to a street, closer to traffic emissions (i.e., open space), compared to a window nine meters away, adjacent to an alley (i.e., semi-closed space). Furthermore, in a neighborhood-wide comparison, nonvolatile PAHs were also significantly higher when measured at the street sides compared with the alley sides of apartment buildings, but not semivolatile PAHs. This study highlights small-scale spatial variations in ambient PAH concentrations that may be related to the structural built environment from which the samples are measured as well as the relative distance from street traffic and may influence personal exposure.
In an effort to understand differences in PAH concentrations related to distance from traffic sources, Nielsen investigated airborne PAHs along a busy street and a few hundred meters away in an adjacent park in Copenhagen, Denmark [13
]. Both nonvolatile and semivolatile PAHs were noted to be higher along the street, closer to traffic emission sources. Our central site findings on the micro level are consistent with those of Nielsen on the macro level, in that PAHs measured closest to street traffic were higher than concentrations measured only about nine meters away at the alley side of the same apartment building. Our findings were more robust for the nonvolatile PAHs which are predominately derived from pyrogenic sources such as traffic and heating oil emissions compared with the semivolatile PAHs [20
]. Thus, we support the work of others that have described horizontal concentration gradient of PAHs associated with increasing distances from major roadways due to dilution effect [16
]. While prior studies have focused on measurement of the dilution gradient at ground level, our measurements were taken from the 5th story apartment window. The height of the building also may influence the dilution effect since the air mass travels up and over the building from ground level emissions, given our previously published evidence on vertical gradients in PAH concentrations [17
]. Our study further illustrates the dilution effect is maintained even above ground level, especially for nonvolatile PAHs whose major sources were traffic emissions at our central site.
In addition, differences in the structural environment surrounding the two sampling locations at our central site may influence the differences in PAHs between the two locations. The street side monitor was placed in an open location at the front of the building, directly adjacent to an active ambulance bay with idling vehicles, while the alley provided a semi-closed space surrounded by other buildings. Lower PAH concentrations from the alley could be due to the possibility that air sampled from the alley was more stagnant because of limited fresh air exchange, thus rendering the PAHs more subject to losses (particle impaction or degradation) compared with the more frequently circulated air sampled from the front, or street side of the building. Unlike other pollutants, such as elemental carbon or metal components, PAHs, in particular nonvolatile PAHs, have been shown to degrade during transport and deposition. This degradation can differ by the volatility, photo-transformation rate and bio-degradation rate of individual PAHs [33
]. In addition, there is a growing body of literature to addresses concerns that “urban street canyons” (streets that are flanked on both sides by tall buildings) have some of the highest concentrations of traffic related particulate pollutants [35
]. Ng and Chau used mathematical models to calculate the potential exposure to pollutants in different micro-environments within urban street canyon and demonstrated that building setbacks, or distance from the road, reduced personal exposure [38
]. Therefore, another potential interpretation of our results is that the structural environment of the alley side of the building offers protection from the urban street canyon effect of exposure to pollutants, similar to that of a building setback.
In our neighborhood-wide comparison, we observed a consistent pattern in that the levels of Σ8
as well as most of the individual nonvolatile PAHs measured from the street side of buildings were also higher than those from alleys. This pattern was more apparent during the heating season, compared to the nonheating season. However, we did not observe appreciable differences in Σ8
. Our lack in significant difference in neighborhood-wide semivolatile PAHs may be related to seasonal variation, which is known to be one of the most important factors affecting ambient PAH concentrations [21
]. For example, semivolatile PAH concentrations are substantially higher during the nonheating season compared to heating season [21
]. Unlike the central site paired comparison between alley and street, most samples from the neighborhood-wide comparison were not concurrent. The lack of appreciable difference between neighborhood-wide street and alley Σ8
may be explained by uneven sample collection from either season (Table 3
. eleven samples in nonheating season, seven samples in heating season), canceling out the effect of the built environment. This was supported by stratified analysis by heating season (Table 3
). Although our small sample size limited our power to detect significant differences, after controlling for heating season, there was a trend toward higher street vs.
alley side Σ8
and individual semivolatile PAHs measured across the neighborhood.
We also acknowledge that the original intent of the neighborhood-wide evaluation of outdoor PAH measurements was not to investigate differences in concentrations by sampling site. Hence, the street side and alley side measurements were not sampled simultaneously. In addition, we were unable to adequately control for variations in measurements by other major emissions sources (e.g., traffic and residential heating oil emissions), floor height, as well as influences from other environmental conditions (e.g., temperature, humidity, and wind speed), that may significantly affect neighborhood outdoor PAH concentrations. These factors may contribute some error in our measurement of PAHs. However, our two week averaged PAH measurements were exposed to various wind speeds and directions as well diurnal variations in daily traffic emissions, thus offering a comprehensive representation of exposure compared to shorter duration measurements. Yet we acknowledge that one limitation of two-week sampling is that samples are more subject to the potential degradation of nonvolatile PAHs by ambient ozone after collection on the filter as shown in our previous publication [21
], likely independent of site of monitor placement. In addition, consideration of factors that contribute to the urban street canyon effect, such as distance between buildings, building setbacks, turbulence of airflow and other micrometeorological conditions should be considered in future replications studies. Despite these limitations, the crude analysis of the neighborhood-wide samples corroborates our findings at the central site and supports our main hypothesis that PAH concentrations measured at the front of urban buildings, adjacent to street traffic, are higher than those measured at the back or alley side of buildings.